THE ANATOMY OF THE ADULT MOSQUITO 83 represented all the parts of the highly evolved, suctorial mouth of the gnats ; the only difference being that on the latter the parts are attenuated and elongated into the far more efficient, though more delicate, suctorial organ represented belovi^.
In the Mosquito the labium is a long, narrow, grooved spine ending in a pen-like point, and the lower lip, though bearing some likeness to that of the sand fly, has become a Fig 18.— Diagram to Illustrate the Mouth Parts op a Female Mosquito.
A, median mouth parts, and gnathites of the left side (their relative position at the base is indicated in a purely arbitrary way) ; B, end of proboscis seen from below ; C, end of proboscis seen from above ; D, the same with the hypopharynx drawn away to the left— with the exception of fig. A, after Ficalbi. In the above, a, labrum; b, mandible; c, labium; k, hypopharynx; w, maxilla; .t, maxillary palp ; y, basal joint of maxillae. mere sheath for the other organs and no longer bears any resemblance whatever to a fly's tongue ; the mandibles and maxillae have become simple slender lancets, formed on the plan of the mediaeval rapier, with a comparatively thick rod-like back, and a broader edged and pointed lamina ; a mere golden-coloured wire of chitine supporting a narrow blade of the same material, so thin as to be absolutely colourless. The hypopharynx has undergone a similar change and has taken the form of a delicate plate, pierced throughout its entire length by a minute canal ; the whole of the mouth parts, except the lower lip, being so delicately
84 GNATS OR MOSQUITOES — CHAPTER YI fashioned as to practically defy draughtsmanship, so that they can only be represented in diagrammatic form.
The upper Hp, or labrum, springs from a bulbar enlargement of the front of the face, placed between the bases of the antennae. This bulb is flexible and elastic, and the Fig. 19. — Semi-diagrammatic eepeesentation op Serial Preparations of THE Anterior part op the Head and Proboscis, x 150 diams. A, median sagittal section of root of proboscis, showing the entry of the salivary duct into the floor of the base of the hypopharynx, with the valvular fold and its muscles, by which the duct is opened and closed. B, transverse section opposite the point of entry of the salivary duct, and the roots of the palpi. C, transverse section of the proboscis, near its base. D, transverse section of the middle of the proboscis. In the above figures : — a, labrum ; b, mandible ; c, labium ; d, buccal bulb ; /, salivary duct ; k, hypopharynx; mx, maxilla; m.at., muscle opening the salivary valve ; m.rt., muscle closing salivary valve ; m.pr. Ibr., protrusor muscle of labrum ; to. rt. Ibr., retractor muscle of labrum; t.o., taste organ (?) ; q, tracheae; x, root of palp.
whole of its median plane is occupied by a fan-shaped muscle, taking origin from its upper surface ; the fibres converging to be inserted into a thickening of the chitine at the base of the actual labium ; while, on either side of this retractor, a number of antero-posterior transverse fibres fill up most of the remaining space. By the action of the
THE ANATOMY OF THE ADULT MOSQUITO 85 retractor muscle, the base of the labium can be invaginated within the bulb, so that its point is withdrawn behind those of the other mouth parts, while by that of the transverse fibres, it can be forced out and extended once more to their level, or be3^ond thein.
