THE ANATOMY OF THE PUPA 75 chiefly downward with reference to the outer world. A sudden loud noise or a very gentle tap upon the vessel containing the pupse causes those at the surface to dart downwards, but as shght sounds produce no effect upon them, Dr. Hurst concludes that the tremor of the surface of the water and not the sound itself is recognised by them. The setse on the first segment of the abdomen are probably the organs by which this movement is felt.
76 CHAPTER VI.
The Anatomy of the Adult Mosquito.
In its anatomy, the Mosquito conforms to the usual insect type, the body being divided into the three sharply separated regions of the head, thorax and abdomen; the first accommodating mainly the organs of sense, the second those of locomotion, and the third the digestive and reproductive organs.
The entire body is covered with a chitinous integument, which is thick and rigid on the dorsum, and over the entire head, and thin and elastic where it connects together the various rigid portions.
This chitinous " exoskeleton," besides covering the exterior of the body, sends inwards plates and trabeculae which form a beautifully intricate framework supporting the bases of the legs and other appendages ; and in addition to this, the entire intracephalic portion of the intestinal canal, and the ducts of the salivary glands are lined with the same material ; while in the form of the tracheae, tubular involutions of this outer covering ramify minutely throughout every tissue, much in the same way as the blood vessels do in vertebrates.
The whole of the outer armour is covered with appendages of various forms, the most characteristic of which take the form of scales.
These scales consist of flat plates springing from a narrow pedicle, and differ a good deal in outline, not only in different species, but on different regions of the same insect ; though in any given species and situation their form is very constant, and affords valuable characters in classification. Each scale consists of an anterior and posterior membrane
THE ANATOMY OF THE ADULT MOSQUITO 77 separated by an almost imperceptible interval, and is minutely ribbed and striated in a manner that requires the best lenses to properly resolve. Perhaps the commonest outline is that of a racquet, but as w^ill be seen from the Fig. 16. — Scales and other chitinous appendages op Mosquitoes. a, Scale of frontal tuft of Atwplieles Rossii, x 375 diams. ; b, detail of membrane of the same scale, x about 1,000 diams. ; c, to show form and arrangement of the scales on one of the veins of the wing of Ciclex fatigans ( Wied.), X 75 diams. ; d, to show form and arrangement of scales on margin of wing of C. fatigans (Wied.), x 75 diams. ; e, scale of nuchal corona of C. hortensis, x 185 diams., after Ficalbi ; /, scale of thoracic tomentum of C. pipiens, x 185 diams., after Ficalbi ; g, scale of alar margin of C. pipiens, X 208 diams., after Ficalbi ; h, scale of thoracic tomentum of C. spathipalpis, X 185 diams., after Ficalbi ; i, scale from the leg of C. pipiens, x 208 diams., after Ficalbi ; j, k, I, various forms of wing scale, after Arribdlzaga; amplification not stated ; m, last tarsal joint of C. pipiens, 9 showing 1, empodium, 2, tarsal claws, and 3, Pulvilli, after Ficalbi ; n, fore, mid, and hind tarsal claws of C. pulchriventer sp. n. c? ; o, single claw of fore leg of An. Rossii, $ .
