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THE

ANATOMY OF THE MOUTH-PARTS

AND OF THE SUCKING APPARATUS

OF SOME DIPTERA.

i

(

DISSERTATION

FOR THE PURPOSE OF OBTAINING

^ i THE PHILOSOPHICAL DOCTOEATE

AT THE LEIPZIG UNIVERSITY,

BY

OF CAMBRIDGE, MASS., U. S. A.

BOSTON:

A. WILLIAMS & CO.

1881.

lit

I

The few diptera whose mouth-parts have been the object of the anatom-
ical studies, the results of which are noted in the following pages, were
chosen, on the one hand, with especial reference to their presenting a series
beginning with a species possessing simple, separate and fully developed mouth-
parts, and ending with a species of which the complexity of the mouth-parts
was due to coalescence and incomplete development of their different elements,
and, on the other hand, with partial reference to forms whose mouth-parts were
of sufficient length to render their study by sections, made with the microtome,
of value in determining their relative lengths, their positions and their attach-
ments. With the above-mentioned objects in view species of the genera Cuk.i;
Bombylius, Installs, and Muxca were chosen. Upon the anatomy of the mouth-
parts of Culcx the following notes probably add most to what has been previously
recorded. No study of the development of the mouth-parts has been made in
preparing the following paper; the results are anatomical only.

The work necessary for the preparation of this paper was done by me,
as student, in the Laboratory of the Zoological Institute in Leipzig, and I gladly
take this opportunity of expressing my sincerest thanks to its director, my
honored instructor, Professor Leuckart, for the advice and encouragement which
he has given me in my studies.

HISTORICAL.

Until FABRICIUS, in J7<5, first called attention to the importance of the
mouth-parts in the classification of insects, little or no valuable progress had been
made in the study of their comparative anatomy. Observations on the anatomy
of the mouth-parts of single insects are to be found in the works of many
writers, previous to the above-mentioned date, but they can be best regarded as
forming only a part of the history of the anatomy of single insects, and, so far
as they treat of insects which are further discussed in this paper, they will be
mentioned later.

1

Fabricius writes, in the preface of his Systema entomologiae, * after dis-
cussing the confusion which existed in the classification of insects at his time,
"Therefore I will try a new method, choosing characters, both of classes and
of genera, from the month-parts. These parts truly offer sufficient and con-
stant characters and far more natural genera." * In his Philosophia cntom-
ologica 2 (p. 37-52), where his ideas are further elaborated, one finds that
Fabricius divided the mouth-parts of insects into palpi, clypeus (now the labrum),
mandibulae, maxillae, galea, labium, lingua, rostrum, proboscis and hanstellum.
The proboscis and haustellum, the parts which concern us most here, are somewhat
confused in Fabricius' classification. The proboscis, he writes (p. 38), is membran-
aceous and bilabiate ; the haustellum is extended, chitinous, and without articu-
lations ; ** with the former is no mention of setae, with the latter, he adds, setae
differing in number. He writes (p. 49), ''The proboscis, so peculiar to the
diptera, is not possessed by all of them," *** and, on the next page he gives
Bombylius as without proboscis. Again he writes (p. 52). "The proboscis
differs from , the haustellum in that the proboscis is always bilabiate, the
haustellum never," f and that, " The haustellum, being the characteristic of
the class, is always present in the diptera; the proboscis is often entirely
absent." ff Furthermore Fabricius writes (p. 30), "In the diptera are
two palpi, a haustellum very often received in a proboscis, while mandibles,
maxillae, clypeus and labium are absent." fff From the above quotations it is
clear that Fabricius meant by the proboscis what is now known to be, for the
most part, labium; by haustellum, following the derivation of the word, he meant
the true suction-tube, whether of one part, or made up of setae. The confusion
which arises in the Fabrician use of proboscis and haustellum is mainly due to
his failing to recognize, in his so-called "haustellum" of Cuh\r. and Bombylius,
parts which correspond to his "proboscis" in Musca, *Syrj>/m* and other diptera.

1 For full titles referred to by superior figures, see Literature at the end of this paper.

N "Novam ideo viam tentabo, characteres et classium et generum ex instruments

cibariis desumens. Praebent sane sufficientes, praebent constantes et genera multo naturaliora."

** "Proboscis membranacea, bilabiata" "Haustellum porrectum, corneum,

inarticulatum."

*** " Proboscis tantum Antliatis propria, nee omnia ilia ea gaudent."

r|- " Proboscis diftert ab haustello, quod proboscis semper bilabiata, haustellum vero
nunquam."

-|-f " Haustellum, classis continens characterem, semper in Antliatis adest, proboscis
saepe omnino deest."

fft "Palpi duo, haustellum saepius proboscide receptum, absque mandibulis, maxillis,
clypeo et labio in Antliatis."

Since Fabricius' time the terms proboscis and haustellum have been used somewhat
indefinitely. Gerstfeldt sought to explain (p. 13 of his dissertation") Fabricius'
meaning, but seems to have failed in clearly comprehending the difference which
the latter intended to establish between proboscis and haustellum. In the course
of this paper proboscis will be used, in its generally accepted meaning at present,
as a designation for such of the mouth-parts of diptera, taken together, as form
their more or less flexible, shorter or longer, sucking apparatus.

According to J. V. Cams (Geschichte der Zoologie, p. 559) Fabricius also
divided the insects into sucking and chewing species, a classification followed later
by Clairville, 4 Lamarck, Gravenhorst, and many others.

SAvifiNY, 5 in 1816, took the next important step in advancing our knowledge
of the mouth-parts of insects, and showed that all the mouth-parts of insects
were reducible to the same plan, that is to the plan which they present in
chewing insects. Most of Savigny's work was done on lepidoptera, because the
lepidoptera were supposed to have mouth-parts farthest removed in structure from
those of chewing insects. Savigny says (p. 10-J1), "I am convinced that, when
one shall have better examined the mouth of the insects properly speaking, that
is to say those with six feet and two antennae, one will find that, whatever
form it may assume, it is always essentially composed of the same elements." . . .
" One discovers that the organ is the same ; the use alone is modified or
changed. See the constant plan of nature. Thus I think I can assert, from
this time on, that the mouth of the diptera is formed of the same parts as that
of the hymenoptera. But to prove this proposition it is necessary to commence
by explaining the organization of the mouth of the hymenoptera." * Savigny
says further (p. 13-14), "The mouth of the hymenoptera is, then, composed of four
unpaired organs, without including the jaw or mentimi; namely, the upper lip,
the epipharynx, the hypopharynx and the under lip ; and of two pairs of organs,
the mandibles and the maxillae. The same organs are all found, either separately
or simultaneously in the mouth of the diptera. The under lip exists almost
always; it constitutes the proboscis, properly speaking. The maxillae exist
likewise almost always; it is these organs which bear the palpi, so that the

* '-Je suis convaincu que, lorsqu'on aura mieux examine la bouche des insectes propre-
ment dits, c'est-a-dire, asixpattes et a deux antennes, on trouvera que, quelque forme qu'elle
affecte. elle esttoujours essentiellement composee des memes elements." .... "On sait que
1'organe est le nieme: 1'usage seul est modifie ou change. Voila le plan constant de la
nature. Ainsi je crois pouvoir assurer des a present que la bouche des Dipteres est formee
des memes parties que celle des Hymenopteres. Mais pour demontrer cette proposition, il
faudrait commencer par exposer I'organisation de la bouche des Hymenopteres."

1*

diptera have two maxillary palpi aiid have no labial palpi. When the maxillae
.are apparently absent, as in the flies properly speaking, they coalesce with the
under lip. The mandibles are found only in certain genera ; they are, for example,
very evident in the breeze -flies, where they have the form of two slender blades.
The hypopharynx and epipharynx are the bristle, or the two intervening bristles.
The upper lip is a bristle or broader scale which covers the others." *

'Saviguy did not stop with the reduction of the mouth-parts of all insects
to a common plan, a result which seemed fairly to complete the work begun by
Fabricius, but, in his second memoir, recognizing the resemblance between the
mouth-parts of hexapods and those of the remaining arthropods (which he terms
apiropods), and the graduated forms between the locomotory and manducatory
appendages of some of these apiropods, he concludes (p. 43) that, "among these
last apiropods [by which he means Limulus, A pus, etc.] the organs which serve
for manducation do not essentially differ from those which, among the first
apiropods [meaning lulus, Scolopendra, and the like], and among the hexapods,
serve for locomotion." ** Here was a direct homologizing of the mouth-parts of
insects with locomotory organs, a theory which Saviguy had publicly advanced
as early as October 1814. *** The comparison of the paired appendages of
spiders, scoloperidra, and Crustacea, which follows (p. 43-60 and 83-101) in his
memoir, has little interest in connection with the study of diptera.

After Savigny had thus led the way, by his theories that all mouth-parts
of insects were modifications of the same general plan, and that the paired mouth-
parts were serial homologs of legs, little was left in the way of general theories

: "Voilii done la bouche jdes Hymenopteres composee de quatre organes impaires.
sans y comprendre la ganache ou le menton; savoir: la levre superieure, 1'epipharynx,
1'hypopliarynx et la levre inferieure, et de deux organes paires, les mandibules et les
maehoires. Les memes organes se retrouvent tous, soit separement, soit simultanement, dans
la bouche des Dipteres. La levre inferieure existe presque toujours; elle constitue la trompe
proprement dite. Les maehoires existe'nt de meme presque toujours: ce sont elles qui
portent les palpes, de sorte que les Dipteres ont deux palpes maxillaires, et n'ont point de
palpes labiaux. Quand les maehoires semblent disparaitre, comme dans les Mouches pro-
prement dites, c'est qu'elles se confondent avec la levre inferieure. Les mandibules ne
s'observent que dans quelques genres: elles sont, par exemple, tres- visible dans les Taons,
ou elles ont la forme de deux lames tres-deliees. L'hypopharynx et 1'epipharynx sont la
soie, ou les deux soies intermediates. La levre superieure est une. soie ou ecaille plus large
j]ui couvre les autres."

* Chez ces derniers Apiropodes, les organes qui servent a la manducation ne dif-
ferent pas essentiellement de ceux qui, chez les premiers Apiropodes et chez les Hexapodes,
servent a la locomotion."

*** See foot-note of p. 43 of Savigny's second memoir.

which later investigators could advance, and we consequently find them confining
their attention to minor anatomical points, or to discussions of how many and
which of the mouth-parts are really homologs of legs, and to homologizing the
coxal, femoral, tibial, or tarsal parts of insects' legs with portions of their
mouth-parts. Savigny had already expressed his opinion (p. 41 of his second
memoir), that, besides the paired organs, the under lip " can be considered as
formed by the union of the two second maxillae." *

ERICHSON, 6 in 1840 (p. 4-7 of . his Entomographieri), discusses the mouth-
parts of insects, devoting the greater part of his space to the consideration of
the under lip, which he regarded as originating in the union of a pair of organs,
in fact, speaking of it throughout his paper as the third pair of jaws. ** He
says (p. 4-5) "The third mandibular pair forms, in insects proper, a constant
part of the under lip, which is formed by the union of this pair with the mentum
and with the tongue (ligula). This third pair has the mentum behind and
beneath, and the ligula above and before it, but is always characterized by
labial palpi which belong to it." *** He further adds (p. 7), " In the two
other orders of insects with sucking mouth-parts, the diptera and hcmiptera, the
coalescence of the third pair extends still further upon the palpi, which form the
tubes of the proboscis, and surround the remaining bristle-formed mouth-parts." f
In the male of Culcs I have found, as will be more fully described later, a
somewhat analogous case of union of the tongue with the labium ; that is, ana-
logous, if one assumes the tongue, of Erichson, to be the hypopharyux, of Sa-
vigny, an assumption which may be incorrect, since Erichson fails to define clearly
what he means by tongue, or ligula. It is furthermore very probable that, as
Savigny stated, the labial palpi are absent in diptera, and I find nothing, in my
own observations, or in those of others so far as I have compared them, to show
that the proboscis of diptera is, in any way, the product of labial palpi. The
homologies found in Ericbson's paper are of doubtful value, at least so far as

* "(^u'on peut considerer comme fornx'e par la reunion ties deux secondes machoires.''
** "Das dritte Kieferpaar."

* li Das dritte Kieferpaar maclit bei den eigentlichen Insecten bestandig einen Theil
der Unterlippe aus, welclie durch die Vereinigung desselben mit dem Kinn (mentum} und
der Zunge (ligula) gebildet wird. Dieses dritte Kieferpaar hat nun das Kinn hinter und
unter, und die Zunge ubcr und vor sich, wird indess immer durch die ihra angehorenden
Lippentaster bemerkbar."

j- "jBei den beiden andern Ordnungen der Insecten mit saugenden Mundtheilen, den
Dipteren und den Hemipteren, erstreckt sich das Yerwachsen des dritten Kieferpaares noch
welter auf die Taster, welclie die Rohre des Riissels bilden, und die iibrigen borstenformigen
Mundtheile umschliessen."

I) -

they concern insects with sucking mouth-parts, because that, up to his time,
and indeed up to the present time, no sufficiently careful work has been done
on these parts to warrant such general conclusions as he pronounces.

The next important paper, in chronological order, upon the mouth-parts of
insects was by BRULLE, 7 in 1844. I have not seen this paper, but I am able
to give a brief resume of the author's views, from the reviews of his paper
which appear in the dissertation by Gerstfeldt 3 (p. 5-9), in a paper by Menzbier s
(p. 18-19), and in Erichson's Bericht iiler die wissenschaftlicJien Leistungen im
Gebiete der Entomologie (1844, p. 3-4). Savigny had regarded the labrum,
epipharynx, and hypopharynx, as unpaired organs. Brulle sought to include these
three organs in the system of paired appendages, which Savigny had established
for the mandibles, maxillae and labium. Brulle considered the insect's head as
made up of six segments, each with a pair of appendages, or with an unpaired
appendage which owed its origin to the coalescence of a pair of appendages. The
first segment consequently bore the labrum, the second the epipharynx, the third
the hypopharynx, the fourth the mandibles, the fifth the maxillae, and the sixth
the labium.

BLANCHARD, 9 in 1850, considered that the most important modifications of
the mouth -parts of diptera were caused by coalescence of parts, and endeavored
to support his views by the origin of the nerves which go to these parts. In
the Asi/idae, where only four setae are present, he regarded the mandibles as
present, but grown together to form what is now termed the hypopharynx. In
the Museidae. and in other diptera with two setae, the mandibles were united
to form the hypopharynx, and the maxillae coalesced with the labium. The
above-expressed view that the so-called hypopharynx is composed of two mandibles
grown together, a theory followed in Cuvier's Regne Animal, does not seem so
improbable as it otherwise would in the light of Weismann's 10 statement (p. 190)
that, in the Muscidae "The mandibular seta arises by coalescence of paired pieces,
which surround, like the two halves of a sheath, a cylindrical chain of cells which
tapers anteriorly. The cell-chain becomes the salivary duct, which in the imago
comes -from behind to the under surface of the seta, so as to unite with it and
to open out a little behind its point with a fine opening." *

GERSTFELDT, S in 1853, (p. 13-47) discusses the mouth-parts of diptera. After

* " Die Kieferborste entsteht durch Verwachsung paariger Stiicke, welche einen cylin-
drischen, nach vorn sich verjiingenden Zellenstrang umschliessen, wie die zwei Halften eines
Futterals. Der Strang wird zum Ausfxihrungsgang der Speicheldriisen, der in der Imago
von hinten her an die untere Flache der Borste tritt, um mit ihr zu verwachsen und etwas
vor der Spitze mit feiner Oeffnung auszumiinden."

preliminary remarks on the nature, size, direction, and general form of the
proboscis, the author considers the parts and their nomenclature according to
previous authors. His compilation of authorities is very complete. In a few
words, the author's own views may be condensed as follows : The proboscis of
the diptera consists of a sheath and its enclosed setae, or bristles. The sheath
is made up, sometimes of the labium alone, sometimes of the labium as the chief
portion, together with other mouth-parts; it can be regarded as consisting of
three parts, the basis, the stem or stalk, and the end-lobes (labellae of Kirby).
The setae or bristles enclosed in the sheath represent the remaining mouth-parts,
and are either given as two, four, or six ; or as one, three, or five ; according
as whether one includes the labrura with them or not. The labrum is more or
less elongated, sometimes with a longitudinal furrow in the middle, supposed by
Gerstfeldt to be an indication that this mouth-part was composed originally of
lateral halves. The other setae, when they are all present, are the mandibles*
maxillae and hypopharynx. Parts of the mandibles or maxillae sometimes unite
with the labium to help form the sheath. The hypopharynx, or tongue, is absent
in very few diptera. The epipharynx perhaps exists in a few cases, according to
Gerstfeldt (p. 21), altho he says he has nowhere seen it. At the basis of the
proboscis are regularly, if not invariably, two more or less developed palpi.
Following the portions of Gerstfeldt's paper, of which the proceeding is an abstract,
is a brief description of the mouth-parts, their mode of coalescence, and their
presence or absence, in different families and genera of the diptera, beginning with
diptera having two setae and ending with those having six.

