THE ANATOMY OF THE ADULT MOSQUITO 91 ventral aspect devoid of any rigid sternal plate, being composed entirely of elastic flexible membrane. The last three, however, are shorter and diminish rapidly in width ; and the last two are specially modified to accommodate the apertures of the digestive and reproductive systems, the anus being placed on the ventral aspect of the eighth segment, while the tergum of the ninth is quite invisible in the usual position of the insect, and all that can be seen Fig. 22. — Transverse Section through the Posterior Part op the Abdomen of a Hibernating Female.
dv, dorsal vessel ; ^jrs, perivascular space ; m, Malphigian tubes ; g, last abdominal ganglion. Of, ovary ; r, rectum ; X about 100 diams.
of it are two lobed appendages, thickly clothed with hairs and scales, and not showing any very promment difference in the two sexes. In both they consist of a pair of oliveshaped protuberances, but in the male, instead of ending in a simply rounded, bristly extremity, they have articulated to each a strong incurved claw, by which the female is grasped during copulation, the whole organ forming a clasper. In the female they are somewhat spatulate and act as an ovipositor, being held together to form a sort of channel along which the ova are passed on to the upper
02 GNATS OR MOSQUITOES — CHAPTER VI surface of the similarly held together hind tarsi, hy which they are guided to the surface of the water on which they are to be launched.
If we dissect the abdomen by teazing with needles or by sectionising, we find that immediately beneath the chitinous exoskeleton there is a soft, cellular layer, the true dermis ; and beneath this again, a very scanty and interrupted layer of longitudinal muscular fibres, which not only serve as flexors, extensors and abductors, but can also, when simultaneously contracted, shorten longitudinally and therefore transversely widen the entire region, an action which cannot fail to have an effect in keeping in motion the air contained within the tracheal system. For although there is no continuous, rhythmic action of the body comparable with the respiratory movements of the higher air-breathing animals, movements of this kind are sufficiently frequent and habitual to exercise a powerful action in preventing stagnation of the air contained in the respiratory tubes. These longitudinal muscles are arranged in sets, each set corresponding to an intersegmental membrane and serving to connect two adjacent segments. There are also a few transverse fibres, arranged mainly in two lateral groups, placed near the middle of the segments, but they are less easily made out.
The best way to exhibit them is to carefully remove all the scales from the abdomen, a preparatory step which should be taken as a routine matter in all dissections, by gentle brushing with a camel hair pencil. Dip the specimen for a moment in spirit, and then by a stroke of a pair of fine scissors split the abdomen into lateral halves. Examined in salt solution, such a preparation, after cleaning away the viscera by means of the needles, shows well the stigmata and parietal tracheae, and subsequently, after appropriate staining in borax-carmine, will give a good view of these parietal muscles. The precaution of dipping the specimen into spirit for an instant, before placing in salt solution, is a necessary preliminary to all dissection of these insects, as otherwise, however carefully one may have brushed, one is sure to be embarrassed by air bubbles, obstinately entangled in the remaining tomentum.
THE ANATOMY OF THE ADULT MOSQUITO 93 Having now sketched the external configuration of these insects, their internal organisation remains to be considered, and will probably be best dealt w4th by taking separately the various systems, digestive, respiratory and reproductive, in their turn.
The Anatomy of the Digestive System. — In the first edition of this book I had perforce to rely mainly, alike for facts and illustrations, on the work of previous writers. My supply of fresh material for verification was very limited, and hence numerous inaccuracies were reproduced. A notable example of this was Arribalzaga's illustration of the general relations of the digestive organs. In this figure the salivary glands are shown protruding from the back of the head, which, as a matter of fact, is just where they are seen in the ordinary course of dissection ; as they are drawn out from their natural position in the soft neck connecting the head and thorax by the act of pulling away the oesophagus from the thorax. He appears to have then hastily sketched in an outline of the insect, and hence these glands are made to appear as if placed in the anterior part of the thorax. The present chapter is, however, the outcome of personal observation, and the illustrations, with a few duly noted exceptions, are reproductions of pen and ink drawings from camera lucida outlines, and so may at least be trusted as to the relative size and position of the parts represented. The digestive tube may be said to commence at the extremity of the proboscis. By the alternate retraction and protrusion of the labrum ; as regards its relative position to the lancets, the whole of the styles of the proboscis are gradually worked into the wound inflicted, slightly in advance, by the mandibles and maxillae ; the hypopharynx, which is a delicate, flexible tube, far too yielding to be forced unsupported into any substance, lying along the groove of the labrum. Imagine a surgeon's " director," with a piece of thin drainage tube carried against its groove, and with four slender bistouries grouped round it, and you will have a fair working model of the malaria-inoculating apparatus of the mosquito.
From this it follows that it is only through the tube
94 GNATS OR MOSQUITOES — CHAPTER VI of the hypopharynx that the sahvary secretion, with its contained raahxrial germs, can be injected into the tissues of the vertebrate alternative host.
In the case of filariasis the route of infection is different. Taken as a family, the filaj'im are essentially parasites of the intercellular lymph spaces, and hence are most commonly found in such situations as the cellular tissues, and the great serous sacs, such as the cavities of the peritoneum and pleura, and that stage of the parasite life-cycle of the filaria inhabiting as an adult the human cellular tissue, vv^hich is passed in the mosquito, forms no exception to this general rule.