above figure, they vary in shape almost as greatly as the leaves of plants, and some are so long and narrow as to be almost indistinguishable from the hairs, w^hich are found everywhere, either interspersed among the scales, or, as in the abdomen of many Anopheletes, constituting the entire
78 GNATS OR MOSQUITOES — CHAPTER VI tomentum. The third class of chitinous appendages are the tarsal claws and epipodium, the latter of which is essentially a compound hair, while the former vary in form almost as much as do the scales. In the females these claws are usually of simple form, and of no great size ; but in the male, those of the fore and middle legs and more rarely those also of the hind pair, are usually proportionally large and armed with one or more accessory teeth, and the two claws of the same leg may differ in size and form. Those of the fore legs are generally the largest, and the difference in size and form in the sexes is suggestive of the function of those of the male being to grasp the female. In many species, at any rate of Anopheles, there is only a single compound claw on the fore leg, the missing claw being apparently represented only by a small projection at one side of its base. Varying as they thus do, there can be no doubt but that the form of the claws would, if generally noted, be capable of furnishing valuable specific indications ; but though valuable notes on this point have been made by Ficalbi, and also by Arribalzaga, information on this point is wanting in the great majority of species, and the circumstance that it is impossible to properly make them out without mutilating a specimen by mounting the legs, for the compound microscope tends to render these characters not so useful as others for the practical purposes of classification. The legs are always thickly clothed with scales, and in the one or two species of the genus Sabethes of Desvoidy, and of Mr. Theobald's new genus Conclnjliates, the sides of the tibiae or tarsi of certain legs are provided with lateral fringes of long scales so as to form a sort of paddle-shaped expansion ; while in Psorophora and Mucidus the entire legs are shaggy. The head is rounded, but wider than long, and bears the usual appendages, all of these being represented, although those forming the mouth parts, being modified to form the style-like proboscis, differ markedly from the ordinary insect foot-jaws, and present perhaps an even wider divergence from the simpler forms than the suctorial mouths of most other Diptera.
THE ANATOMY OF THE ADULT MOSQUITO 79 In this region of the body no sign of segmentation can be made out, nor is there any visible separation between the dorsal and ventral chitinous shields, such as is found in the other regions of the body.
The greater part of the sides and front of the head are occupied by the facetted eyes. These are alw^ays large and well developed and in certain species may even touch each other in front aiod nearly so below. Their anterior border is usually somewhat hollowed back to lodge the bases of the antennae, so that they tend to a reniform outline. There are no ocelli, or simple eyes, such as are found in the majority of Diptera, and though some authorities assert that they are present in a rudimentary form, I have not been able to satisfy myself as to the existence of any trace of them in the adult insect, and for practical purposes, at any rate, they may be considered as absent.
Immediately in front of the eyes will be seen the antennae. These are of the moniliform type, and although of fundamentally similar construction, differ greatly in appearance in the two sexes, owing to the organ in the male being so richly provided with long, silky hairs as to form a pair of singularly beautiful plumes ; while in the female these hairs are less numerous and down-like, so that the joints of the antennae itself are plainly visible, and are the portion of the organ that catches the eye. In both sexes the antennae is typically formed of fourteen joints, the basal one of which is much the largest and of globular form, the constriction at its base being almost fused with the cephalic shield so as to be capable of little motion. The greater part of the front of this large basal joint is occupied by a soft but tightly stretched membrane, and into the centre of this is articulated the base of the second joint, which, like those that follow it, though but little shorter, is not onefifth of the diameter of the basal joint. The chitinous wall of the latter is grooved on its interior for a richly-developed system of nerve threads, which are symmetrically arranged in its lining membrane like the wires of a birdcage. The structure of this joint has been described in great detail by Dr. Christopher Johnstone of Baltimore {U.S. Quart. Journ.