Gerstfeldt's work was based, as is apparent from' his introductory remarks
(p. 4-5 ), on the view of Brulle that fill the mouth-parts of insects were reducible
to the modification of six pairs of appendages, themselves serial homologs of the
thoracic ambulatory appendages. The theory seems to have been assumed at
the outset of the author's work, and, therefore, to have had too much influence
on the conclusions which Gerstfeldt drew from his othenvise, in general, accurate
observations, for Gerstfeldt examined a large variety of material in the preparation
of his paper, and examined it carefully, as far as the instruments at the disposal
of anatomists of his time would allow. His failure, however, to recognize the
epipharynx in diptera led him to make a curious mistake. He has described, as
Menzbier 8 correctly observes (p. 65), the epipharynx of Musca freed from the
labrum as the hypopharyux, and failed to discover the true hypopharynx. He
regarded the longitudinal "suture" (probably in reality the channel) of the
epipharynx (his hypopharynx), as an indication that the hypopharynx owes its
origin to the coalescence of paired organs. Another mistake which Gerstfeldt

made in regard to the mandibles of (W/V,r will receive notice later, in the
description of those parts.

Passing over a number of works, which dealt with the anatomy of single
species, or which had for their object the elucidation of the anatomy of special
organs, the next important discussion of the mouth-parts of diptera is by Menzbier.

MENZBIKR,* in 1880, devotes the first twenty pages of his paper to a critical
historical synopsis of the works in which the mouth-parts of diptera have been
especially considered, and sixteen pages further to a critical historical summary
of the literature which treats of the development of the cliitin covering and
appendages of insects, and to the results of the studies of Weismann, Kunckel
d'Herculais. and others, upon the histoblasts, or imaginal disks. Succeeding the
historical part of his paper, Meuzbier details the results of his own investigations.
After enumerating the regions of the head, according to his ideas of its structure,
he explains (p. f)3-(>6) the mouth-parts of Haematopota, Si/rpkn.^ Empis, Mum-n
and Sargus, these genera representing a series, in which the first has all the
typical mouth-parts of a chewing insect, and the last only the labrum, labium,
and maxillary palpi. Menzbier seems to have been the first to recognize the
usual intimate union of the labrum and epipharynx in diptera, where he finds
them often separable by maceration in caustic potash. The following tabulation
gives, at a glance, the more important points in Menzbier's paper, in regard
to the mouth-parts of those diptera which he studied.

Labrum and
Epipharynx.

Hypopharynx

Mandibles.

Maxillae.

Labium.

Haematopota \
and Clirysops. \

Separable in
caustic potash.

lamelliform
with channel
on upper sur-
face.

lamelliform

acicular.
with palpi.

muscular.

Syrphus taeniatus.

do.

do.

do.

palpi only.

do.

Empis livida.

Not separable in
caustic potash.

do.

absent.

united to form
an unpaired
needle, with
palpi.

Bilobed
at tip.

Musca species,
Sarcophaga carnaria
Stomoxys calcitrans.

Separable in
caustic potash.

do.

do.

palpi only.

muscular.

Sargus nubeculosus.

Labrum present,
epipharynx absent.

absent.

do.

do.

fleshy.

The fulcrum, which Gerstfeldt regarded as united maxillae, Menzbier considers
to be chitinized processes of the oesophagus.

That the maxillae are grown together in Empis livida, is, perhaps, an error.
My having no specimens of Empis prevented my proving this point. Gerstfeldt
writes (p. 3.1-32), "Four setae which always represent the maxillae, the upper
lip and the hypopharynx, while the mandibles are united with the sheath of the
proboscis - - are found in the Etnpklae (in Einph the maxillae are also shorter
than the hypopharynx, but this exceeds the upper lip; all four setae are pointed
and horny)." *

After recording some of his observations on the development of the epipharynx
and hypopharynx, Menzbier closes with a summary of his conclusions, which
would he superfluous, if transcribed here.

With this short resume of the advances made in the study of the general
structure of the mouth-parts of insects, especially those of diptera, I will pass
on to the recording of results of my own observations on single genera, prefixing
to my own remarks, brief notices of the work done by others in the same direction.

ANATOMY OF THE MOUTH-PARTS OF CULEX.

From early times Cule.v has attracted the attention of mankind, but, until
about two centuries ago, little or no progress was made in the knowledge of the
inner structure of its mouth-parts.

SWAMMEKDAMM, 11 who studied Culc.v in 1668. distinguished the male and
female, but evidently supposed that the structure of the proboscis of both sexes
was the same. He correctly distinguishes the long maxillary palpi of the males
from the short palpi of the females. He clearly recognized that there were six
mouth-parts enclosed in a sheath, but erroneously supposed them to be protrusile
from the end of the sheath, without flexion of the latter, and thus figured them.
The largest of these enclosed mouth-parts (really the combined labrum and
epipharynx) he supposed enclosed the other five, and he says (p. 147 of the
German edition of 1752), "I regard it that these five setae serve, like as many
sharp little awls to make the opening in the sweat-pores of the skin. When
this is done they draw themselves back again into the inner sheath. This then

1 "Vier Borsten - - die inimer den Maxillen, der Oberlippe und dem hypopharynx ent-
sprechen, wahrend die Mandiblen mit der Riisselscheide verschmolzen sind - - besitzen" . . .
"die Enqiidi'ii (bei Empis sind die Maxillen ebenfalls kiirzer als der hypopharynx, dieser
iiberragt aber die Oberlippe; alle vier Borsten sind zugespitzt und hornig)." . . .

10

enters (according to my idea) into the wound with its sharp cavity, and the
mosquito sucks through it the blood, which ascends alongside and between the
little setae into the belly of the mosquito." * The apical opening of this inner
sheath he correctly thought to be turned toward the ventral side.

LEEUWENHOEK next endeavored to settle the structure of the proboscis of
Culex. All that I know of his work is from what Reaumur 12 (13 h memoire,
t. 4, p. 401-403) says of it. Leeuwenhoek found only four setae in the sheath
of the proboscis, and doubted that the inner sheath, described by Swammerdamm,
existed as a closed tube. BARTH, 13 on -the other hand, in 1737, according to
Reaumur (1. c., p. 403), thought the inner sheath was a closed tube and not a
channel.

REAUMUR, 12 in 1738, published his description of the mouth-parts of Culex,
and of their mode of biting; to the latter subject he gave much attention, and
was the first to describe correctly how the sheath was disposed of during the
use of the setae within, in the act of biting. He also calls attention to the
poisonous effects of the bite, and, contrary to the views of Leeuweuhoek, who
thought the inflammation following the bite of Cule,r was due to the peculiar
nature of the wound itself, that is, that they were the natural consequences of a
wound made by an instrument of a particular form, Reaumur regarded the sub-
sequent inflammation of the place bitten by Ctde.r to be due to a poisonous liquid
which the insect injects into the wound, in order to cause the blood to flow
faster. Reaumur found only five of the six setae which the proboscis contains.
He favored the idea that the inner sheath, which Swammerdamm had described,
was not cylindrical, but only a channel open on one side. Reaumur also arrived
at the idea that the maxillary palpi of Cule.r could, in some cases, help to form
the sheath which encloses the setae, but he does not clearly say that they always
do so in the males.

Since Reaumur's time but little has been added to our knowledge of the
mouth-parts of Culex, some writers following the statements of Swammerdamm,
others those of Reaumur, or of Leeuwenhoek, in regard to the number of setae.
Among others I will cite Sulzer 14 (176J), who says "four to five pointed tubules;"**

*

* "Ich halte davor, diese fiinf Angelgen dienen dazu, als mit so viel spitzigen Pfriem-
gen die Oeft'nung in den Schweisslb'chern der Haut zu machen. Wenn dass gethan, so
ziehen sie sich wiederum in die innere Scheide zuriick. Diese dringt dann (nach meinem
Begriff) mit ihrer spitzigen Hb'hle in die Wunde hinein, und die Miicke saugt durch sie das
Blut in sich, das neben und zwischen den kleinen Angeln bin in den Bauch der Miicke
hinaufsteigt."

** "45 spitzigen Rohrchen." (p. 169.)

JJ

>

Fabricius l (1775), who writes, "sheath exserted, uuivalvular, flexible, with five
setae;"* Jordens 18 (1801) describes four setae;** Gravenhorst 15 (1817), "The
proboscis long, setiform, five-parted;*** Meigen 16 (1818) describes four setae and
figures five;f Gerstfeldt 3 (1853), "The sheath is formed of the under lip alone,
and contains six setae;" ft Packard 17 (1869), "These six bristle-like organs are
folded together within the hollowed labium;" ftt Clausal 876) writes, proboscis
"extended with four setae."

EoFFKEDRi's paper, 20 in 1769, upon the mouth-parts of Culex, I know only
by title.

Without being unduly influenced by one or another of the authorities above
mentioned, I will set forth the structure of the mouth-parts of Cide.e, both male
and female, as I have found them in my own observations, in the following pages.

THE MOUTH-PARTS OF CULEX, FEMALE.

The mouth-parts which form the proboscis of the female Culcx, as I have
found them by study of C. rufus, C. ciliatus and C. pipiens, represent all the
typical mouth-parts of different insects, and consist of a labrum (PI. 1, fig. 1, A 1 .),
an epipharynx (e), a hypopharynx (/*), two mandibles (w) and two maxillae (m.r),
all sheathed, when in repose, in the labium (/), which receives them into a groove
on its upper side. Each maxilla has a maxillary palpus (m p) which lies outside,
and at the base 'of, the labium; the latter has no palpi, unless one regards, as
Erichson did, the lobes at its tip to be palpi. Of these mouth-parts the labium
and maxillary palpi are covered with hair and scales, the others are naked, light
brown, setiform and transparent; they all originate at the anterior basal portion
of the head, below the eyes and antennae (a), and are, with the exception of the
maxillary palpi, of about equal lengths, that is, about three to four times the
length of the head. The maxillary palpi, in the females of Cule.r proper, are
about the length of the head. Those mouth-parts which are without scales, and

* "Vagina exserta, univalvis, flexilis, setis quinque." (p. 799.)
** P. 162, of Band I.

*** "Der Riissel lang, borstenfb'rmig, fiinftheilig." (p. 44.)
f P. 2 and pi. i of Theil I.

ff "Die Scheide wird von der Unterlippe allein gebildet und enthalt sechs Borsten."
(p. 32.)

tff P. 369-370.

" Vorgestreckt mit 4 Stechborsten." (p. 682.)

these are the ones that penetrate the skin in biting, are not jointed; the labium
of the female Cidex is not jointed throughout its length, but the lobes, or
labellae (Fig. 3 and 4, ll>), are jointed to it, by a true joint, near its tip; the
maxillary palpi are sometimes four-jointed, sometimes five-jointed, probably varying
according to the species. Unlike the mouth-parts of most diptera, those of
Cidea; with one exception, are free to the base ; this exception is that the labrum
and epipharynx are united their whole length, forming a piece, which is shown
in section in fig. 6, d. The labium and palpi are the only mouth-parts which
contain muscles. The epipharynx forms the channel through which Cidex sucks
up blood ; the hypopharynx probably furnishes a channel for the discharge of a
saliva-like fluid. A pumping organ, trianguloid in cross-section (fig. 10, />), for
sucking up blood into the pharynx, is formed by a dilation of the oesophagus
behind the oesophageal nerve-ring. Each of the above parts will be described
more in detail later. In comparative size and strength the mouth-parts would
be arranged as follows, the largest and stoutest first: labium, labrum-epipharynx
(the name by which I shall designate this compound piece), hypopharynx, maxillae,'
and mandibles.

The general arrangement of the mouth-parts, relative to each other, is shown
best in fig. 8, which is of a cross-section through the middle of the proboscis
of a female Cule.r rufus, while in repose, with the setae sheathed in the labium.
The labium (7), clothed on the outer side with its scales and hairs, wraps itself
nearly around the other mouth-parts. In it lay the two maxillae (w.r), partly
enclosing the parts above them, and thus helping to bind the parts together;
above the maxillae are the two mandibles (m), and immediately above the man-
dibles, in a median line, is the hypopharynx (/*), with a thickened middle
portion. Resting on the hypopharynx is the labrum-epipharynx (/-> artd <>) ;
the epipharynx is omega-form in section, and above it, delicately attached, is
the labrum. The determination of the positions of the hypopharynx, mandibles
and maxillae, in a section, is difficult, because of their minuteness and
transparency, and because that they are very closely packed together, much
more closely, relatively, than they are represented in figure 8. Their
position was finally determined by the following process, which I repeated
several times to insure accuracy. The section of the proboscis, still in the par-
rafine in which it was cut, was put on a microscopic slide, covered with a
cover-glass, and a piece of blotting-paper laid at that side of the cover-glass
toward which the labium was turned. Then, while watching the parts carefully
with the microscope, turpentine was passed under the cover-glass, at the side
opposite the blotting paper, and allowed to flow slowly about the object until it

was imbibed by the paper at the other side. Thus a constant stream of turpentine
flowed against the upper side of the mouth-parts, and they were dissolved away,
in the field of the microscope, one after the other, in the order, labrum-epi-
pharynx, hypopharynx, etc. The position of the parts can be also determined by
pressing the head of Cnle.i 1 laterally between a cover-glass and a slide; in
perhaps one case out of every twenty tried, the parts will arrange themselves as
they did in the specimen from which fig. 1 was drawn. The changes in position
which the mouth-parts of Cnh'.r undergo as they approach the head can be best
described in the subsequent description, in detail, of each separate part.

The labrum-epipharynx (figs. 1, 5, 6, 7-8; lr and <?) of Culex consists of the
thin labrum resting upon and fastened to the epipharynx ; it tapers gradually
from base to apex. The epipharyux is omega-form, being a channel rather than a
tube, a tube being formed by the pressing of the hypopharynx upon its under
side. The tube thus formed is the channel through which the blood, which
Cule.i- sucks, passes into the oesophagus. At its base or proximal end the
epipharynx is supported and moved by strong muscles having their insertions on
the upper side of its wings or lateral portions, and upon the upper side of its
tube. These muscles extend upward and posteriorly, and have their origin on
the inner surface of the clypeus. (See fig. 9 and 11.) These muscles (;/<) by
their contraction, elevate, and perhaps slightly retract, the epipharynx, and the
labrum, to which they are also attached. These muscles probably aid in suction,
for, when the setae are all stuck firmly in the skin, the contraction of these
muscles would only serve to raise the base of the epipharynx from that of the
hypopharyux; this action would tend to produce a vacuum between the two (see
fig. 9), and thus cause the blood to be drawn up in the tube of the epipharynx.
The probability that these muscles aid in suction is augmented by the fact that
the corresponding muscles in other flies, which cannot raise their epipharynx so
freely from their other mouth-parts as is seen in fig. 1, these muscles are devoted
to suction; and further, that in the male Culex, which does not possess - - as
does the female a pumping apparatus behind the oesophageal nerve-ring, these
muscles are the ones that must serve for suction. The section, represented in
fig. 9, was taken near the base of the clypeus; a few sections further on,
posteriorly, the channel for the passage of food turns upward and then backward
again, passing in its course a place (fig. J 1 , r) where its walls approximate
dorsally and ventrally. This narrowing of the walls is probably a valve to
prevent the return of fluids to the mouth during the pumping process. I term the
portion in which this sucking process is carried on, the portion of the tube which
is between the mouth (where the mouth-parts unite to form a closed tube), and

14

this valve, the pharynx; posterior to this valve, oesophagus. These limitations
of the oesophageal region will be found convenient later, when I compare the
mouth-parts and the sucking pharynx of Gulex with those of other diptera.