Necessarily it is along the groove of the labrum that the embryonic filarial find their way, along with the ingested blood into the mosquito's stomach ; but once there, they hasten to reach a more congenial situation, by boring through the walls of the organ into the cellular tissue around it, and are then free to wander in its interstices all over the body, including that of the sheath of the proboscis or labium, which is a comparatively thick fleshy organ, containing between its inner and outer walls a considerable amount of cellular tissue supporting the tracheae, muscles, and nerves, with which it is liberally supplied. Now, as Grassi has recently pointed out, filarice, though not usually very destructive parasites, do cause a certain amount of irritation and effusion ; and the result of the intrusion of filarice into the substance of the labium will be to raise a tense swelling of the delicate inner covering of the organ. As already pointed out, when the mosquito bites, the labium is sharply doubled into a loop, and if the organ be swelled, owing to the presence in its substance of filarice, the result of this doubling up will be to rupture its delicate integument, and so set free the contained filarial. Once free they can scarcely fail to be carried by capillarity among the blades of the lancets, and so introduced with them into the cellular tissues of the new victim in which they propose to grow to maturity.
Passing through an opening between the trabeculte which form the framework of the head, and to which it is
PLATE IV.
PLATE IV. —To Illustrate the Anatomy op the Digestive System op THE Imago.
A. Outline figure of a female Culex, to show the relations of the intestinal canal and its appendages, x 15 diams.
B. The rectal glands lying in situ en the large intestine, with their tracheal supply. X 75 diams.
C. Isolated cells of rectal glands, x 300 diams.
D. Chitinous pharyngeal bulb, x 75 diams.
E. Blind extremity of a Malphigian tube, x about 200 diams. F. Semi-diagrammatic representation of the parts about the mouth, x about 76diams.
G. The salivary glands and their ducts lying in situ on the pharynx and oesophagus, x 75 diams.
H. Salivary duct of one side and its branches, the middle one of which, though mainly denuded, has attached to it two clusters of salivary cells. X 430 diams, K. Normal salivary cells somewhat more highly magnified. In the above figures : a, labrum ; h, mandible ; c, labium ; d, buccal bulb ; e, oesophagus; /, salivary duct; g, nerve; k, " hypopharynx " ; m, Malphigian tubes ; n, chitinous pharyngeal bulb ; o, spiral valve of large intestine ; p, pneumatic vesicle (dotted outline of) ; q, trachea supplying the cellular tissue of the labium ; r, rectum ; s, salivary glands ; t, rectal glands ; v, stomach.
The chitinous framework of the head is indicated by coarse oblique shading ; the outline of labrum in dotted, and that of the labium in broken dottedlines.
96 GNATS OR MOSQUITOES — CHAPTER VI connected by a delicate membrane, reflected on to them, and thence to the floor of chitinous pharynx, the hypopharynx, as already described, ends as a blind sac lying in the root of the labium, just in front of the buccal cavity. The anatomy of the latter is by no means simple, and so far as I know, has hitherto remained unnoticed. As I have endeavoured to indicate in fig. F, plate IV., the framework of the mouth springs from the strong tergal plate running along the middle line of the head, and from which springs the base of the labrum. From this, on either side, runs down a strong lateral piece, which shortly divides into two rods, enclosing an oval space, the anterior of which appears to be really the basal piece of the mandible. Reuniting below, these lateral pieces are continued downwards to join with the strong but narrow sternal plate that lies in the ventral middle line. This sternal plate, however, is only single and median for a short distance ; after which it divides into lateral rods, united by a delicate membrane, which run back between the lower borders of the ejes. Through this transparent membrane, if we can successfully clear away the opaque mass of structures lying above them, may be distinctly traced the single median common duct of the salivary glands, running forward to open into the fundus of the hypopharynx. The pharynx, or intracephalic portion of the digestive tube, is entirely chitinous and terminates in a cup-shaped expansion, which forms the back of the roughly spherical frame of the buccal cavity. This, as already described, is in no way rigidly connected with either the framework of the head, or the end of the hypopharynx, continuity of the tubes being maintained only by a very delicate membrane, and into the space between the two structures a mass of mesoblastic tissue is pushed forwards, especially below, and expands between the cupped end of the pharynx and the delicate connecting membrane into a thick mass composed mainly of pecuHar spindle-shaped cells. It is along with this mesoblastic intrusion that the salivary duct and two large nerves are conducted to the interior of the buccal bulb, and the presence and large size
THE ANATOMY OF THE ADULT MOSQUITO 97 of the latter make it probable that the function of this thickening which fills up the greater part of the chitinous sphere, is connected with the sense of taste. The anterior part of the lining membrane, especially below, is closely armed with minute spines, the back of the cavity being lined with the soft tissues of what is evidently a nervous end organ, and is probably that of taste.
At the point where it springs from the cup-like expansion, the pharyngeal tube is somewhat constricted, but after this Pig. 23.— Transverse Section of the Head of An. Culicifacies, sp. n. X 120 diams. , showing the pharyngeal bul,b, with its dilator muscles. Immediatelyabove is seen the mass of the brain, or supra-CBSophageal ganglia ; /, salivary duct ; g, ventral nerve cords ; n, pharyngeal bulb ; q, trachese. it runs for a short distance as a tube of uniform thickness, through the mass of the oesophageal nerve collar, and then expands into a fusiform dilatation of considerable size. As usually seen (in fig. D) it is longitudinally furrowed so that its transverse section would have the outline of a wallflower, but when filled by imbibition of glycerine these infoldings disappear.
This bulb unquestionably serves as the pump, by means of which the insect draws up the fluids of the tissues into which its proboscis is plunged ; as it is provided with a 7