Firmness is imparted to this arrangement by a pair of strong spines, which project from the lateral trabeculse of the front of the head into the bulb. Beyond the bulb, the labrum becomes a strong, rigid style of a form not unlike that of a surgeon's " director," but with a more complete groove ; the transverse section forming at least three parts of a circle ; and this incomplete canal is converted into a tube by the hypopharynx, which, throughout its length, lies in contact with its lower surface. It is through this channel that blood or other food reaches the mouth cavity. The hypopharnyx springs from the anterior part of the floor of the buccal bulb, and is a tubular organ with, at first, a very broad but shallow lumen. This, however, rapidly contracts to a minute tube, of about the same diameter as the salivary duct, of which it is virtually a continuation, and on either side of the tube are developed the membranous wings, which serve to convert the groove of the labrum into a tube. Into the floor of the broad cul-de-sac of the commencement of this tube opens the salivary duct, the mouth of the latter being guarded by a remarkable arrangement, which serves as a valve. The floor of the buccal valve at this point is strengthened by a strong chitinous plate ; and this, though of course continuous with the thinner membrane near it, forms a veritable articulation with a separate plate in the floor of the base of the hypopharynx, in which is placed the termination of the salivary duct. As the rest of the lining of the cul-de-sac is delicate and flexible, this plate can be moved forward and back, and is provided with two special muscles for the purpose. In the latter position the mouth of the duct is effectually closed, while it is freely open when drawn forward by the anterior muscle, which springs from the chitine of the ventral wall of the labium. Grouped round the lower surface of the hypopharynx and the sides of the labrum are
86 GNATS OR MOSQUITOES — CHAPTER VI the four lancets, which are simple, rapier-like blades, simpl}^ pointed, and unprovided with teeth or serrations of any kind, though a false appearance of the sort is lent to the points of the inaxillse by the plate of their blades being fibrillated. The niaxillse are usually stated to be the smaller and more delicate, but, as a matter of fact, they are considerably the stouter, and except in this the mandibles and maxillae closel}^ resemble each other. Neither pair of lancets appears to be provided with any special musculature, and they are probably forced into the skin simply by the thrust of the head ; the labrum being drawn back while they make their puncture, and then forced into the wound they have made. Lastly, the labium is a comparatively stout, fleshy organ, soft, flexible, and clothed with scales on its outer surface, which is wrapped round the other parts so as to form a sheath, open only above. Between its scaly ventral surface and its delicate dorsal lining is interposed a considerable amount of loose connective tissue, in which lie a pair of tracheae, and immediately beneath them two large nerves. The labium ends in a pair of valveshaped, articulated lobes, capable of being moved by certain special muscles. The function of these lobular organs, which are said to represent the labial palps of certain other arthropods, is probably to grasp the styles as they pierce the skin, while the stem of the labium is being looped down out of the way.
The above account differs, it may be noticed, from previous descriptions, inasmuch that it has hitherto been assumed that both food and saliva passed through the proboscis by the same channel ; but there can be no doubt of its correctness, which I have verified by several series of sections, in both Culex and Anopheles ; and I gather from Professor Grassi that he holds the same view of the functions of the various structures, who I trust will understand that his priority in the matter is herein fully recognised ; though he must not be held responsible for the existence of the salivary valve, and the other structural details of the above description.
A little reflection will, moreover, show that an arrange-
THE ANATOMY OF THE ADULT MOSQUITO 8/ ment of the sort is a physiological necessity, as it would be quite impossible for the saliva to be forced down a tube while the food is being drawn up through it. When the Mosquito brings the proboscis into action to pierce the cuticle of the plant or animal from which it seeks its food, all these parts, except the sheathing labium, are forced into it, while the labium is bent down into a sort of loop which progressively narrows as the piercing organs penetrate more and more deepl}'. On either side of the base of the proboscis may be seen a pair of jointed appendages, the maxillary palpi. These organs have been described by several authorities as labial palps, but a closer examination shows that they are really connected with the base of the maxillary lancets through the intermediation of a piece which, in the usual position of the parts, is hidden within the base of the labium. In most cases the palpi consist of five visible joints, but they present great sexual and generic differences within the family, and require especial notice in systematic work, as the classification of these insects is largely based on their characteristics. In some species they greatly exceed the proboscis in length, while in the genus ^des they are quite rudimentary in both sexes and appear to consist of only a single short joint.
The head is connected with the thorax by a soft flexible neck corresponding in function to the intersegmental membranes of the abdomen, though by some authorities it is considered as a part of the prothorax, which allows of a considerable freedom of movement between these two divisions of the body.