The tip of the labrum-epipharynx seems to turn upward (fig. J, lr-e), altho
the opening is upon the ventral surface, as may be seen in fig. 6, l>, whiqh
represents the ventral view of the tip of this part. The tip of the labrum-
epipharynx is comparable to a quill-pen with three tips near each other, the
middle one of these three tips being slightly shorter than the other two. The
two lateral portions of the epipharynx, as seen in section, when they near the
tip, lay themselves closely upon the sides of the tubular portion, passing upward
upon it, as seen in fig. 5, lr-e; they thus serve to strengthen the two outer
points of the tip of the epipharyux, while] the labrum continues to a sharp point
at the tip, and, united with the upper surface of the epipharynx tube, forms the
middle point of the tip. The channel, or slit, along the under side of the epi-
pharynx, widens toward the tip, leaving thus an opening for the passage of fluids
into the tube of the epipharynx.

The labrum itself is a thin lanceolate lamella of chitin, concave along the
under side from the basal portion to the tip, and its concavity rests upon and
fits- to the convexity of the tubular part of the epipharynx, to which it is so
lightly attached that they readily separate by application of caustic potash. The
outer edges of the labrum roll slightly inward toward the epipharynx along most
of its length. (See fig. 6, d.) At its base the labrum sends a chitiuous support
beneath the clypeus, where it separates more from the epipharynx and has its
own muscles, indicating that the labrum has a degree of motion independent of the
epipharynx, a motion allowed, perhaps, by the elasticity of the connection between
the labrum and epipharynx. The muscles of the labrum (fig. 9, p>n ') are inserted
upon the upper side of its base and have their origin on the inner surface of
the roof of the clypeus. These muscles are, at least in the females of Cule.i'
rufus, divided into three portions in their upper part, as shown in fig. 9.

The hypopharynx of the female of Culex is a linear, lanceolate, transparent
lamella of chitin, with a longitudinal rod through the middle, the nature of which
will be discussed later. At its base the hypopharynx forms the continuation of
the under wall of the pharynx. (See fig. 11, /;.) The hypopharynx is closely
pressed upon the under side of the epipharynx, completing the tube nearly formed
by the epipharynx. No muscles have their insertion on the base of the hypo-
pharynx. Its tip is simply lanceolate (fig. 5, A). In CuUx pipicnx and C. rufus
nothing further is visible (with a magnifying power of five hundred diameters),
in sections of the thicker middle portion of the hypopharynx, than a simple

- 15 -

rod of chitin ; but, in C. ciliatus, a North American species of which the mouth-
parts are larger, this rod appeared to be tubular. Is it a rod or is it a tube?
Menzbier 8 writes (p. 25) that in diptera " neither the labrum nor the hypopharynx
possesses a completed tube, but only a channel" * which leads into the salivary
duct. That Menzbier is incorrect in affirming that the hypopharynx has no
complete tube I have clearly proved in my observations on Bomlylim and
Ei'istalis; but the question still remains unsettled whether Culex has any passage,
either tube or groove, through the hypopharynx. Reaumur 12 (tome 4, part 2,
p. 396) discusses the probability of a poisonous fluid being secreted by Culex, to
cause the blood to flow more readily when it bites, and since his time writers
have, on the one hand, accepted this statement, without proving the presence of
such a fluid or of the glands to secrete it, or they have, on the other hand,
denied the existence of such a fluid, and affirmed, as Leeuwenhoek did, that the
swelling subsequent to the bite of Culex was due to the irritation produced by
the tearing of the mouth-parts in the skin, without the aid of a poisonous
secretion. After having experimented a large number of times with the living
mosquito, I am convinced that there is use made of a poisonous saliva; for,
when biting, if the mosquito fails to strike blood, which it often does On parts
of the back of my hand, it may have inserted its proboscis (labium of course
excepted) nearly full length, in from one to six directions, in the same place
and withdrawn its proboscis; indeed it may have inserted its proboscis, as often
occurs, in extremely sensitive parts; yet in such cases, if no blood be drawn, no
more effect is produced upon my skin than is produced by the prick of a sharp
needle; a red point appears only to disappear in a few hours. Certainly there
has been as much tearing of tissues in such a case as the above-mentioned, as
there is when Culex settles on a place richer in blood, and, with a single probing,
draws its fill. When the insect is allowed to draw its fill on the back of my
hand, the subsequent swelling lasts from forty to forty-eight hours, and tha
amount of poisonous effect upon me, as proved by numerous experiments, is in
direct proportion to the length of time which the Culex has occupied in actually
drawing blood. The above-mentioned facts would indicate a constant outpouring
of some sort of poisonous fluid during the blood-sucking process, and would
necessitate a tube or channel for its conduction. Now no other channel exists
through which saliva could pass from the base to the tip in the mouth-parts which
Culex inserts in the skin, and this, together with the position occupied by the

* "Dass weder das Labrum noch die Hypopharynx eine vollstiindige Rohre besitzt,
sondern nur einen Kanal."

salivary duct in other diptera, leads me to believe, without as yet being able to
give anatomical proof of it, that the hypopharynx of Culex contains a duct that
pours out its poisonous saliva. Not having fresh specimens of Culex ciliatiis,
and the extreme minuteness of the hypopharynx in the species of Culex available,
has precluded my determination of the actual presence of glands in connection
with this mouth-part.

The mandibles (figs. 1 and 8, w), the most delicate of the mouth-parts of
Cnli'.r, are two very thin linear-lanceolate lamellae of transparent chitin, which
rest with their inner edges beneath each half of the hypopharynx, their outer
edges projecting beyond its outer edge, on each side. The mandibles are so thin
and transparent, and so tightly pressed upon both the hypopharynx above and
the maxillae below, that they easily remain undiscovered in sections of the pro-
boscis. At the base of the proboscis they appear to have no muscular attach-
ments, but to lie imbedded in the connective tissue, beneath the pharynx and
above the maxillae. (See fig. 9.) They are slightly tapering from the base to
the tip, but are of equal thickness throughout their breadth ; at tLa tip they
have a slight thickening, in form of a letter V, with its opening turned toward
their very delicate, almost invisible tip. (See fig. 5, m.)

The maxillae (mistaken by Gerstfeldt * for the mandibles), are tapering
lamellae of chitin, apparently serrate at the tips. Each maxilla is thicker near
the inner edge, the thickening being formed by a solid chitinous shaft, which is
fixed longitudinally upon the upper side. (See fig. 5 and 8, mx.) The bases of
the maxillae join the stouter maxillary palpi just before passing under the cly-
peus, and immediately afterwards they join the labium, and become imbedded,
with the mandibles, in connective tissue. (See fig. 9, MX.) Their continuations
in the head are two delicate chitin-supports, each of which ends in a strong
muscle; this muscle, the retractor maxillae (fig. 10, rt>i), passes backward and
downward through the 'head, beneath the infraoesophageal ganglion, and has its
origin in the posterior basal part of the head. The maxillae probably have no
protractor muscle, their forward motion being due to the elasticity of the chitin
frame-work of the head. The shaft of the maxillae is very transparent, except
near the inner side where the chitin-rod runs; here it is brownish and more
opaque. Out from the above-mentioned chitin-rod extends a very delicate
feathering, or corrugation, of chitin to the edge of the most transparent portion

'" Gerstfeldt 3 (p. 33) says, "of which the mandibles, toothed at the end, are somewhat
broader but of the same length as the toothless maxillae." In the original, "von welchen
die am Ende gezahnten Mandibeln ctwas breiter, aber ebenso lang als die zahnlosen
Maxillen s ; nd."

of each maxilla, as seen upon the basal portion of fig. 5, w.r. In some species
of Cuh'.i' this feathering is right-angled with the direction of the chitiii rod; in
other species oblique-angled, with the angle pointing forward, so as to
form a series of barblike corrugations. The tip of the maxillae (fig. 5, /,')
is very acute, has none of the before-mentioned chitinous corrugations, but, in
their place, near the outer edge, is a row of papillae, which have their tips
slightly recurved toward the head, and consequently appear serrate. These papillae
are different in number in different species, and probably in different individuals
of the same species. That they are true papillae, and not points of a serrate
edge, is not always at first apparent, but becomes so by observing them in all
directions, and is still further shown, in certain species of Culex, where they are
situated near the middle of the blade of the maxilla. These papillae are upon
the upper surface of the maxillae, as can be readily seen, by preparing the mouth-
, parts by lateral pressure, as in fig. 1.

The maxillary palpi (figs. 1 , 2, and 9, mp.~) are four-jointed in some species
of Cule.r, five-jointed in others. At first sight they appear to be three-jointed,
but more careful examination serves to show that the apparent basal joint is
made up of two joints, and oftentimes to reveal a very short, knob-like joint at
the extremity of what appears to be, at first, the apical joint. At their base the
maxillary palpi join the maxillae just before the latter pass beneath the clypeus,
and, with the maxillae, join the other mouth-parts, as shown, in section, by fig. 9.

The function of the maxillae is, probably, to draw the other mouth-parts into
the skin, when (. 'nh>,i- bites, for if one watches the maxillary palpi of Cuk.v, while
the 'setae are entering the skin, the setae seem to pierce the skin, and enter it
with a slow gliding motion, as if drawn from below, instead of pressed from
above ; meanwhile, if one observes carefully, with a lens, the maxillary palpi can
be seen to be in an alternating motion, as if the maxillae to which they are
attached, pressed, first one then the other, into the skin, and then pulled the
other parts after them. The muscles, retractores maxillarurn, already described,
lend weight to this view of the functions of the barbed maxillae.

The labium .(figs. 1, 2, and 3, /), the largest of the mouth-parts of Culex,
and the only one of them, helping form the proboscis, which contains muscles,
forms a sheath opening along the upper side, and receiving in its channel the
other mouth-parts (excepting the maxillary palpi), as seen in cross-section in
fig. 8; it tapers from base to tip, is flexible, has a delicately aunulated structure.,
and is clothed with hair and scales. At its base it unites with the maxillae,
mandibles, and hypopharynx, and continues into the under surface of the head.
Throughout its length it contains, on each side, muscles, which have their origin

2

1.S

in the base of the head and serve to control the motions of the lahium. (See
figs. 8 and 9, ml.) At the sides of the tip of the labium are attached two
lobiform appendages, the labellae, which are seen at II in fig. 3 with the true
tip of the labium proper between them. These terminal lobes are jointed to the
labium, a little distance behind its tip, as can be seen in fig. 7, which is a
cross-section of the labium a trifle anterior to the actual centre of motion of
these joints. The section of that portion of the labium which extends forward to
form its tip is seen in the middle of the figure, just below the section of the
maxillae (<,/). Outside the section of each lobe is seen the section of the exterior
edge of the labium itself, which here forms a double socket, or pair of acetabula,
into which the heads of the two labellae are set. The reason why this exterior
edge of the labium does not appear, in the section, as entirely surrounding the
sections of the labellae, is that the labium extends forward further on the dorsal
and ventral sides than at other points. Each of the lobes of the labium,
the labellae, is provided with an extensor and flexor muscle. (See fig. 7, me.,
and mf.) The extensor muscles (me) are toward the outer side of the cavity of
each lobe, and serve, when simultaneously contracted, to separate the tips of the
labellae ; the flexors (mf) are upon the inner sides of the cavity of each lobe,
and serve to approximate the tips of the labellae. Near each flexor is -to be seen
(fig. 7) the section of the thickening of the chitin-walls, which, continued as a
chitinous rod, extends a distance back into the labium, and serves as attachment
for the flexor. In Cidex, then, the labellae are attached to the labium by true joints.
The labium has for function, for the most part, the protection of the fine
setae which form the true piercing organ of Cnles. In the females of Cidex
proper, the protective sheath is formed by the labium alone. When the mosquito
has found a place which suits its taste for piercing for it often tries different
places on our skin with its labellae, probing right and left, before finding a
place where it decides to remain. - it plants its labellae firmly upon the spot,
and a moment later the labium is seen to be flexing backward in its middle, the
setae, firmly grouped together, remain straight and enter the skin, while the two
labellae guide them, much as a carpenter guides his bit with his fingers, while
boring a piece of plank. When the setae of Culex have entered the skin to
nearly their full length the labium is bent double beneath the body of the insect,
the labellae still holding the base of the setae at the point where they enter the
skin. When the mosquito wishes to withdraw the setae it probably first with-
draws the two barbed maxillae beyond the other setae, that is, so that their
barbs, or papillae, will be kept out of action by the mandibles and hypopharynx;
then it readily withdraws the setae, perhaps aiding their withdrawal by the

19

muscles of the labium, for, during the process of extracting the setae from the
skin, while they are slowly sinking back into the groove upon the upper side of
the straightening labium, the mosquito keeps the labellae pressed firmly upon
the skin. Reaumur described carefully, in his memoir, the process of biting in
Cule.r, and was the first, probably, to describe and figure how the insect disposed
of its labium during the operation.

The mouth-parts of Cule.r, as* above described, are suspended under a clypeus,
or epistom, which is figured from the side in fig. 1, c; from above in fig. 2, c;
in length-section in fig. 11, c; and in cross-section in fig. 9, c. This clypeus is
the hood-shaped forward continuation of the lower part of a A -shaped piece of
chitin which forms the framework of what may be termed the "face" of Gulex;
right and left of the upper portion of this framework pass out the antennal
nerves, the antennae being supported by the framework itself.

The pharynx (fig. 11, p), the tubular continuation of the epipharynx above
and the hypopharyux below, as it passes backward, beneath the centre of the
A -shaped framework, turns somewhat upward, is narrowed to the valve prev-
iously described, then widens slightly again, and, as oesophagus (fig. 11, oe)
passes through the oesophageal nerve-ring, in which it is supported by three
delicate chitinous rods, which lay, one longitudinally on its ventral surface, and
two to the right and left on its dorsal surface. Just posterior to the oesophageal
nerve-ring, directly above the nerve-commissure which connects the infraoesophageal
ganglion with the first thoracic ganglion, the oesophagus suddenly expands into
an oesophageal pump, or bulb, the longitudinal section of which is shown in
fig. 11, ft; the cross-section in fig. 10, b. This bulb, which is the chief sucking-
organ in the female Cnh'.r, is supported b three longitudinal chitinous rods, which
are stouter continuations of the three rods supporting the oesophagus through
the nerve-ring. These rods (fig. 10, r) have between them chitin-plates (fig. 10,
which are suspended from the rods by elastic membranes. On the dorsal plate
is inserted a double muscle, or a pair of muscles (ft*//), the origin of which is
in the dorsal part of the chitiuous shell of the head. Each of the lateral plates
has inserted on it a muscle (bm'), the origin of which is in the chitin of the
lower lateral regions of the head. The origin of each of these muscles is in the
so-called occipital region of the head, that is, behind the eyes. By the simul-
taneous contraction of these muscles (bm and ftw'), the lumen of the oesophageal
bulb is enlarged, and the blood flows into the bulb from the pharynx, and,
upon their relaxation, the elasticity of the chitinous walls of the bulb drives the
blood, which cannot return to the pharynx because of the closing of the valve
at v (fig. 11), into the stomach.

20

THE MOUTH-PARTS OF CULEX, MALE.

The mouth-parts of the male of Culex have not been described, as far as I
know, with any degree of accuracy, altho, since Swammerdatnm's time, the males
have been distinguished from the females, by all scientific entomological writers
on the subject, by means of their feather-lifte antennae and maxillary palpi.
Jordens 18 (Bd. 1, p. 165) thinks, contrary to the opinion of most writers, that
the male mosquito can bite, and writes "But since the male is also provided with
a sucking seta, it is not comprehensible why it should not use it for the same
purpose." * He describes (p. 162), however, the setae of the male Culex as four
in number, the same as he describes those of the female.

