THE ANATOMY OF THE PUPA 67 trumpets are protruded, the abdominal tracheae appear to collapse, and the animal floats with the anterior end upwards, the new syphons coming to the surface. The soft parts of the old respiratory syphons are withdrawn from the cuticle and invaginated into the eighth abdominal segments while the lining of the tracheal trunks breaks up into pieces, which in the abdomen correspond to the segments, and is cast off, with the other larval exuviae, by means of the mechanism that has been already described. The pupa which thus escapes, differs greatly from the larva. It is in the larger species a little under 1 cm, in length when fully extended, and consists of a bulky, laterally compressed mass, made up of the head and thorax with their appendages, and of a slender flexible abdomen, which when at rest is carried curled under the thorax. In a specimen 9 mm. long the thorax was 2'5 mm. and the abdomen 6"5 mm., but the thorax appears much longer on account of the wings, which extend downwards and backwards from its sides. The head adds nothing to the length, as it is carried tucked down under the thorax. It is broad from side to side, short from back to front, while ventrally it is drawn out into a long process, which extends backwards under the thorax as far as the anterior part of the abdomen, where it curves upwards. This process is made up of the mouth parts, and includes all the parts represented alike in the adult and larva.
On throwing off their larval chitinous covering, the parts retain their larval masticatory type, but during the four days of pupal existence the various parts mould themselves and develop into the basis of the adult condition, so that by the time the chitin of the adult is ready for induration, they have altered their form to that of the adult mouth. From the sides of the epicranial region, the antennae run outwards to the sides of the thorax, one beneath the anterior margin of each wing. The head and all its appendages are immovable during the pupal stage. The thorax is rounded, but somewhat compressed from side to side. From the sides of its summit arise the respiratory syphons, a pair of conspicuous organs whose position
68 GNATS OR MOSQUITOES — CHAPTEE V and form has led to their being termed horns or trumpets. The wings are nearly flat, oblong plates, arising behind the bases of the syphons, and extending downwards and backwards.
Immediately behind them are a pair of triangular plates, enclosing the halteres of the future gnat. The legs are mostly hidden by the wings, but the femur, tibia and first tarsal joint of the first leg, and the tibia and first tarsal of the second are visible. The respiratory syphons are nearly cylindrical, narrowed at their bases, and curved forwards to be attached by flexible membranes to shght prominences on the sides of the prothorax. Above, they are obliquely truncate and open, and the margin is slightly notched on the inner side. The outer surface is marked so as to resemble imbricated scales, each with a minute spine at its apex. The cavity of the syphon communicates directly with the tracheal trunk at its base. Palmen (" Zur Morphologic des Tracheen systems," Helsingfors, 1877) has denied the communication of the syphons with the tracheae,, and imputed to them the function of "tracheal gills" ; but apart from the fact that their dense chitinous structure renders them entirely unsuitable for the performance of any such function, the reality of their communication with the tracheEB can easily be proved by watching the imbibition of suitable fluids through the syphons into them. All these appendages originate as protrusions of the epidermic layer, enclosing mesoblastic tissue. Those of the dorsum are all at first flat, wing-like plates, but those of the mesothorax alone retain this form as the wings of the adult, while the halteres become club-shaped, and the anterior appendages become rolled up to form the syphons of the pupa, only to disappear on attaining the adult form. The legs, on the other hand, appear from the first as cylindrical processes. They are at first unjointed, but by the end of the pupal period have segmented themselves into the various joints of the adult.
The abdomen is flattened dorso-ventrally, and when at rest is curved under the thorax. It is jointed and flexible, and forms with the pair of large fins, borne by the eighth
THE ANATOMY OF THE PUPA by segment, the only locomotor organ of the pupca, the wings and legs lying immovable, and even adhering to each other, though they are easily separated in specimens preserved in alcohol.
Nine segments are easily recognised in the abdomen, and the last one, though it is probably composed* of no less than three condensed and highly modified segments, is the smallest. Each segment has a chitinous tergum and sternum, and setae are sparingly distributed over them, those present being mostly on the hinder part of the terga. Of these a pair placed on the hinder part of the first segment alone require mention. Each consists of a triangular basal plate, articulated to the tergum by a soft membrane, and distally divided into a number of bars, which by repeated sub-division give rise to about a hundred setae, all lying in one plane parallel with that of the median, of the body. When at rest the pupa floats with the tips of these setae and those of the respiratory syphons at the surface of the water, and the setae probably assist in maintaming equilibrium, as well as serving as sensory organs for the perception of disturbances of the water.