The thorax is the most bulky portion of the body, as, though but little broader than the head, it is more than twice as deep. It is composed of three segments, but the great preponderance of the middle of the three, or mesothorax, and the fact that the three are fused together into a single rigid mass, makes it not altogether easy to make out the lines of division between the component parts. Viewed from above, almost all that is visible is the tergum of the mesothorax, but in some mosquitoes, as in Psorophora,
88 GNATS OR MOSQUITOES — CHAPTER VI a portion of the prothorax projects in the form of a pair of lateral protuberances, the shoulder callosities ; and on either side of the scutellum, which appertains to the mesothorax, can be seen, even in the extended insect, a portion of the better-developed metathorax, while in flexion the whole width of its tergum can be easily seen. Laterally, however, the three component segments are easily distinguishable by the fact that each gives origin to a pair of legs, the coxse of which guide the eye to the corresponding Fig. 20. — p, prothorax ; w, mesonotum ; h, metanotum ; k, scutellum pp, propleura ; mp, lobes of metapleura ; 7ntp, metapleura ; s, s, s, thoracic stigmata ; -lo, root of wing (after Arribalzaga). portion of the thorax. Just in front of the root of the coxa of the front leg can be seen, in the great majority of species, an oblong or oval scale, which covers the stigma of the prothorax.
The stigma of the mesothorax is also well developed, and can easily be seen just in front of the origin of the wings; but those of the metathorax, on the other hand, are small or not recognisable. From the propleura springs the coxa of the front leg, and between it and that of the middle leg is a comparatively large area of mesopleura. This latter is more or less divisible into three areas, one of which carries the stigma, the second the middle leg, and the third and hindmost the wings. From the metapleura springs the coxa of the hind legs, and above it may be seen
THE ANATOMY OP THE ADULT MOSQUITO 89 the halteres or poisers, a pair of small, club-like organs, consistiDg of a delicate stem terminating in a pear-shaped head, which represents the rudiment of the hinder wings of other insects.
There appears to he good reason for regarding these modified wings as sense organs, for although mere rudiments, as far as their original functions are concerned, they are supplied by one of the largest nerves in the body, and have been regarded by Keller, Hicks, and Bolles Lee as auditory organs.
Pig. 21. — Semidiageammatic Dbawing op a Transverse Section of the Thorax of C. pipiens (L).
a, alar muscles ; b, portion of chitinous framework supporting the wing ; c, coxal muscles ; d, dorsal vessel ; e, CBSophagus ; /, flexor of abdomen ; 7i, nerve cords ; p, pneumatic vesicles.
According to the last-named authority (" Les Balanciers des Dipteres," Becueil Zool. Suisse, ISSo), the organ consists of a varying number of rows of minute vesicles placed at the base of the organ, each vesicle being perforated and containing a minute hair. Sense organs have been discovered in a variety of very unexpected situations in insects, and it is quite possible that, while the verticillary hairs of
90 GNATS OR MOSQUITOES — CHAPTER VI the antennae of male gnats may serve as chordotonal organs, whereby they are enabled to localise the whereabouts of the female, ordinary hearing may be subserved by these peculiar structures at the base of the halteres.
Almost the whole of the interior of the thorax is occupied with the powerful muscles that actuate the wings, legs, and halteres, but the detailed consideration of their arrangement hardly lies within the scope of a work like the present. The general structure of the Dipterous leg and wing have already been sufficiently alluded to in the remarks on the terminology of the Order, but as regards their special characteristics in the CuUcidce, it may be noted that the former are always proportionally long and slender, and that the hips, or upper sections of the coxae, take the form of obconical processes, immovably cormected with the corresponding pleurae ; while the trochanters are much smaller^ and take the form of short, globular, or oviform pieces. Of the remaining joints, the femur, tibia, and first tarsal joint are long and linear, and, as a rule, differ but little in length, though their relative proportions furnish valuable specific indications. Not unfrequently the first tarsal is the longest of the three. The remaining four tarsal joints taken together do not, as a rule, equal either of the above linear joints, and generally progressively diminish in length, the last being often scarcely wider than long, and carrying a welldeveloped epipodium and pulvillus, and a pair of claws, the characters of which have already been sufficiently noticed. The third division of the body or abdomen is the longest of the three, but is much more slender than the thorax. It is more or less cylindrical, but depressed, being broader than deep, and is composed of nine segments, none of which are provided with locomotor appendages, although the last, bears a pair of jointed appendages which serve as external organs of generation. Each segment is composed of a dorsal and a ventral, rather rigid, chitinous plate, united at the sides by a softer membrane, in which are placed the stigmata or external respiratory apertures. The anterior segments closely resemble each other, except the first, which is a good deal shorter than the rest, and has its