The proboscis of the male of Culex pipiens, the only species the male of which
I have studied, is slightly longer and slenderer than the corresponding organ in
the female. The five-jointed maxillary palpi are covered with long hair at the tip.
The setae are fewer in number and less completely sheathed by the labium than
in the female; they consist of a well-developed labrum-epipharynx and two slightly
developed maxillae. The mandibles are absent, and the hypopharynx coalesces
with the labium (fig. 12, h and /). The labium and maxillary palpi are more
densely covered with hair and scales than they are in the females, and they
contain muscles; the other mouth-parts, the setae proper, are naked, chitinous,
and contain no muscles. In comparative length the mouth-parts may be arranged,
longest first: maxillary palpi, labium and labrum-epipharynx, maxillae; in
comparative size they may be arranged, largest first: labium, maxillary palpi,
labrum-epipharynx, maxillae. The relative position of the mouth-parts of the
male, determined in the same way as for the female, is different from that in
the female (compare fig. 8-U with 13-15) in that the short, rudimentary maxillae
are pushed out sidewise to allow the hypopharynx to coalesce with the labium.
In the male the oesophageal pump, or bulb, behind the nerve-ring fails, and the
sucking of fluids must be done by the pharynx alone, as it is done in most diptera.

The labrum-epipharynx is nearly the same in general form and structure in
the male Culex as it is in the female, it is a trifle longer and slenderer, but
the same figures (5 lr-e, and 6) will serve for the tips of both. In section (fig.
12, I >'-?), the labrum shows a groove on its upper surface, which deepens as it
nears the base (fig. 13, lr-e). At its base the labrum-epipharynx unites with the

* " Da aber auch die mannliche mit ebcn dm Sau-stachel versehen ist, so ist nicht
einzusehen, warum sie sich desselben nicht zu gleicher Absirht bcdienen sollte."

- 21

maxillae and their palpi before it unites with the labium, as shown by tig. 14.
The apical four-fifths of the labium contains no other seta than the labrum-epipharynx,
as seen in fig. 12, which is a section at about the middle of the proboscis. At
the base of the labrum-epipharynx are inserted pharyngeal muscles similar to those
found in the female, and with similar insertions and origins, except that the
median muscle (fig. 15' pm') is not divided into three parts as in the female
(fig. 9, p')-

The hypopharynx is, throughout its whole length, joined to the labium, and
thus necessarily pushes the maxillae, which would normally lay between it and
the labium, to one side. (See fig. 13, h and ?/u.'.) The hypopharynx shows, in
section (fig. 13-15 /*), the same chitinous rod through the middle as in the
females, but I was unable to detect any channel for saliva through this rod.

The maxillae are very thin lamellae of transparent chitin, about one fifth
as long as the labium, and so delicate as to be easily overlooked. Altho as broad
at the base as is the tube of the epipharynx, they taper regularly from their
base to their fine tips. When the maxillary palpi are carefully pulled from the
head of the male Culea-, the maxillae usually remain attached to their base.
The attachment of the maxillae to the maxillary palpi is readily seen in sections
(fig. 13, mx and mp); that they are not mandibles is evident.

The maxillary palpi are five-jointed, very hairy toward the tip, much longer
than they are in the female, and when at rest their basal portions cover the
labrum-epipharynx and maxillae in the sheath of the labium.

The labium of the male Cidex is similar in general structure to that of the
female, if one considers it together with the hypopharynx. It is, however,
slenderer, more densely covered with scales, has a shallower groove for the
reception of the labrum-epipharynx, and has a joint near the middle. The
slenderness of the labium] in the male extends itself to the labellae. (Compare
fig. 4, lb with fig. 3, lb.) The groove of the labium of the male increases in
shallowness from tip to base; at the middle of the proboscis (fig. 12) it is so
shallow that it fails to fully protect the labrum epipharynx, and at its base
(fig. 13) it is so shallow that the other mouth-parts rest only on top of the
labium. To make up for this deficiency of protection by the labium, the maxillary
palpi, as was previously mentioned, cover over the upper side of the enclosed
parts (see fig. 13), and thus, altho free from the labium, form a part of
the protective sheath, which, in the female, is formed by the labium alone.
Whether the joint near the middle of the labium of the male Culex is true or
false I cannot say, since I have never seen it bent by the insect itself; its
appearance is that of a true joint. Like the labium of the female, that of the

22

fcfy

male has two longitudinal main tracheal stems (figs. 12-14, ft 1 ), and two rows
of longitudinal muscles.

Whether the male Culex can bite, or not, is a question to which I can give
no decisive answer ; but I do not believe it can. I have tried to have the male
mosquitos bite me when in the field, where they were abundant, but they did
not seem attracted, as the female mosquitos were, to my person ; they flew away
indifferently without lighting upon me. I have often taken male mosquitos, with
all possible care to prevent disturbing them, beneath a glass cover upon my hand,
letting them remain long enough to be as tranquil as they were Avhen upon the
leaves and grass of the field, but they would neither bite nor show any desire
to do so, nor have T been able to feed male mosquitos with water, saliva or
fresh blood, all of which liquids the females often drink with avidity.

Upon anatomical grounds I believe that male mosquitos take liquid food,
altho I have never dissected their stomachs to see what this food was. They
have mouth-parts and pharynx developed sufficiently to suck liquids; but the
absence of barbed maxillae, of a free hypopharynx, and of an oesophageal bulb,
leads one to suppose that they take a smaller quantity of food than the females
do, and that they do not obtain it by piercing the skins of animals.

THE ANATOMY OF THE MOUTH-PARTS OF BOMBYLIUS.

The mouth-parts of the Bombyliidae have been studied only superficially.
Sulzer 14 says (p. 174) "The proboscis" of Bombylins "is as long as and longer
than the thorax, extended horizontally, bristle-like, more flexible apically, and is
only the sheath for the true sucking-seta, which comes out through its upper
side, which, as in Conops, is split longitudinally." * Fabricius writes (Syst.
entom., p. 802), "Mouth haustellate, without proboscis. Haustellum very long,
straight, setaceous, bivalvate" ** and (Phil, adorn., p. 50), "setae three."***
Gerstfelclt 3 writes (p. 31-32), "The Bombyliidae also have four setae, which
always represent the maxillae, the upper lip and the hypopharynx, Avhile the
mandibles are united with the proboscis-sheath in Bombylius the hypopharynx

s "Der Riissel 1st so lang, und langer, als die Brust, horizontal ausgestrekt, borsten-
ahnlich, vorne biegsamer, und nur die Scheide zu dem rechten Saugstachel, welcher durch die
obere Seite, die, wie bei dem Pferdstecher, der Lange nach gespalten ist, heraus kommt."

* "Os haustellum absque proboscide. Haustellum longissimum, rectum, setaceum,
bivalve."

*** "Setae tres,"

23

exceeds the upper lip in length, and the maxillae are still shorter than the
latter; all the setae appear hard and sharp."*

The species of Bombyliidae which I have chosen for study is Bombylius major.
Its proboscis is about three times the length of its head (See PL 2, figs. 1 and 2),
and extends directly forward from the ventral side of the anterior portion of its
head, in such a position as to be parallel with any plane on which it alights.
Gerstfeldt 3 (p. 14) incorrectly regards the proboscis of Bombylius as extending
first directly downward, and then bending, knee-formed, forward. The pharynx
at the entrance of the head, turns slightly upward (fig. 3, ^>), but the proboscis
itself is straight. The tip of the proboscis is divided into two labellae, which
separate and press themselves, with a rubbing motion, upon whatever Bombylius
eats. When the labellae are separated the tip of the hypopharynx is visible (as
in fig. 2) between them. The basal half of the proboscis often appears split into
an upper and under portion. At each side of the base of the proboscis arises
a one-jointed palpus, at the base of which the maxilla can be seen (fig. 1, mx)
to pass downward and into the side of the proboscis, between the above-mentioned
upper and under portions. The different parts of the proboscis can be separated
from one another with dissecting needles, and are five in number, but their determ-
ination, as regards length, position, and homology, is better revealed by sections,
a series of which (fig. 1, a-l') are connected by dotted lines to the place in the
figure of the proboscis and its base, from which the sections, which are repre-
sented in the figures, were taken. As shown by these sections the mouth-parts
consist of three unpaired organs, labrum-epipharynx, hypopharynx, and labium,
and of a pair of maxillae with maxillary palpi at their bases. These parts will
be described in detail further on. The comparative length and size of the sepa-
rated labrum-epipharynx, hypopharynx, and maxillae, can be seen in the upper
portion of fig. 1, where they are represented in their proportionate lengths and
sizes, as compared with the entire proboscis figured below them. All the mouth-
parts of Bombylius are without art iculations and are more or less pubescent, altho
the hairs are very minute on the maxillae. (See fig. 4.) When at rest the
organs of the proboscis are thus arranged (Compare fig. 1, 17 and#): the hypo-
pharynx (h) lays in a groove on the upper side of the labium (I) and is covered
by the labrum-epipharynx ; beneath or at the side of the labrum-epipharynx (Ir-e)
are the maxillae (?n,) and maxillary palpi (mp).

* " Vier Borsten - - die immer den Maxillen, der Oberlippe und dem hypopharynx
entsprechen, wahrend die Mandibeln mit der Riisselscheide verschmolzen sind besitzen
auch die Bombyliarien (bei Bombylius iibe.rtrifft der hypopharynx die Oberlippe an Lange
und die Maxillen sind noch kiirzer als letztere, alle Borsten aber erscheinen hart und spitz)."

9!
/C't

The labrum-epipharynx (seen in cross-section in fig. I . fi-t, /!-< : from beneath
in fig. 1, lr-c) is composed of the labrum, which is a direct continuation of the
front walls of the head, and of the epipharynx, which is a direct continuation of
the upper chitinous lining of the pharynx. These two parts (fig. 1 , */, Ir and e)
are so combined that the upper convex surface of the epipharynx fits into the
under concave surface of the labrum, while the edges of the epipharynx are
connected to those of the labrum by a less strongly chitinized membrane,
whose infolding between the two can be distinctly seen on sections t-i. The
enclosed space between the labrum and epipharynx contains longitudinal muscles,
tracheae, and connective tissue. The muscles are at each side of the tube of the
epipharynx, and are represented in cross-section on fig. 1, >]-i. Below the muscles
on each side, near the fold between the labrum and epipharynx, is a tracheal
stem, seen in cross-section in sections -x. The combined labrum-epipharynx
tapers from base to tip, is shorter than the hypopharynx and labium, but longer
than the maxillae. Of its two component parts the labrum undergoes much
more change in form, from base to tip, than does the epipharynx. The epi-
pharynx forms a channel, open along the under side by a narrow slit; this slit
widens as it approaches the tip, and bears on its two sides hairs, pointing
inward and backward, to prevent the return of food (See fig. 1, lr-e); for it is'
through the epipharynx, as in all diptera which I have studied, that the food passes.
At its basis the labrum-epipharynx, by a sprt of reflexion of its margins, forms
chitin-plates, which serve to support it in the forward part of the head (see fig.
1, A'), and to contain the pharyngeal muscles (fig. 1, /.', ^m and fig. 3, pin), which
elevate the upper elastic wall of the pharynx, in the action of sucking. As will
be seen later, in the portion of this paper which I devote to the comparison of
the parts of the different species studied, this reflexed wall of the pharynx is
the homolog of the greater part of the so-called fulcrum of the Mmcidae.

The hypopharynx of Bombylius is a flexible tube of thin chitinous membrane,
containing within itself a more rigid chitinous tube, which opens on the upper
side near the extremity of the hj^popharynx. The tip of the latter is thus of
the form of a pen, with its concave side upward. (See fig. J, A.) The inner
tube of the hypopharynx is continuous at its base with the salivary duct; the
outer flexible tube only serves to make the hypopharynx fit more closely to the
form of the surrounding mouth-parts. The upper surface of the hypopharynx is
naked ; its lower surface hairy. Its section (fig. I , >] and #, h) is variable in
form on account of the flexibility of its outer tube. Just before it reaches the
mouth, that is just before it unites with the epipharynx to form the mouth, the
hypopharynx joins, on each side, with folds from the united maxillae and maxillary

25

palpi. (See tig. I, /.) The tip of the epipharynx rests between the labellae,
as is seen iii fig. 1, , and in fig. 2.

The maxillae (fig. 1, -w.r) are slender, solid, chitinous rods, the outer or apical
halves of which lay along each side of the hypopharynx ; their basal portions pass
more and more outward, so that their bases are at each side of the labrum-epi-
pharyux. (See fig, 1, >>, *.'.) In cross-section the maxillae are reniform, with
the concave sides inward, gradually changing to triangular towards their tips,
where their outer sides are very finely and densely pubescent, (fig. 4.) Between
the portions represented in sections # and /, the maxillae are joined by the
maxillary palpi, and, at about the same time, they join a fold (/') of the labium,
which fold, as seen in section, surrounds the labium itself. The bases of the
maxillae f.r) extend deep into the head.

The maxillary palpi (fig. 1, wp) are slender, hairy, cylindrical, and lay just
outside, and at the base of the proboscis. Their bases lay at the sides of the
labrum-epipharynx ; their tips usually directly over it.

The labium (fig. 1, /) is slender, hairy, and its outer fifth is divided into
two labellae. Throughout its length, the labellae excepted, the labium forms a
channel (see sections y-i) for the reception of the hypopharynx and maxillae. It
contains two longitudinal tracheal stems, and longitudinal muscles; it is especially
flexible toward the tip. At its base the upper surface of the labium joins the
under surface of the hypopharynx, and its under surface continues, with two folds
(I' and I" of figs. 1, L-Y., and of- fig. 3), directly into the lower surface of the
head. The labellae are not jointed to the tip of the labium by a true joint, as
in Cide.v, but are the continuations of the two lateral halves of the labium. The
tip of the labium projects between them, however, as can be seen in section //,
where the cross-section of the tip of the labium is shown, at its first point of
connection with, and between the two labellae. Each labella contains a flexor
and extensor muscle, the flexor a little above the extensor in position. The
labellae are usually pressed closely together, but can be separated, as seen in
fig. 2, or even wider, so as to be at right angles to the axis of the proboscis.
On the inner side of each labella are three longitudinal grooves or channels, held
open by semi-rings of ehitin at right angles to their axes, and toothed on each
side. These, on account of their general resemblance to tracheae, were termed
by Suffolk' 2 pseudotracheae, in his description of them as they appear in Mnx,-a
vomitoria. These psendotracheae are seen in section in fig. 1, . Fig. 5, a, is
a perspective view of the pseudotracheae of Bombi/fiiix major, when at rest, with
their teeth turned inward (fig. 6, a is cross-section of the same) ; fig. 5, b, shows

26 -

them in perspective, when Bombylius is feeding, with their teeth turned outward
(fig. 6, I), is a cross-section of the same).

The functions and finer structure of these pseudotracheae, together with other
points in the anatomy of the mouth-parts of Bombylius, will be further discussed
in the part of this paper devoted to a comparative study of the mouth-parts of
diiferent diptera.

ANATOMY OF THE MOUTH-PARTS OF ERISTAL1S.

Little has been done in the study ofj the anatomy of the mouth-parts of
Syrphidae, of which I have chosen Eristalis horticola for anatomical investigation.
Gerstfeldt 3 (p. 28-29) gives a general description of the mouth-parts of the
Syrphidae, which can be condensed as follows : labrum blunt, with the tip separ-
ated into several points ; hypopharynx always present ; mandibles more or less
rudimentary and coalesced with the sheath of the proboscis; maxillae fairly well
developed and setiforrn; labium with well-developed terminal lobes. Meuzbier 8
(p. 57-60) describes the proboscis of Syrphw taeniatus. His results, as regards
the typical structure of the mouth-parts of Syrphus, may be summarized thus:
labrum and epipharynx united ; hypopharynx with a channel for salivary secretion;
mandibles present; only remnants of the maxillae are the two maxillary palpi;
labium with well developed terminal lobes. Menzbier, in describing the proboscis of
Syrphidae, divides it into three portions, a basal, a middle, and a terminal portion.
Leaving all reasons for my own views to be given in detail later, I will only say
here, that this division of the proboscis of the Syrphidae, Mmddjie, and other
families of diptera, into basal, middle, and terminal portions, has served to render
the study of the mouth-parts of these insects more complex and more difficult than
it would otherwise be, and that, while retaining the term basal portion, for reasons
to be given later, as a convenient designation for a part of the proboscis, I lay
little value on such a division of parts. The so-called middle and terminal portions
of the proboscis of the Syrphidae and Muscidae are really a single portion.

