George Dimmock.

The anatomy of the mouth-parts and of the sucking apparatus of some Diptera online

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are diflferent 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 Gdea:, where they are
situated near the middle of the blade of the niaxilla. 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 Culex, 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 extremit}' 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 ftmction of the maxillae is, probably, to draw the other mouth-parts into
the skin, when C%de,t bites, for if one watches the maxillary palpi of C%dex, 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 maxillarum, already described,
lend weight to this view of the functions of the barbed maxillae.

The labium (figs. 1, 2, and 3, /)» ^^^ largest of the mouth-parts of Cwfer,
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 annulated 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

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in the base of the head and serve to control the motions of the labium. (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 lb 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 bebw the section of the
maxillae (mx). 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, nie,
and 7nf.) 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 Cvlexj 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 Cidej^. In the females of Ctdesr
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 fuU length the labium is bent double beneath the body of the insect,
the labellae stiU 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



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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
Cukxy and was the first, probably, to describe and figure how the insect disposed
of its labium during the operation.

The mouth-parts of Cvlex^ as above described, are suspended under a clypeus,
or epistom, which is figured ifrom 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 Culex^
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 epipharjmx above
and the hypopharynx 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
nefve-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, fc; the cross-section in fig. 10, b. This bulb, which is the chief sucking
organ in the female CuLexy is supported by 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 nduscle, or a pair of' muscles (&m), the origin of which is
in the dorsal part of the chitinous shell of the head. Each of the lateral plates
has inserted on it a muscle {Jym% 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 hm% 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.

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THE MOUTH-PAKTS OP CULEX, MALE.

The mouth-parts of the male of Ctdex have not been described, as far as I
know, with any degree of accuracy, altho, since Swammerdamm's time, the males
have been distinguished from the females, by all scientific entomological writers
on the subject, by means of their feather-like antennae and maxillary palpi.
J5rdens^* (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 pipiem, 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. 8-9 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 game 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 Ir-e^ and 6) will serve for the tips of both. In section (fig.
12, Ir-e), the labrum shows a groove on its upper surface, which deepens as it
nears the base (fig. 13, Ir-e). At its base the labrum-epipharynx unites with the

• "Da aber auch die mannliche mit ©ben dera Saugstachel versehen ist, so ist nicht
einzuseben, warum sie sich desselben nicht zu gleicher Absicht bedienen sollte.''



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maxillae and their palpi before it unites with the labium, as shown by fig. 14.
The apical four-fifths of the labium contains no other seta than the labrura-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, pm^.

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, A and ina;.') The hypopharynx shows, in
section (fig. 1^^-15 A), 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 Cule^^ the maxillae usually remain attached to their base.
The attachment of the maxillae to the maxUlary palpi is readily seen in sections
(fig. 13, mx and mp); that they are not mandibles is evident.

The maxiUary 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 Culea; 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
slendemess 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 30 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 Culea 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



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male has two longitudinal main tracheal stems (figs. 12-14, tr), 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 when upon the
leaves and ^ass of the field, but they would neither bite nor show any desire
to do so, nor have I 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^* says (p. 174) *'The proboscis" of BonibijUm "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
aide, which, as in Conops, is split longitudinally." * Fabricius writes (Si/sL
entom., p. 802), ** Mouth haustellate, without proboscis. Haustellum very long,
sti-aight, setaceous, bivalvate"** and {P/dL entom,, p. 50), "setae three."***
Gerstfeldt* writes (p. 31-32), "The Bombyliidae also have four setae, which
always represent the maxillae, the upper lip and the hypopharynx, while the
mandibles are united with the proboscis-sheath — in Bombylius the hypopharynx

* '*l)er Rtissel ist so lang, und langer, als die Brust, horizontal ausgestrekt, borsten-
ahnlich, vome biegsamer, und nur die Scheide zu dem rechten Saogstachel, welcher durch die
obere Seite, die, wie bei dem Pferdstecher, der Lange nach gespalten ist, heraus kommt."

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

**♦ "Setae tres."



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exceeds the upper Kp 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 Bomhylius 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* (p. 14) incorrectly regards the proboscis of Bombylim as extending
first directly downward, and then bending, knee-formed, forward. The pharynx
at the entrance of the head, turns slightly upward (fig. 3, p), 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 ofben 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-A') 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-epipharyni, 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 ai 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, ?? and d') : the hypo-
pharynx (A) lays in a groove on the upper side of the labium (/) and is covered
by the labrum-epipharynx; beneath or at the side of the labrum-epipharynx (/r-e)
are the maxillae {mx) and maxillary palpi (wp).

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



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The labrum-epipharynx (seen in cross-section in fig. 1 , /:f*^ Ir-e ; from beneath
in fig. J, Ir-e) 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 e-t. 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, r]-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 t-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, /r-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 sort of reflexion of its margins, forms
chitin-plates, which serve to support it in the forward part of the head (see fig.
1, 1% and to contain the pharyngeal muscles (fig. 1, l\ ^wi and fig. 8, 7>m), 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 diflFerent 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 Bomhylim 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 hypopharynx. 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. 1, ly 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



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palpi. (See fig. 1, x.) The tip of the epipharynx rests between the labellae,
as is seen in fig. 1, a, and in fig. 2.

The maxillae (fig. 1, nuv) 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-
pharynx. (See fig, 1, ^, ma;,) 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 e, the maxillae are joined by the
maxillary palpi, and, at about the same time, they join a fold (I') of the labium,
which fold, as seen in section, surrounds the labium itself. The bases of the
maxillae (^) extend deep into the head.

The maxillary palpi (fig. 1, mp) 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-t) for the reception of the hypopharynx and maxillae. It
contains two longitudinal tracheal stemfa, 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
(/' and /" of figs. 1, t-x, 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


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Online LibraryGeorge DimmockThe anatomy of the mouth-parts and of the sucking apparatus of some Diptera → online text (page 3 of 7)