80 GNATS OR MOSQUITOES — CHAPTER VI Micros. Science, iii., pp. 97-102), and he contends that the entire antennae serves as an auditory organ, the atmospheric vibrations being received by the long hairs of the antennae, and so transmitted to the drum-hke membrane which, as already described, closes the front of the basal joint, and thence through its contained fluid to the nerves lining its cavity. According to this theory, the anterior membrane is an actual memhrana Ujmpani, and the fluid w^ithin corresponds in function to the endolymph, contained in the cavities of the internal ear of the higher animals. It has been further pointed out that the hairs of the verticils of the male Mosquito respond to the musical note given by the vibrations of the wings of the female insect. Professor Lubbock (" The Senses of Animals," p. 115) seems to regard with some favour Johnstone's idea on this point, but antennae having this form of basal joint are not very common in insects, and if it be really a tympanum it is certainly a very exceptional arrangement. The articulation between the first and second antennal joints is capable of very free motion, so that at this point the entire organ can be moved to considerable angle in any direction, while the extent of mobility between the latter and the succeeding ten joints is much more limited. Except in the very aberrant Deinukerides cancer, in which the second is as long as several of the succeeding ones, from the second to the twelfth pieces inclusive, the joints closely resemble each other, forming a moniHform series of short cylindrical pieces of a length but little exceeding their thickness. From the base of each springs a verticil of hairs, numerous (about forty) and long in the <^ , and shorter and fewer in the ? . It is more than probable that these hairs in the <? are really chordotonal, auditory organs, as there is no doubt that they respond to the note of the female wings, and it is the function of the male in these insects to seek out the female, but the acceptance of this does not involve that of Johnstone's theory of a tympanic function for the basal joint. The last two joints greatly exceed the others in length, forming in the male, together, much more than a third of the entire length of the organ. The basal verticil of the penultimate
THE ANATOMY OF THE ADULT MOSQUITO 81 is well developed, but that of the last is ill developed, and both are closely covered with short, downy hairs, and exhibit also certain pits and specialised hairs, which probably are sense organs, olfactory, tactile, &c. For further details on the sense organs of insects, the reader is referred to Lubbock's work, already quoted. Projecting from the middle of the head {vide fig. 13), below the antennae, is the characteristic proboscis which, although it at first sight appears to be merely a cylindrical, trunk-like projection, is really a very complex organ, being composed of no less than nine separate pieces. The organ springs from a sort of groove on the lower aspect of the head, through the intervention of a flexible membrane, which admits of a certain amount of protrusion and retraction as well as of flexion and extension in the vertical direction; the bases of both the upper and lower lips being furnished with sets of longitudinal and vertical transverse muscular fibres to effect these motions.
In all the Diptera the mouth parts of the adult insect are modified, almost beyond recognition, from the obviously trenchant jaws which have been already described in the larva, and which persist, with but little alteration, throughout life in some orders, such as the beetles. During the three or four days of pupal life a marvellous series of changes takes place, whereby the masticatory mouth of the larva is transformed into the suctorial apparatus of the adult.
To understand the dipterous mouth it must be remembered that, in most of the order, all that is visible to the eye is the lower lip, the other parts being hidden, wrapped up within it. Everyone who has handled a microscope is familiar with the stock popular object of the " blow fly's tongue." This "tongue" is the labium, and with the exception of the maxillary palps, little else of the mouth parts is in evidence, the other foot-jaws being but little developed, as the fly licks rather than sucks its food, and the " tongue " is provided with an armature of chitinous rasps, not unlike those that stud the lingual rib, and of gasteropod molluscs. If now we examine a horse fly, we 6
82 GNATS OR MOSQUITOES — CHAPTER VI find that the upper lip is developed into a strong spine ; and that, hidden between it and the lower lip are four short, stout blades, each of which has exactly the form of a Roman sword. These blades are the mandibles and maxilloe, and are capable of inflicting a sharp bite or rather puncture, but the lower lip remains practically identical with that of the blow fly.
Fig 17.— Mouth parts of an Indian Sand ply {Phlebotomus), the Labrum HAVING BEEN REMOVED AND PLACED SEPARATELY TO THE LEFT. a, labrum; b, mandible; c, labium; 7c, " hypopharynx " ; tv, maxilla; X, maxillary palp, that of the left side being shown bent ; as habitually held during life when at rest.
If we now examine the further modified mouth of a sand fly, we find that the lower lip, though obviously approaching in form to that of the Mosquito, is still recognisably the same organ as the " tongue " of flies. The four miniature, sword-like blades of the mandibles and maxillae are present, and almost identical in shape with those of the horse fly. There is, however, now an additional median organ lying in the midst of this bundle of weapons, in the shape of a conical tube, sharp at the point and very broad at the base, prolonged from the mouth cavity. This organ is the hypopharynx, and with this addition, we have now