The proboscis of Eristalis horticola (side view in PL 3, fig. 1) is hung upon
the under side of the head, and, unlike the proboscis of Cukx or of Bombylius,
is extensible and retractible. When extended the proboscis of E. horticola is as
long as the head, and points nearly directly perpendicular toward the surface on
which the insect is resting. Its retraction is accomplished by means of joints,
near the points indicated on the figures by d and g. If the retraction is begun,
by a partial revolution toward the head, of the portion of the proboscis between
d and g, around the extremity d as axis, then the end g will follow the course

- 27

represented by the dotted segment of a circle fjn ; -and if, during the time that
the above mentioned revolution is progressing, a revolution of the portion of the
proboscis gk around the axis </ takes place, then the tip of the proboscis, A, will
follow the direction indicated by the paraboloid curve kd. When these two partial
revolutions, around the axes of the joints at d and g, are completed, as far as
possible, the points g and ?i, and k and d, will have been closely approximated,
and the proboscis will have been folded upon itself by a fold near its middle
point, and, folded thus, the whole proboscis will have been retracted into a groove
on the under side of the head. When fully retracted the dorsal * portion of the
basal half of the proboscis is closely pressed against the dorsal portion of its
distal half. Fig. 2 represents a longitudinal section of the head of Eristalis
horticola with the basal half of the proboscis wholly retracted, and its distal half
partly retracted. A section through the. distal half of the proboscis of Eristalis
(Fig. 1, /?') shows all its mouth-parts in their normal positions. They consist
of a labrum-epipharynx (lr-e), closed beneath by a hypopharynx (/*). To each
side of the above named parts lay the only paired mouth-parts, the maxillae (mx)
and their palpi (>np). Beneath these parts the labium forms a channel for all
the other mouth-parts, a channel into which they fit when the proboscis is retracted.
No mandibles are present. A section through the proximal half of the proboscis
(fig. 1, d'} shows none of the true mouth-parts. In the middle of this section
is the channel for the passage of food, the pharynx (;>), surrounded by the
chitinous distal end of the so-called fulcrum, and its muscles (pm). At each side
of the pharynx is a chitinous rod (./) which supports at its distal extremity the
maxilla. Around these parts is a thin elastic membrane (c I') which is continuous,
proxirnally, with the chitinous covering of the head ; and, distally, on the upper side,
with the labrum, and on the under side with the lower walls of the labium.
This elastic membrane, surrounding the forward extremity of the fulcrum, folds
itself together, on the under side, between n and g (fig. 2), when the proboscis
is retracted ; on the upper side, when the proboscis is retracted, it receives in its
folds the dorsal surface of the labrum.

The labrum-epipharynx is, in section (fig. 1, p' and /', lr-c), nearly the shape
of a horse-shoe, convex above, and is composed of the labrum, the continuation
of the flexible walls of the upper side of the basal portion of the proboscis, and
of the epipharyux, the continuation of the upper walls of the pharynx; but, in

' I shall use dorsal and ventral, upper and under, distal and proximal, and anterior
and posterior, of the parts of the proboscis, in the same way and with the same meaning
as if the proboscis were extended out from the front of the head.

28

the labrum and epipharyux are not connected, as in Bombylivs, by
infolding delicate membranes, to mark, in section, their line of union. The basal
half of the labrum-epipharynx contains muscles passing obliquely upward and
backwards from the epipharynx to the labrum ; these muscles are only a forward
continuation of the system of pharyngeal muscles in the fulcrum, which the base
of the labrum-epipharynx joins. At its tip the labrum-epipharynx is divided into
six parts, as seen in h'g. 5, which represents the tip unrolled, from within, and
these are hairy, the rest of the labrum-epipharynx being naked. The hairs
on" the inner side of the outer parts of the tip have the appearance of being
sense-hairs, but I have only studied them superficially, and cannot therefore speak
with certainty of their nature. The division of the tip of the labrum into several
parts is regarded by Meigen 10 (Theil 3, p. 381) as a characteristic of the
Syrphidae, and the researches of Gerstfeldt, as well as my own examination of
the laftrum of about a dozen species, tends to confirm Meigen's statement.

The hypopharynx (fig. 6 ; in cross-section, fig. 1, fi' and y', A) is lanceolate,
naked, and strongly chitinized. It is a rigid tube, opening apically on the upper
side, and is the outlet for the saliva. Its proximal end is slightly broadened,
and unites, with a true joint, to the under Avails of the distal end of the fulcrum,
or pharynx. The margins toward the tip and the tip itself of the hypopharynx
are transparent; the remainder of the hypopharynx is opake.

The maxillae (fig. 4, w, and, in section, fig. 1, /?', m.r) are thin naked
blades of chitin, concave on their inner sides, convex on their outer sides, and
uniting at their bases with the maxillary palpi. The bases of the maxillae lay
directly outside of the labrum-epipharynx, when the proboscis is extended, but the
maxillae curve downward, and inward, distally, so that their tips are between the
hypopharynx and labium. The margins and tip of the maxillae are very trans-
parent, the middle a little thickened. The maxillary palpi (fig. 4 tup; fig. 1,
/?', tup) are cylindrical, one-jointed, hairy, a trifle longer than the maxillae, and
lay along the sides of the labrum-epipharynx, when the proboscis is extended
(fig. 1). The chitinous supports which bear the maxillae at their tips, extend
back along each side of the pharynx, where they can be seen, in section, at .
in fig. 1, 6 V and e'.

In Enstalis hort/cola I was unable to find the least traces of the mandibles,
either as free rudimentary structures, or as portions united to the labium. What
Menzbier considered to be mandibles in St/rphm tacti/atii*, are, undoubtedly,
maxillae. Menzbier 8 writes (p. 60) " At the sides of the basal portion of the
labium and united with it, lay two thick chitinous structures, which project right

Of)

and left of the opening of the mouth in the form of three-cornered blades, or
sharp prongs. These blades or prongs resemble so much the well-developed
mandibles of chewing insects that we can rightly regard them as such. Syrphn*
has, then, besides labrum, epipharynx, hypopharynx and labiimi, a pair of man-
dibles. Near the mandibles, united with the basal portion of the proboscis, are
two palpi. Since neither maxillae nor anything similar to them, are to be found
on the mouth-parts of Syrphus, we must consider the aforesaid palpi to be the
only remnants of the maxillae, since the latter always have palpi." * The above-
quoted remarks give no valid reason for regarding the parts under consideration
as mandibles, for the form which a part assumes in an animal has little value
in determining its homology in comparison with the worth, in such determ-
inations, of the position and attachments of the organ, the homology of which is
in doubt; and, in this case, the situation of the maxillary palpi so near the base
of the organs in question, together with the fact that the maxillae are present,
and attached at the base to their palpi in Eristalis (fig. 4), leads me to suppose
that the parts, which Meuzbier regarded as mandibles in Syrphus, were really
maxillae. This view is further supported by the figure ** which Menzbier gives,
and by Gerstfeldt's statement that the mandibles are more or less rudimentary
and united to the sheath of the proboscis, in the Syrphidae.

The labium of Eristalis horticola (side view in fig. 1; section in fig. 1, fi'
and 7', /) is larger than the other mouth-parts, muscular, pubescent on the outer
or under side, and throughout its length run two tracheal stems and numerous
longitudinal muscles. Its under surface and the middle of its upper side are
strongly chitinized, but between these two portions the walls of the labium
are very flexible, as can be seen in the sections ft' and y 1 of fig. 1. At its
base the upper or inner surface of the .labium joins the under side of the
hypopharynx, at the point where the latter is jointed to the fulcrum; the under
or outer surface of the labium .passes (with a fold or two when the proboscis

* "An den Seiten des Basalkegels der Unterlippe und mit ihm verwachsen liegen zwei
dicke Chitingebilde, die rechts und links von der Mundoffnung in Gestalt dreieckiger Sclinei-
den oder scharfen Hacken vorspringen. Diese Klingen oder Hacken gleichen so selir den
wohlentwickelten Mandibeln der kauenden Insekten, dass wir mit vollem Rechte sie als
solche deuten konnen. Syrplms besitzt also ausser Labrum, Epi- und Hypopharynx und
Labium noch ein Paar Mandibeln. Neben den Mandibeln, mit dem Basalkegel verwachsen,
sitzen zwei Palpen. Da weder Unterkiefer noch etwas ihnen almliches an den Mundtheilen
von Syrplius zu finden 1st, so miissen wir die erwahnten Palpen als einzige Reste der Unter-
kiefer betrachten, da Letztere immer Palpen besitzen."
** See Menzbier' s pi. 3, fig. 3.

- 30

is not fully extended) directly into the under surface of the basal portion of the
proboscis. On the upper side the labium is hollowed out to form a channel into
which the labrum-epipharynx, hypopharynx and maxillae fit, when the proboscis
is -retracted, and usually when it is extended. To each side of the tip of the
labium is attached a labella (in section in fig. 1, a'). The two labellae are
short, fleshy, and hang below the level of the lower wall of the labium, often,
when they are in -use, seeming to be at an angle to it; thus they have more
the appearance of distinct organs than they have in Bombylius. They are,
however, nothing more than two hanging lateral wings of the labium, joined to
it only by flexible chitinous walls, not by true joints. The labellae fold together
so that the interior face of the one presses upon the interior face of the other,
when they are not in use, and when the proboscis is folded in its channel beneath
the insect's head. This seems to be the normal, or resting position, of the
labellae, from which they are brought into action by a pair of extensor muscles,
one in each labella (as seen in fig 1, a' in section); their further action will be
described more fully later. Each labella has on its inner side pseudotracheae, all
branching from a main pseudotracheal stem, which extends anteriorly and
posteriorly along the upper side of each labella. The anterior portion of this
pseudotracheal stern has about twenty-four branches, the posterior portion about
eighteen branches. The branches of these pseudotracheal stems extend, approxim-
ately parallel to one another, from the upper to the under margin of each
labella, and are much finer than, but of similar structure to. the pseudotracheae
of Bombylius.

The structure of the so-called fulcrum, which occupies the central portion of
the basal part of the proboscis, the use of the pseudotracheae, the mode of
expanding the labellae, and other points pertaining to the structure and use of
the proboscis in Eristalis will be discussed later, and more appropriately, in that
part of this paper which will be devoted to a . comparison of some of the cor-
responding organs in Culex, Bombylius and Musca.

ANATOMY OF THE MOUTH-PARTS OF MUSCA.

The proboscis of Musca domestica, the common house-fly, and that of M.
vomitoria, the blow-fly, have attracted the attention of naturalists, from the time
when Aristotle wrote, down to the present day, and to attempt anything like a
complete notice of the different papers which, wholly or in part, treat, more or
less fully, more or less scientifically, of this subject, would be an extravagant

31

expenditure of space and a waste of time. I select, therefore, for notice a few
of the papers which treat upon the subject of the proboscis of Mutca, or of its
mouth-parts, such papers as seem to me to have furnished the more important
contributions to our knowledge of the subject, or such papers as have so fully
dealt with the subject that they ought not to be overlooked.

The earliest paper which I have thought worthy of notice here, that
deals with the anatomy of the proboscis of Musca, is REAUMUR'S memoir 12
upon the trunks, with swollen and fleshy lips, which belong to two-winged
flies. * This paper by Reaumur, - - altho published in 1740, over thirty years
before Fabricius had reduced the terminology of the oral organs of insects to
anything like a system, and over seventy years before Savigny had settled the
question that the mouth-parts of insects of all the different orders were homo-
logous, - is a very complete and accurate presentation of many anatomical facts
in regard to the proboscis of Musca. Reaumur, whose observations in regard to
Miis fa were made, for the most part, on M. vomitoria, studied the proboscis, as
was his usual method of study, with especial reference to its functions, and to
the functions of each of its parts. Reaumur believed the proboscis of Musca to
be essentially an organ for sucking, altho he thought that the food was aided,
perhaps, by the undulatory motion which he observed in the proboscis, in its
passage upward from the mouth, which was, according to him, between the two
labellae. By watching a fly rolling a bit of dry sugar between the labellae,
gradually moistening it, and thus slowly dissolving it away, Reaumur came to the
conclusion that the fly had a salivary secretion, but, altho he describes the
hypopharynx, by him termed the spur (aiguillori), as channeled above, he does
not seem to have discovered that the channel of the hypopharynx was the
outlet for the saliva. Reaumur correctly recognized the labrum-epipharynx as
the part of the proboscis through which the food passes, and he therefore termed
it the sucker (suqoir'). He called the labellae lips (levres), and writes of them
that they are traversed by parallel channels, or flutes (cannelures^), which extend
toward the middle of the labellae; but, further on, he incorrectly assumes that
this channeling is due to a large number of parallel vessels, which distend with
liquid, when one presses the head of a fly, or when a fly wishes to use its
labellae. Altho Reaumur does not directly say, in any place, that the proboscis
of the fly is extended by means of injecting blood or air into it, yet the idea
that the proboscis was extended by inflation was evidently often in his mind,

: Tome 4, part 1, p. 256-297, pi. 16-18. "Des trompes a levres grosses et charnues
des mouches a deux ailes."

32

while writing about the proboscis, and he often uses the term inflate (gonfler) in
regard to the proboscis or its parts. He writes (p. 259), " The fly can increase
the volume of its trunk and can diminish it to a certain extent." * He further
writes (p. 260), " One easily compels a fly to show its entire trunk, finely extended
and well inflated; one has only to press between the two fingers the thorax, '
either laterally or from above and below ; it seems as if one obliged the fly,
immediately, to put out the tongue." ** Reaumur regarded the proboscis of
Musca to be composed of, or folded into, two joints.

GLEICHEN, 2 '' in 1764 (p. 19-21), describes the proboscis of Musca domestica.
He regards the proboscis as made up of three parts, longitudinally, namely a
basal sac, a middle tube, and the proboscis proper, the latter being only the
labellae. He writes (p. 19) that " The extension of the proboscis probably arises
from the air, which is driven by the fly into the sac, and from this into the
[middle] tube, finally into the lips. The elevation and depression of the proboscis
is, however, brought about by the process with which the [middle] tube is sup-
ported." *** Gleichen writes further (p. 21), " The fly can indeed, as I have
already mentioned, drive air between the membranes of the proboscis into the
lips, but cannot take in air with them." f

Gleichen did not succeed in finding the part now known as hypopharyux in
Musca domestica. He regarded the basal sac, or basal part of the proboscis, as
a pumping organ.

GERSTFELDT-S statements 3 (p. 24-26) regarding the mouth-parts of Musca may
be noticed briefly as follows : the under lip, according to him, was what will be
later described as labrum-epipharynx ; his hypopharynx, as mentioned in the
historical notes (p. 7) of this paper, was really the epipharynx, separated
by the action of caustic potash, from the labruni ; the real hypopharynx was not
discovered by Gerstfeldt; the rudiments of the mandibles appear, according to
Gerstfeldt, as the dorsal corner, on each side, of the fulcrum ; the maxillae form

* "La Mouche peut augmenter le volume cle sa trompe & le diminuer jusqu'a un
certain point."

** " On force aisement une Mouche a montrer sa trompe toute entiere, bien etendue &
bien gonflee; on n'a qu'a presser entre deux doigts, soit les deux cotes, soit le dessus & le
dessous du corcelet; il semble qu'on oblige sur le champ la Mouche a tirer la langue."

*** "Das Ausstrecken des Riissels riihret vermuhtlich von der Luft her, die von der
Fliege in das Sackchen, und von diesem in die Rb'hre, biss in die Lefze, getrieben wird.
Das aufrichten und senken des Riissels aber wird mit dem Hornbeinchen, womit die Ro'hre
gefasset ist, bewirket."

f "Die Fliege kan zwar, wie ich bereits erwahnet, die Luft zwischen den Hauten
des Riissels in die Lefzen treiben, aber nicht dainit Luft schb'pfen."

33

the lateral portions of tlie fulcrum and support above the maxillary palpi; the
labium, as submentum, mentum, etc., extends anteriorly from the base of the
fulcrum and terminates in the labellae. Gerstfeldt's views of the homologies of
the different parts of the fulcrum seem utterly untenable, as will appear later.

LowNE, 23 in 1870 (p. 41-51, pi. 2-3), describes and figures the proboscis and
its parts in Musca vomitoria. He describes fully, and in general accurately, the
fulcrum, which he regards to be homologous with the fulcrum of bees. The
labrum-epipharyux, Lowne thinks, is composed of the labrum united with the term-
inal lobes of the maxillae, and he terms the organ as a whole the operculum,
because it shuts like a lid over the other mouth-parts. Lowne correctly describes
the hypopharynx under the name of tongue. The labium, which he calls the
canula, is considered to be the united labium and mentum. I do not here go
into further details in regard to Lowne's work, because I shall have occasion to
notice it often in the following pages.

MACLOSKIE, in 1880, describes the proboscis of the house-fly, Musca domestica,
using for the most part 'Lowne's terminology, and discusses briefly several points
about its homologies and functions. Among other things he again affirms the
statement that inflation plays a prominent part in the extension of the proboscis,
a statement criticized by Suffolk, 21 in 1869. Macloskie writes (p. 157-158),
"It is easy to dispose of Mr. Suffolk's hasty criticism. Immerse the head of the
fly in caustic potash, which destroys the muscles, the chitine of the membranous
sheath and the tracheal tubes remaining intact, and you can still protrude the
organ by slight pressure. Further, when the proboscis is pressed out and all its
parts distended, pierce with a needle the swollen air sacs under the tip, and at
'once the tip collapses upon the mentum. If you tear the membrane about the
base of the proboscis that part collapses. If you press the head over much, the
membrane-sheath sends out bulging processes which soon burst, sending bubbles
of air through the water in which you are examining it." I have verified the
experiments mentioned by Macloskie above, besides trying others, and I think that
an important factor in the cause of the extension of the proboscis of Musca is
the injection of air into it. Macloskie also homologizes the fulcrum of the diptera
with the endocranium of the cockroach (Blotto), a view that I ana not prepared
to affirm or to deny; but his conclusion that the so-called middle section is the
true base of the proboscis, the so-called basal portion being a part of the head,
and its organs internal, rather than true mouth-parts, this latter conclusion I
hope to be able to prove in the concluding part of this paper.

MENZBIEE (p. 62-66) describes the organs which constitute the proboscis of
Musca, and which are, according to him, the following: a labrum united to an

3

34

epipharynx, a hypopharynx, a labium, and two maxillary palpi; mandibles and
maxillae are entirely absent. Menzbier says that the fulcrum is of "chitinized
processes of the wall of the pharynx." * Menzbier criticizes severely (p. 26-27)
several absurd statements made by Lowne, in regard to the proboscis of Musca,
but says nothing further of Lowne's work, leaving one under the impression, if
they depended for their information upon Menzbier's criticism, than Lowne's work
was only a mass of absurdities.

Turning now from this brief historical summary of the progress of our
knowledge of the mouth-parts of Musca to the results of my ; own studies, based
almost wholly on the examination of the proboscis of M. vomitoria, I will describe
briefly the proboscis and its parts in the above-named species, omitting many
minutiae of structure, which it would be of little interest to mention in this
connection, because they would have neither value in determining homologies or
functions, nor use in the subsequent comparison of the mouth-parts of Musca
with those of the other diptera which I have examined.

The proboscis of Musca vomitoria (side view of proboscis fig. i, of pi. 4) is
suspended from the under side of the head, and, like the corresponding organ of
Eristalis, can be extended and retracted by means of joints. The general external
aspect of the proboscis of Musca is like that of Eristalis ; the mode of its folding
by joints at the anterior under portion of the head and at the middle of the
proboscis, is, in almost every respect, the same as it is in Eristalis, and need not
be described here in detail. The most noticeable external difference between the
proboscis of Musca and that of Eristalis, a difference of limited morphological
value, - which one sees in a hasty examination of these two diptera, is that
the maxillary palpi of Musca are much larger, proportionally, than those of
Eristalis, are inserted much nearer the base of the proboscis, and are borne much
more erect from the proboscis, when the latter is extended, than they are in
Eristalis. Another noteworthy difference in outward appearance, but not one of
morphological worth, which is discovered between the proboscis of Musca and
Eristalis, is that, in the former of the two genera, the labrum-epipharynx is never,
by voluntary effort of the fly itself, extended above and free from the labium, so
that one sees, in a side view (fig. 1) of the apical half of the proboscis of
Musca, only the labium ; while in a side view (pi. 3, fig. 1) of the same portion of
the proboscis of Eristalis, one often sees the labrum-epipharynx, maxillae and labium.

A cross-section through the proboscis of Musca, made at the point indicated
at a in fig. 1 and figured in fig. 1, a', exposes the labium (/) in the form of a

* " Chitinisirte Fortsatze der Schlundwand."

35

trough, in the hollow of which is the labrum-epipharynx (lr-e) with the hypo-
pharynx (A) closely pressed beneath it. A cross-section of the same proboscis at
fi (figured in /?'), fig. 1, reveals the bases of the maxillary palpi (>np) above, and
within the proboscis a section of the distal half of the fulcrum, which contains
the pharynx Q>) and the pharyngeal muscles (pin). At each side of the pharynx
is the section of a chitin-piece (x) and its surrounding muscle, the functions of
both of which will be discussed later ; directly beneath the pharynx, in the median
line, the salivary duct (sd) is sectioned. If a section of the fulcrum be made
still further toward its proximal end, its upper side will be found to be closed
.together, as in fig. 4.

The labrum-epipharynx of Musca vomitoria is a little longer than the hypo-
pharynx, tapers gradually from base to tip, is strongly chitinized, is clothed with
fine short hairs on the upper surface of its basal half, is- in cross-section through
the middle (fig. J , a', lr-e) shaped like a horse-shoe with both its aides broadened,
and is composed of a labrum and epipharynx, separable from each other in caustic
potash. From the labrum to the epipharynx, in the basal half of the labrum-
epipharynx, extend muscles. The extreme tip of the labrum-epipharynx has the
shape of a quill-pen with its point evenly rounded off, is very thin and transparent,
and is lined on its inner side with numerous very fine short spines. The base
of the labrum-epipharynx is jointed to the distal end of the fulcrum in such a
manner that the labrum is continuous with the upper surface (fig. 1, /?', <') of
the basal half of the proboscis, and the epipharynx with that portion of the
fulcrum which directly covers the pharynx dorsally. The tip of the labrum-epi-
pharynx is so completely surrounded by the sides of the labium, which, at the
point where the labrum-epipharynx ends, are so developed dorsally as to form a
tube, that the fly. itself probably cannot raise the labrum-epipharynx out of the
trough of the labium. The two can be separated, however, with a pair of needles
without tearing the parts, if the operation is performed very carefully.

The hypopharynx of Musca vomitoria (fig. 1, a', h, in section) is thin, tapering
gradually from its base to its tip, which is acute and sparsely clothed with very
fine hair. The basal portion of the hypopharynx contains a tube, which opens
into a groove on the upper side of the distal half of this organ; this tube is
the outlet for the saliva, which is conducted to its basal end by a duct which
has rings similar to those of tracheae. The course of this duct is shown by the
dotted line sd, on figs. 2 and 3, and its position, in section, at sd of fig. 1, /?'.
The hypopharynx lays closely pressed to the under side of the epipharynx, from
which it is not always easily separated, and with which it forms the tube for the
passage of food to the pharynx. The base of the hypopharynx unites beneath

36

with the labium, somewhat anterior to the point where it joins the epipharynx
above, to form the mouth.

Mandibles and maxillae, as free mouth-parts, are absent in Musca vomitoria.
The one-jointed maxillary palpi (fig. 1, mp), two club-shaped appendages clothed
with a few hairs, are attached to a slight thickening, or band, of the flexible
membrane which bridges the upper anterior portion of the fulcrum. This band
is marked by longer hairs than the rest of the membrane. The attachment of
the maxillary palpi is shown on fig. 1, /?'.

The labium of Musca vomitoria (side view in fig. 1 ; cross-section in fig. 1,
', /) is a deep trough or channel, the distal end of which is closed entirely over
so as to form a tube; to the lateral margins of the distal end of this tube are
attached the labellae. The walls of this trough (fig. 1, a', I) are double, that is,
the trough itself can be compared to a cylinder of which the upper side has been
so invaginated as to form a channel. These double walls extend into the tubular
distal portion of the labium, so that the latter portion, structurally considered, is
made up of one tube inside another, the greater part of the space between the
tubes being beneath the inner one. The labellae are two lobiform hollow sacks;
their inner margins are continuous with the inner tube above-mentioned, and
their outer wall with the outer tube, so that their inner cavity, is, on each side,
a continuation of the space between the two. tubes. The basal channeled end of
the labium is attached to the under side of the base of the hypopharynx above,
and is continuous below with the lower half of the membranous sheath around
the fulcrum. Through the labium extend stout muscles and two tracheal stems
(fig. 1, a', /). From the lateral edge of the inner chitinous tube, or wall, of
the distal end of the labium there extends into the cavity of each labella a
delicate curved chitinous band; on the bases of each of these bands are inserted
muscles, which originate in the labium further toward its base. These muscles,
altho not extending into the labellae, help to separate the latter from each other.
The labium and labellae are somewhat hairy on the outer surface; especially is
this true of the margin of the labellae. Each labella bears, on its inner surface,
about thirty pseudotracheae, arranged nearly parallel to one another from the
outer to the inner margin of the labella, where they, in part, join anteriorly a
large pseudotracheal stem, extending toward the middle of the disk formed by
the labellae, and, in part open directly into the triangular central opening between
the labellae, their basal ends being supported by rods which radiate from a
strongly chitinized ring around this opening. Between the bases of the rods
supporting the inner ends of the pseudotracheae are teeth, which are concealed
when the labellae are not opened very widely.

37

The structure and functions of the fulcrum, and the minute structure and
mode of use of the pseudotracheae are left for further discussion later.

Leaving the part of this paper which is more exclusively devoted to the
descriptions of the structure and arrangement of the mouth-parts and suctorial
apparatus of the four diptera Culeu, Bombylius, Eristalis and Musca, which I have
studied, I turn to the portion of my paper devoted to the

COMPARISON OF THE MOUTH-PARTS AND SUCTORIAL APPARATUS
IN DIFFERENT FAMILIES OF DIPTERA.

In this division of my paper, while I shall, for the most part, make use of
results derived from my own examination of Culex, Bombylius, Eristalis and
Miisca, for these four genera make a fine series for most purposes of comparison,
I shall not hesitate to take advantage of the results of the researches of others,
so far as I think them trustworthy and suited to my purpose, but, in all cases,
giving credit, as far as possible, to those from whom I obtain anatomical facts.
I shall try, in this portion of the treatment of my subject, to present briefly,
together with my own ideas, the views, theories, homologies and nomenclature,
which have been adopted by various authors for the mouth-parts of diptera, so
far as their views pertain to parts under my consideration.

The first organ which I shall examine comparatively in this paper is the
fulcrum. In Eristalis, the fulcrum (side view, pi. 3, fig. 7; cross-section, fig. 1,
', d'fpf) consists of a hollow frame of chitin, comparable to a somewhat
irregular, much compressed, truncated cone, with a deep sinus in each of the
flattened sides of its larger, or basal, end. This fulcrum is suspended, by the
longer of the two processes (fig. 7, d') of its basal end, in the anterior angle
(fig. 1, d~) of the front of the head, - with its apex reaching nearly to the
middle joint of the proboscis, - in such a way as to swing on the point d as
an axis. In figs. 2 and 3 (of pi. 3) it would occupy the space filled by the
pharynx (p) and the pharyngeal muscles (pw); pendulous around the point d,
in the extension and retraction of the proboscis. The under, or ventral, portion
of the inside of the fulcrum is occupied by the pharynx (fig. 1, e', p); its upper,
or dorsal, portion by the pharyngeal muscles (pm). In Musca vomitoria the
fulcrum (in cross-section, pi. 4, fig. 4) is of similar general outline as in
Eristalis, as can be seen in pi. 4, figs. 2 and 3, where it would occupy the
space filled by the pharynx (p) and the pharyngeal muscles (/>w); its posterior

38 -

margin, more sinuate than it is in Eristalis, is indicated by the dotted line dz.
Its posterior ventral horns here project backward on each side of the oesophagus.
In Musca the fulcrum is not as much compressed laterally as it is in Eristalis,
as can be seen by comparing its sections in the two. (Compare pi. 3, fig. 1,
', d' fpf, and pi. 4, fig. 4.) In both Musca and Eristalis the fulcrum contains
the pharynx and pharyngeal muscles, but in Musca the latter are divided into
two portions (pi. 4, fig. 4, pm~). Another peculiarity of the fulcrum in Musca, a
peculiarity not very evident in Eristalis, becomes very marked; that is, that its
walls fail on the upper side of its anterior extremity, so that a cross-section of
the fulcrum, as seen in pi. 4, fig. 1, /*', ///, is nearly U-shaped, and the upper
opening is closed over by the elastic membrane (<) which forms the outer walls
of that portion of the proboscis. In Eristalis the upper wall is really absent
from the anterior end of the fulcrum, altho its absence is not as marked as it
is in Musca. In pi. 3, fig, 1, <5', the chitinous portions and the muscles directly
around the pharynx (/>) are parts of the extreme anterior end of the fulcrum
and its contents, and are without upper covering-walls. The above-mentioned
absence of the upper wall on the anterior end of the fulcrum of Eristalis
and of Musca allows, when the proboscis is retracted, the labrum to sink its
convexity into the flexible membrane, which bridges from one side of the fulcrum
to the other, in the basal portion of the proboscis. Cross-sections of' the
fulcrum (pi. 3, fig. 1, e', pfd'p, and pi. 4, fig. 4) show that its ventral part
consists of two plates of chitin, the outer plate firmer than the inner, united
above on each side, that is, the inner plate is sprung into the concavity of the
outer one, so that, at rest, on account of the elasticity of the inner plate, but
little space remains between the plates ; this space is the pharynx. Upon the
upper surface of the chitiu plate which forms the upper wall of the pharynx are
inserted the pharyngeal muscles, their origin being in the dorsal portion of the
fulcrum. By their contraction the pharyngeal muscles enlarge the lumen of the
pharynx, which is again diminished, upon their relaxation, by the elasticity of
its upper wall.

The function of the fulcrum is, in the first place and most essentially, to
furnish the frame-work of a suctorial organ, of which the motive power is the
enclosed pharyngeal muscles, as dilators of the pharynx, opposed by the elasticity
of the upper pharyngeal wall, as contractor of the pharynx. The function of
suction was attributed by Keaumur 12 to the proboscis of Musca; he regarded
the "trunk as a sort of suction-pump;"* but Gleichen 22 was the first to locate

* "... trompe corame une sorte de pompe aspirante." (p. 268.)

39

more definitely the pumping organ in the basal half of the proboscis, " which"
he writes (p. 20), " may be indeed the real pump of the whole machine." *
Lowne 2:? (p. 41-42) gives an excellent description of how the parts of the fulcrum,
which he calls the pharynx, act in pumping. A second function of the fulcrum
in Musca and Eristalis, and one which will be seen later to be less constant
throughout the diptera than the first function, is to support and control the
motions of the basal half of the proboscis, of course controlling through its basal
half many of the motions of the whole proboscis. On the posterior ventral pro-
cesses of the fulcrum (pi. 4, fig. 2 and 3, ~) are inserted muscles, which have
their origins in the anterior, lateral part of the head; these muscles, by their
contraction, project the base of the proboscis, by revolving the fulcrum about the
point d, where it is attached.

The next question which naturally arises is, whether the fulcrum, more or
less completely developed, is found, or not, in Cule.r. and Bombylius. A hasty
examination of the median longitudinal section through the head of Bombylius
(pi. 2, fig. 3) shows that the portion comprised, in section, by the pharynx (/>)
and the pharyngeal muscles (pm) is an exact morphological counterpart of the
section of the fulcrum in Eristalis and Musca. (Correspondingly lettered in pi. 3
and 4, figs. 2 and 3.) A cross-section of the fulcrum of Bombylius, near its
distal end (pi. 2, fig. 1, I', //) shows that it consists, like the corresponding
section in Musca, of the upper and under walls of the pharynx, extending upward
and divergent, from each side of the pharynx, and having between its diverging
walls the pharyngeal muscles (pm). The differences in structure between this
fulcrum of Bombylius and that of Musca and Eristalis are such as are due to
the fact that the proboscis of Bombylius is not extensible, and, consequently,
that its fulcrum need not be movable about an axis at d (pi. 2, fig. 3). This
absence of motion of the fulcrum around an axis allows the fulcrum to be less
independently suspended in the head than it otherwise would be; its upper side
consequently fails, its lateral chitinous walls are attached to the anterior wall
of the head, and the pharyngeal muscles have their origin on the wall
of the head. The distal extremity of the fulcrum of Bombylius serves, as it
does in Musca, for attachment for the bases of the labrum-ephipharynx and
hypopharynx. Here, then, in Bombylius, the fulcrum retains its essential
anatomical characters, and its function as a suctorial organ; but is less
independently movable than in Musca and in Eristalis, and its position is inside
the head.

* "... Welches wol die eigentliche Pumpe der ganzen Machine seyn mag."

40

In Culex the part which corresponds to the fulcrum of Miisca is a trifle less
easily recognizable than it is in Bombylius. By an examination of the longitudinal
median section of the head of Culex (pi. 1, fig. 11) it is easily discoverable that,
just behind the place where the mouth-parts join to form the mouth, a set of
pumping muscles exists similar to those found in Musca and Eristalis. These
muscles are, however, enclosed in a chitinous case or box (fig. 11, c) of their
own in front of the head. By studying cross-sections through this box (pi. 1,
fig. 9, in female; fig. 15, in male), the muscles {pm and pm''), which have their
origins in the upper and lateral portions of this chitinous case, are inserted on
the basal supports of the labrum-epipharynx, in the same general way as they
are attached to the pharyngeal walls in the fulcra already described; and that,
by their contraction, these muscles would serve, in the same way as has been already
described, to separate from each other the upper and lower walls of the pharynx.
The upper and lower walls of the pharynx themselves have a similar form, in
cross-section, to that which they have in Musca and Eristalis ; their lateral portions
turn upward and form a slight internal chitinous framework about the pharynx;
on the upper walls of the pharynx originate the pharyngeal muscles. Here we
have, then, as in all the diptera previously compared, a fulcrum, only that, in
Culex, its upper portion is more rudimentary than is usual, the necessary rigidity
for the origin of the pharyngeal muscles being supplied by the forward wall of
the head (fig. ll, c). The position of the fulcrum in Culex is not essentially
different, because located in a case apparently external to the head, from the
position which it occupies in the basal part of the proboscis of Musca, when the
proboscis is extended.

The relative position or condition of the fulcrum in the genera of diptera
which I have most studied is this: in Culex it is permanently extended; in
Bombylius, permanently retracted ; and in Eristalis and Musca, capable of extension
or retraction at will.

Before essaying to discover the homological significance of the fulcrum itself,
it is well to turn attention to the parts which surround it. In Bombylius the
chitinous wall (pi. 2, fig. 3, c) of the head, forming the covering to the upper,
or dorsal, open side of the fulcrum, bearing, on its inner surface, the pharyngeal
muscles, and continuous with the labrum in front, would, without doubt, be recog-
nized as the clypeus (epistom of some authors). The chitinous wall which
surrounds the fulcrum beneath, continuous at the sides with the clypeus above,
is the under surface of the head. The under surface of the head is continuous
anteriorly with the labium, but often, between the two, exhibiting folds, which
(pi. 2, fig. 3, I', I") may be reckoned as belonging to the head or to the labium.

- 41

At the side one of these folds encloses the basal chitinous support of the maxillae
as can be seen in pi. 2, fig. 1, * and A', where the basal supports of the maxillae
are lettered #, and the folds of the labium /' and I". In CWe.r the same relative
position and connection of parts are found, as can be seen in pi. 1, figs. 9 or 15,
where the pharynx and its muscles are surrounded above by the clypeus (c),
extending, at the sides, into the bases of the maxillae (mx) and of the maxillary
palpi (mp), and continuing beneath into the walls of the basal portion of the labium.
In EristaMs and Musca, when the basal half of the proboscis is retracted, the
conditions are relatively the same as in Bombylius. (Compare pi. 3, fig. 2, and
pi. 4, fig. 3, with pi. 2, fig. 3.) The clypeus (c on each pi.) extends downward,
from the frontal angle (rf) of the head, to the middle joint of the proboscis; at
the sides, as can be seen in figures of numerous sections, the bases (.<) of the
maxillae, where maxillae exist, are in the base of the labrum (7) or in its fold (/').
The question, whether, or not, the portion represented in section in pi. 4, fig. 1,
/?', x, is the hornolog of a basal portion of a maxilla is left for further con-
sideration.

Having discussed the surroundings and functions of the fulcrum in Culex,
Bombylius, Eristalis and Musca, the question remains, what is the fulcrum itself,
homologically considered? Repeating, in some cases, what I have already said,
in earlier parts of this paper, I will briefly state the opinions of several authors
on this question. Gerstfeldt 3 figured the fulcrum of Musca, without assigning it
a special name, and apparently without discovering that it was, strictly speaking,
an internal organ. He considered the fulcrum to be made up of the maxillae,
for the most part, united with the mandibles above and with the subnientum of
the under lip beneath. The incorrectness of Gerstfeldt's views of the construction
of the fulcrum are evident when one sees, as already explained, that it is only
the outer sheath around the fulcrum that is continuous with the under lip, and
that the bases of the maxillae lay entirely independent and outside the walls ol
the fulcrum. The use of the name fulcrum, probably originated from the following
remark of Lowne 23 (p. 42): "Although I believe this organ is homologous to the
fulcrum in bees, I prefer the term pharynx, from its double connection with the
mouth and oesophagus, as well as its peculiar function." Macloskie 24 says (p. 154) :
"One of Lowne's terms for it (pharynx) is incorrect; it is rather a case
surrounding the pharynx. I shall refer to it by the name fulcrum." Macloskie
later (p. 159-160) expresses the view that the "mid-segment" of the so-called
proboscis, " is the true base of the fly's proboscis ", and homologizes the fulcrum
with the endocranium of the cockroach (Blattd) he also says of the fulcrum
"It seems to be general indiptera; even the mosquito possesses it," but he does

- 42

not further describe it, in other diptera than Musca. Menzbier 8 writes (p. 64)
of the fulcrum, without attempting further to homologize it, that, " it is chitinized
processes of the walls of the throat." * It is probably the fact that the fulcrum
is only a modified form of chitinization of the walls of the pharynx, which are
turned upward at the sides, and fastened, more or less firmly, to the under side
of the clypeus; in cases where independent motion of the entire pharynx takes
place in the extension and retraction of the proboscis, the lateral upward con-
tinuations of the pharyngeal walls may unite again above, to a greater or less extent,
so as to form a cylinder for the firm attachment above of the pharyngeal muscles,
which thus act with equal ease to whatever extent the proboscis may be withdrawn
or stretched out. The whole plan of structure of an independently movable fulcrum
is well shown in the section of the fulcrum of Musca figured on pi. 4, fig. 4.
The fulcrum is, then, an internal skeletal piece, the chitinized expanded portion
of the walls of the pharynx, and not a mouth-part in the strict sense of the
term. The reason will now be more apparent, why, in an earlier part of this
paper (p. 26), I objected to laying too much stress on the division of the
proboscis into basal, middle, and terminal portions. While applying the term
proboscis, in its ordinary sense, to the whole organ, I agree with Macloskie that
the so-called middle portion of the proboscis is the true basal segment of the
fly's proboscis. I may, perhaps, add, at this point, that my reasons for using
the somewhat clumsy term labrum-epipharynx, in place of the name operculurn,
applied by Lowne to the same part in Musca, are that, first, the name operculurn
is inappropiate in some diptera, and that, secondly, the name labrum-epipharynx
saves all confusion of parts by indicating, at first sight, what mouth-parts are
meant.

If one considers, in order, the different mouth-parts of diptera, they are as
follows :

The labrum-epipharynx is composed of the labrum above and the epipharynx
beneath, united at their margins by a delicate membrane, which is often infolded
(Bombylius). The labrum and epipharynx may be separable in caustic potash
(Mused) or inseparable (Empis according to Menzbier). The epipharynx may be
absent, according to Menzbier (Sargus). At its base the labrum-epipharynx may
be united to the apical end of the fulcrum by a true joint (Musca), or without
joint (Culex); the labrum is always continuous with the clypeus at the' base, the
epipharynx always continuous with the upper wall of the pharynx. The labrum-

* " . .es chitinizirte Fortsatze der Schlundwand siud.

43 -

epipharynx raay have a single point at its tip (Mwca } Bombylius) or may end,
apically, in several points (Culex, and probably all Syrphidae). Tlie labrum-epi-
pharynx usually forms a cover for the channel of the labium below, and may be
separable from the labium, at the will of the insect (Cidex), or may remain always
tightly closed over the labium (Musca). The labrum-epipharynx is usually, second
to the labium, the largest mouth-part of a dipteron. Within it may contain
muscles and tracheal stems (Bombylius and Musca). The channel of the epi-
pharynx is never completely cylindrical, but is slightly open along its under side ;
a tube is formed by the pressing of the hypopharynx upward against this open-
ing ; through the tube thus formed the food is sucked up to the mouth.

The hypopharynx is usually present in diptera (according to Menzbier absent
in Sargus~), and contains a tube, opening by a channel on its upper surface; this
channel extends back, more or less, from the tip, and is the outlet for the salivary
secretion. The tip of the hypopharynx may be naked and used as a lance
(Haematopoia, according to Menzbier), or may be hairy (Musca). The upper side
of the base of the hypopharynx is continuous with the lower wall of the pharynx ;
its under surface may entirely coalesce with the labium (Cidex, male), may join
the labium more or less anterior to the mouth (Musca), or, if either mandibles or
maxillae are present, its base may join them (Culex, female).

The mandibles are the mouth-parts which are least developed, or most often
absent, in diptera. They are present in Culex, female, and, according to Menzbier,
in Haematopota ; they are absent in Eristalis, Bombylius, Musca, and many other
diptera. When present they are usually delicately lamelliform.

The maxillae are, next to the mandibles, the oftenest absent in diptera; but
when both maxillae and mandibles are present (Culex, female), the maxillae are
more developed than are the mandibles. Maxillary palpi are usually, probably
always, present in diptera, usually joining the maxillae at the base; they are
from one-jointed (Bombylius, Eristalis, Musca) to five-jointed (species of Culex),
are more or less hairy, and lay outside the proboscis, attached near or at the
mouth. The position of the maxillary palpi in Musca has evidently created much
confusion in the homology of the parts of the proboscis of these insects. Their
location on the membranous bridge between the two upper edges of the distal
end of the fulcrum (in section, pi. 4, fig. 1 , /?', mp) seems to have led Gerstfeldt :i
to regard the fulcrum itself as chiefly made up of the maxillae. Gerstfeldt
described two processes extending back from the base of his hypopharynx (really
the epipharynx), and figured them as inside the fulcrum. These processes he
regarded as the basal portions, or " cardines," of the hypopharynx; a view which

- 44

is necessarily incorrect because his hypopharynx was the real epipharynx separated
by caustic potash from the labrum. Lowne 23 (p. 43) mentions and figures these,
processes which he terms " apodemes," aud, as he incorrectly supposed the
labrum-epipharynx, his operculum, to be made up of the labrum with a maxilla
coalesced on each side, he regards these " apodemes " to be the basal portions of
the maxillae. Thus from incorrect premises Lowne arrived at what I consider
to be a correct conclusion, that is, that these processes are the basal portions of
undeveloped maxillae. Macloskie' 24 regarded these same chitinous rods as the "great
tendons" of the mandibles, and then added (p. 159) the impossible idea, " This will
make the operculum represent two united mandibles, probably enclosing the labrum."
Macloskie correctly says, however, of these "great tendons," that they are to
the right and left of the fulcrum, and that they are slightly articulated to the
operculum. Menzbier 8 writes (p. 65) of these same parts, in a very indeterminate
manner, that, "they are simple muscle-tendons, which were less developed in
Syrplms, and consequently not described by us." * That these processes really
are the remnants of the basal chitinous supports of the maxillae is very probable,
because they are the only chitin-rods which occupy the relative position that is
occupied by the corresponding parts in other diptera (compare pi. 4, fig. 1, /?',
x with pi. 1, figs. 9 and 15,.ww,- and mp; pi. 2, fig. 1, i-k', x\ and pi. 3, fig. 1,
/-', x)\ because they are imbedded in a fold of the base of the labium;
because they are not firmly joined to the labrum, or to any other mouth-part ;
and lastly, because the maxillary palpi, altho not joined to these processes, are
above their distal ends, a position they would scarcely reach if these chitinous
tendons belonged to the mandibles or labrum. The maxillary palpi of Musca
are not joined to any firm supporting piece; the bridge or band of chitin, seen
at their base in pi. 4, fig. 1, is only a slight corrugation (as can be seen by
the section represented in pi. 4, fig. 1, /?', c-), which is probably due to the
hinderance to irregular folding, which would be otherwise caused at that point
when the proboscis retracts, by the presence of the palpi themselves. The
maxillary palpi of Musca, then, by some cause, are displaced from their basal
supports, which remain in their places, without maxillae or palpi, performing, by
the muscles attached to them, another function, that is, the lateral swinging of
the proboscis when the insect is feeding.

The labium of diptera, their most fully developed mouth-part, and one that
is always present, joins the head below the other mouth-parts; its under wall

* "Es sind einfach Muskelsehnen, die bei Syrphus weniger entwickelt und deshalb von
uns nicht beschrieben sind."

45

is continuous basally, sometimes with folding, into the under wall of the head.
The labium is usually somewhat fleshy, containing muscles and two longitudinal
trachea! stems, and is hollowed out along the upper side to form a trough for
containing other mouth-parts. At each side of the tip, the labium, probably in
all cases, has more or less developed labellae, on the inner surface of which are
usually the channels termed pseudotracheae. These pseudotracheae may be absent,
however, as in Culex. The labellae are fleshy in Musca, less so in Eristalis, and
least so in Bombylvus and Culex. While the labium of all insects is undoubtedly
the product of a coalesced pair of appendages, as Savigny supposed, and its parts
can be still further safely hornologized, in many insects, with particular portions
of the maxillae, and altho it has been divided in Orthoptera and some other
insects into subrnentum, mentum, glossa, etc., I do, not believe that in .the
diptera, our knowledge of the structure of the labium suffices for us to enter
safely on homologies of its parts, and I have, consequently, refrained from doing
so in the proceeding pages.

The mode of separation and approximation, of the labellae is somewhat diffe-
rent in different diptera, and is, perhaps, worthy of a comparative notice in this
place. As already mentioned (p. 18), the labellae of Culex are attached to
the tip of the labium by a true joint, and have, each, a flexor and extensor
muscle. The labellae of Bombylius, described briefly on p. 25, altho not
connected to the labium by a true joint, have each a flexor and extensor
muscle. When, however, one examines, by sections, the labellae of Eristalis,
which I have briefly described on p. 30, one finds only an extensor muscle
in each labella, and in the sections of the labellae of Musca one finds no muscles.
How, then, is the separation of the labellae effected in Musca, for both in Musca
and in Eristalis their normal condition seems to be that of opposition ? Examining
the sections of the labellae of Eristalis (pi. 3, fig. 1, ') one sees that a chitiuous
band passes through the middle of each labella, and that, to the outer side of
this band, the extensor muscle is attached. The bands serve as supports for the
labellae, and as attachments for their extensor muscles; and these bands are
themselves jointed at their bases to the inner or upper chitinous wall of the
labium. An examination of cross-sections of the labellae of Musca shows this
same chitinous band, but no muscle. The reason for the absence of the muscle
is that, as has been already described (p. 36), the muscle does not extend into
the cavity of each labella, but is only inserted on the base of these chitinous
bands. The labellae of both Musca and Eristalis are opened, then, at least in part,
by muscular force ; but probably the more complete separation of the two labellae
is caused by the same inflation witli air, which, as previous writers have supposed,

46

.

serves to protrude, and to press firmly, the inner surfaces of the labellae, upon
any surface from which the insect wishes to scratch or dissolve substances for
food. That it is air, at least for the most part, and not water, which expands
the inner surfaces of the labellae, is easily proved by pressing carefully the head
of a fly between the fingers until the proboscis is fully extended, and the labellae
fully inflated, then, putting the fly under water, and pricking with a needle
the inner surfaces of the labellae ; they will at once collapse, bubbles of air
escaping, at the same moment, from the opening made in their surfaces by
the needle.

The pseudotracheae on the inner surfaces of the labellae of Musca are
cylindrical channels, sunk, more or less deeply, into the surface of the labellae,
according to the amount that that surface is inflated, and they open on the surface
in zigzag slits. These channels are held open by partial rings, more strongly chitin-
ized than the rest of the membrane of the cylinder. As seen from above in Musca
domestica, the pseudotracheae (pi. 4, fig. 6, 6) appear to be supported by partial
rings, one end of each of which is forked. Such a ring, if isolated, would appear
as in fig. 6, d. These rings are apparently arranged so that one has its fork
on one side of the opening of the channel, the next ring the fork on the opposite
side of the channel, and so on, in alternation. This, I say, is the apparent
structure, for if one expands the membrane of the inner surface of the labellae,
to a sufficient extent, the channels, or pseudotracheae, are flattened out and their
true structure is revealed. PI. 4, fig. 7, represents a portion of such a flattened
out pseudotrachea of Musca domestica, the structure of which is immediately
evident; at the right-hand side of the figure is represented an irregularity, such
as now and then occurs in pseudotracheae. If such a piece of flattened pseudo-
tracheae as is seen at the left, in fig. 7, be formed into a cylindrical channel,
its appearance will be as in pi. 4, fig. 6, b- In Musca vomitoria the pseudo-
tracheae have a structure somewhat different in detail, but not in principle, from
those already described, as can be seen from fig. 5 without further description.
The pseudotracheae of Eristalis horticola are so nearly like those of Musca
vomitoria, that I have not figured those of the former. PI. 4, fig. 8 represents a
section through a portion of the inner surface of a labella of Musca vomitoria,
cut at right angles to the direction of the pseudotracheae which pass through it;
in this section the intrapseudotracheal spaces are represented as protruded beyond
the edges of the pseudotracheae themselves ; this is the natural position of the
pseudotracheae in repose, but when the fly inflates his labellae in the process of
eating, the soft intrapseudotracheal membrane is stretched, and the margins of
the pseudotracheae are protruded beyond the general level of the surface of the

47

labella. These margins of the pseudotracheae consist of the lobes, or teeth, to
be seen at each side in pi. 4, fig. 7. The ring-like structure of pseudotracheae
has the same function as the similar structure in the tracheae of insects, or in the
wind-pipe of vertebrates, that is the function of holding tubes open, and preventing
their sides falling in upon one another. The same peculiar ring-like structure
is als9 found, with like functions, in the salivary ducts of diptera. If the labellae
are now alternately inflated, a little more and then a little less, the width of
the zigzag openings of the pseudotracheae is alternately increased and diminished,
and the teeth along the margins of the cleft naturally scratch on the surface
on which the fly has pressed his labellae. The same scratching effect is produced
when the labellae are simply moved a little back and forth upon any surface.
The labellae are, then, files of which the teeth are the serrate edges of the
pseudotracheae. During the scratching process the pseudotracheae are, in Musca,
moistened with saliva, with which they wash the surfaces of the substance to be
scratched or dissolved away. Lowne 23 says of the pseudotracheae (p. 47-48)
"these form a fine strainer through which the insect is enabled to filter the fluid
from the solid portion of the substances on which it feeds." In Bombylius
which offers excellent facilities for studying the pseudotracheae, because they,
altho constructed on the same principles as, are more firm and consistent
than, in Musca, I have found that, after feeding the fly with a mixture
of sugar and gum-arabic, colored with carmine, and then plunging it
suddenly into strong alcohol to fix the colored solution in its mouth-
parts, that, later, when I cut sections of its labellae, the pseudotracheal teeth,
instead of having their ordinary position (see pi. 2, fig. 5, a; in section, fig. C.
a), were turned outward from the pseudotracheal channel, into the position seen
in pi. 2, figs. 5 and 6, b, and that the colored solution of gum-arabic had not
entered the pseudotracheae. Boinbylius also rubs the labellae over the surface
of the substance on which it is feeding, in a way similar to that which Musca
does. It can be seen that the more purely liquid food any species of diptera
eat, for a constant diet, the less necessary and the less developed are the
pseudotracheae; Culex, for example, eating a purely liquid food, has no pseudo-
tracheae. From the above facts, I am led to think that the primary function,
at least, of the pseudotracheae of diptera is to file away substances on which
they feed. That the pseudotracheae may have other functions, I do not wish to
affirm or deny.

The pharyngeal sucking organs of the four different genera of diptera which
I have studied have been so fully compared in the comparison of the fulcra in

48

these genera, that I need not further refer to them. The oesophageal bulb, as
sucking apparatus, behind the oesophageal nerve-ring, exists, as far as I know,
only in Culex, and cannot therefore be compared with any other like structure ;
and I will here end my notes on the comparative structure of the mouth-
parts and suctorial apparatus of the diptera, whose examination and dissection
furnished me the materials for this paper.

LITERATURE.

1 Fabricius, Johann Christian. Systema entomologiae, sistens insectorum classes, ordines,

genera, spe"cies adiectis synonymis, locis, descriptionibus, observationibus. Flensburgi
et Lipsiae, 1775.

2 Fabricius, J. C. Philosoplria entomologica sistens scientiae fundamenta adiectis defini-

tionibus, exemplis, observationibus, adumbrationibus. Hamburg! et Kilonii, 1778.

3 Gerstfeldt, Greorg. Ueber die Mundtheile der saugenden Insect-en. Ein Beitrag zur ver-

gleichenden Anatomic, welchen zur Erlangung der Magister-Wiirde . . . zu Dorpat.
Dorpat, 1853.

4 Schellenberg, Johann Rudolf. Helvetische Entomologie . . . Ziirich, 1798, 1806.

According to Hagen (Bibliotheca entomologica, I, p. 131 ; II, p. 120) this work was translated
into French by J. de Clairville and published, the same years, in German and French. Percheron
(Bibliographic entomologique, I, p. 68) cites only Clairville as author. Gerstfeldt credits Clairville
with having first divided insects into suckers and chewers.

5 Savigny, Jules Cesar. Memoires sur les animaux sans vertebres. Premiere partie . . .

Premier fascicule. Mem. 1-2. Theorie des organes de la bouche des Crustacea et
des Insectes. Insecta, Linn. . . . Paris, Janv. 1816.

6 Erichson, Wilhelm Ferdinand. Entomographien, Untersuchungen in dem Grebiete der

Entomologie . . . i. Ueber zoologische Characters der Insecten, Arachniden und
Crustaceen. Berlin, 1840.

'' Brulle, Auguste. Recherches sur les transformations des appendices dans les Articules.
(Ann. des sciences natur., 1844, ser. 3, t. 2, p. 271-374, tab. 1.)

8 Menzbier, Michael Alexander. Uber das Kopfskelet und die Mundwerkzeuge der Zwei-

fliigler. (Bull. Soc. imper. natur. de lloscou, 1880, t. 55, no. 1, p. 8-7J, tab. 2-3.)

9 Blanchard, Emile. De la composition de la bouche dans les insectes de 1'ordre des

Dipteres. (Compt. rend., .1850, t. 31, p. 424-427.)

I am dependent, for the contents of this paper on Gerstfeldt (p. 20), Menzbier (p. 19-20), and
Schauin's Bericht iiber die wissenschaftlichen Leistungeu im Gebiete der Entomologie (1850, p. 100).

10 Weismann, August. Die Entwicklung der Dipteren. ... i. Die Entwicklung der

Dipteren im Ei. n. Die nachembryonale Eutwicklung der Musciden. [Abdruck aus
der Zeitschrift fiir wissenschaftliche Zoologie. xm. und xiv. Band.] Leipzig, 1864.

4

50

11 Swammerdamm, Johann. Buch tier Natur . . . Leipzig, 1752.

In all cases where I have had occasion to cite Swammerdamm's Bybel dcr Natuure I have
cited the above German translation.

12 Reaumur, R. A. F. Memoires pour servir a 1'histoire des insectes.

Citations are given in this paper from the edition of this work published in Amsterdam, 1737-1748.

18 Barth, Johann Mathaeus. Dissertatio de culice. Ratisbonae, 1737.
I have not seen this work.

14 Sulzer, Johann Heinrich. Die Kennzeichen der Insekten . . . Zurich, 1761.

15 Gravenhorst, J. L. C. Grundziige der systematischen Naturgeschichte . . . Breslau, 1817.

16 Meigen, Johann Wilhelm. Systematisch'e Beschreibung der bekannten europaischen zwei-

fliigeligen Insekten . . . Aachen, 1818-1838.

17 Packard, Alpheus Spring, Jr. Guide to the study of insects . . . Salem, 1869.

Citations are from the first edition.

18 Jordens, Johann Heinrich. Entomologie und Helminthologie des menschlichen Kb'rpers . . .

Hof, 180J.

19 Clans, Carl. Grundziige der Zoologie . . . Marburg und Leipzig, 1876.

20 Roffredri, D. Moriz. Memoire sur la trompe du cousin et sur celle du taon . . . (Miscell.

Soc. Taurinens., 1766-1769, t. 4, p. 1-46, tab. 3.)
I have not seen this paper.

21 Suffolk, W. T. On the proboscis of the blow-fly. (Monthly microscopical Journ.,

June 1869, v. 1.)

I have not seen this paper.

22 Gleichcn, Friedrich Wilhelm. Geschichte der gemeinen Stubenfliege . . . Niirnberg, 1764.

23 Loivne, Benjamin Thompson. The anatomy and physiology of the blow-fly (Musca

vomitoria, Linn.) . . . London, 1870.

24 Macloskie, George. The proboscis of the house-fly.- (Amer. Naturalist, March 1880,

v. 14, p. 153-161, fig. 1-3.

VITA.

The only child of George Monroe, and Elizabeth Learned Dimmock, I, Greorge Dimmock,
was born the 17th of May 1852, in Springfield, Massachusetts, U. S. A.

After several years of study in the public schools of my native city, I was fitted, at
the Harvard School, a private school in Springfield, to enter Harvard College. Cambridge,
Massachusetts, which I entered in 1873, and from which, on graduation in 1877, I received
the degree of bachelor of arts, with honors in natural history.

In October 1879, I was matriculated, as student of natural history, in the University
of Leipzig, where I remained until the autumn of 1881. In Leipzig my time was chiefly
devoted to the study of zoology in the laboratory of Professor Leuckart, in addition to
whose lectures, I attended lectures by Professors Carstanjen, Carus, Credner, Schenk and
Wiedemann, and by Doctors Chun, Fraisse, von Ihering, Luerssen and Marshall.

EXPLANATIONS OF THE PLATES.

The following letters have the same signification on all the plates:

c, clypeus ; c 1 , its folds. m, mandibles.

e, epipharynx. mp, maxillary palpi.

/, fulcrum.

h, hypopharynx.

i, infraoesophageal ganglion.

I, labium; I', I", its folds.

Ib, labellae.

Ir, labrum.

Ir-e, labrum-epipharynx.

mx, maxillae.

oe, oesophagus.

p, pharynx.

pm, pm 1 , pharyngeal muscles.

s, supraoesophageal ganglion.

tr, tracheal stem.

x, basal support of maxillae.

The number of diameters enlargement is indicated against each figure.

Greek letters are used to indicate where sections are made in probosces,
and the corresponding sections are indicated by the same letters, often with
an accent (').

Shaded parts of sections are portions filled with connective tissue,
nerves, air-spaces, and other parts having no significance in connection with
points discussed in the dissertation.

EXPLANATIONS OF PLATE I.

Fig. 1. Side view of head of Culex rufus, with extended mouth-parts;
a, antennae.

Fig. 2. Same from above with mouth-parts partly cut away.

Fig. 3. Tip of labium of female Culex.

Fig. 4. Tip of labium of male Culex.

Fig. 5. Tips of separated setae of mouth-parts of Culex.

Fig. 6. Tip of labrum-epipharynx seen from beneath and in section.

Fig. 7. Cross-section through the labellae and tip of labium of female Culex.

Fig. 8. Cross-section near the middle of the proboscis of female Culex.

Fig. 9. Cross-section through pharyngeal region of the forward part of the
head of female Culex.

Fig. JO. Cross-section through the posterior part of the head of female
Culex, to show the sucking bulb of the oesophagus, b, lumen of
the oesophageal bulb, bm and 6m', .muscles to dilate the bulb.
r, chitinous rods which support the oesophageal bulb, rm, retractor
muscles of the maxillae, at their point of origin, t, elastic plates of
sides of bulb.

Fig. 11. Longitudinal section of the head of a female Culex. b, oeso-
phageal bulb, g, point where the clypeus appears cut off from the
rest of the head, v, valve between pharynx and oesophagus.

Fig. 12. Cross-section near the middle of the proboscis of a male Culex.

Fig. 13. Cross-section at the base of the proboscis of a male Culex.

Fig. 14. Cross-section further into the base of the proboscis of a male Culex.

Fig. 15. Cross-section through the pharyngeal region of the head of a male
Culex.

Pltlti- /.

nip

EXPLANATIONS OF PLATE II.

Fig. 1. Side view of the head of Bombylius major with its proboscis; at the
upper right-hand corner are figured the tips of the labrum-epipharynx,
hypopharynx and maxilla, to illustrate their comparative sizes and
lengths. Below ( 2') is a series of cross-sections of various parts
of the proboscis and its base.

Fig. 2. View of the head of B. major from above, to show how the labellae
open.

Fig. 3. Longitudinal section of the head of B. major, z, location where the
ventral posterior process of the fulcrum lays, d, location of the
dorsal posterior process of the fulcrum.

Fig. 4. Tip of maxilla of B. major.

Fig. 5.- Perspective view of the chitinous portions of the pseudotracheae of
B. major; a, while at rest; b, while feeding.

Fig. 6. Cross - sections of the pseudotracheae of B. major when the two
labellae are opposed to each other; a, at rest; b, while feeding.

EXPLANATIONS OF PLATE III.

Fig. J. Side view of the head of Eristalis horticola; dotted lines indicate
the courses of the tip of the proboscis, k, and of its middle joint,
g, in the retraction of the proboscis, d, angle of the head in which
the dorsal posterior process, d' (fig. 7), of the fulcrum is attached.

Fig. 2. Longitudinal section of the head of E horticola, with partly retracted
proboscis. Lettering as in fig. 1.

Fig. 3. Longitudinal section of the head of E. horticola, with extended pro-
boscis. Lettering as in fig. 1 and 2.

Fig. 4. Detached maxilla and maxillary palpus of E. horticola.
Fig. 5. Tip of Iabru7ii-epipharynx of E. horticola, spread out and viewed
from beneath.

Fig. 6. The hypopharynx of E. horticola, from above.

Fig. 7. The fulcrum of E. horticola; d', its dorsal posterior process; z, its
ventral posterior process.

Plate ///

EXPLANATIONS OF PLATE IV.

Fig. 1. Side view of the head of Musca vomitoria, with extended proboscis.

Fig. 2. Longitudinal section of head of M. vomitoria, with extended proboscis.
sd, salivary duct, d, dorsal posterior process of the fulcrum, z, ven-
tral posterior process of the fulcrum. [In this and the next figure
the section of the frontal sac is omitted to avoid complicating the
figures.]

Fig. 3. Longitudinal section of the head of M. vomitoria, with partly retracted
proboscis. Lettering as in fig. 2.

Fig. 4. Section of the fulcrum of M. vomitoria.
Fig. 5. Pseudotrachea of M. vomitoria.

Fig. 6.- Pseudotrachea of M. domestica; d, one of the apparent segments of
the pseudotrachea-

Fig. 7. A portion of a pseudotrachea of M. domestica spread out to show
its real structure; toward the right it is abnormal.

Fig. 8. Cross-section of three pseudotracheae of M. domestica to illustrate their
position in the surface of the labella.

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