The eighth segment bears the fins, a large pair of thin oval plates about 1'2 mm. in length, each of which is strengthened by a midrib, which projects as a spine beyond its hinder border. Beneath and behind them is the ninth segment, a small though probably composite segment, which contains the anus, and is provided on either side and in front of it with a pair of blunt processes, larger in the male than in the female.
The digestive canal differs but little in any stage of the insect, the main change being the casting off of the thick large-celled lining of the stomach and the substitution of the more delicate mucosa of the adult. The cast-off larval mucous membrane appears to be disposed of by digestion. During this period also is developed the peculiar chitinous dilatation of the anterior part of the thorax, already described in the adult. Mr. Hurst describes it as triangular in section with incurved sides, to the concavities of whose sides are attached muscular fibres originating from the sides of the
70 GNATS OR MOSQUITOES — CHAPTER V ^ head, the mechanism being specially well developed in the female. The peculiar air-containing sac already described is also developed at this time. It is obvious euough that the former mechanism is capable of being actively dilated by muscular action, and may therefore assist in suction, but it is difficult to understand how any one can have fallen into the error of ascribing such a function to the latter organ. The circulatory system consists of a long dorsal vessel, which is broad and actively contractile in the abdomen and contracts into an " aorta " in the head and thorax. From its sides membranes, the alee cordis, which serve to suspend it, run out between the extensor muscles and the stomach, to attach themselves to the tracheal trunks. Each ala consists of a dorsal and ventral lamina, and the space between them has been called the pericardium. It contains the pericardial cells and communicates freely with the body cavity by the spaces between the alse. There is no distinct constriction of the heart into chambers, and the paired ostia or slits, which put it in communication with the "pericardium," open backwards in the first segment, and forwards and inwards in segments three to seven.
In the space between the ahe cordis are also the pericardial cells, which are brown in colour and arranged in ovoid masses, of which there are four pairs in each abdominal segment, two of which are in its anterior and two in its posterior portion. The protoplasm of these cells is extraordinarily spongy and contains numerous granules which stain deeply with borax carmine. The nuclei vary in number from 3 or 4 to 10 in each mass, but the boundaries between the cells cannot be made out. The glandular character of these cells has been shown by Kowalevsky (" Biolog. Centralblatt," ix., 1889), their function being probably somewhat analogous to that of the lymphatic and other ductless glands of the higher animals. I reproduce more fully tbe histological characteristics of these masses, as, alike from their position close to the walls of the stomach, from which they are separated only by the ventral layer of the ala cordis, and from their general appearance they might easily be confused with the parasitic " coccidia " of malaria recently
THE ANATOMY OF THE PUPA 71 described by Major Koss, I. M.S., by an observer not personally conversant with the appearances of the two structures ; and I shall not be at all surprised to find descriptions of these bodies appearing in the form of notices of the occurrence of the parasites in the pupal stage. The possibility of the communication of the disease among Mosquitoes through infected ova has already been mooted, and investigators working upon this line should be on guard against this fallacy.
The so-called aorta runs from the anterior end of the dorsal vessel forwards, above the stomach and oesophagus to the head, where it terminates in an open end. In transverse sections of the thorax the aorta appears as a laterally compressed tube, and does not appear to give off any branches.
The respiratory system, during pupal life, undergoes the changes which prepare the rudimentary stigmatic trunks of the thorax and abdomen to take on functional characters in the adult. With the exception of the first abdominal pair, however, none of the stigmata are open except the prothoracic openings which form the respiratory syphons. These first abdominal stigmata open into the air space which exists under the pupal skin beneath the thorax, and in which the legs are undergoing development. This cavity must exercise a hydrostatic function, and the patency of these stigmata must be in this case necessary for the conveyance of air to the cavity. From the base of each syphon tracheae run to various parts of the body and head. Among these may be mentioned specially one transverse trunk running across the thorax between the alimentary canal and the nerve chain, which puts the two syphons in direct communication with each other ; and a pair of longitudinal trunks running back to the hinder end of the body, and giving off branches to the various organs, and also a branch to each of the future stigmata.
The cuticular lining or intima of the chief trunks and their branches is well developed even at the beginning of pupal life, and has the usual spiral thickening. The trunks connecting the stigmata with the main trunks are the only
72 GNATS OE MOSQUITOES — CHAPTER V ones that undergo any marked change. These widen round their separated and collapsed intima, and a new and strongly thickened intima is developed. In the main trunks no new intima is formed, and when the imago escapes no portion of the intima is shed, saving the portions connecting the syphons and the first abdominal stigmata with the main trunks. These fragments are, in the case of the syphons, well developed, and have a fully-developed spiral thickening. The portions connected with the first abdominal stigmata, though better developed than the other abdominal branches, have the spiral thickening only slightly developed. The terminal portion is beset with very numerous small spines. The nervous system is particularly interesting. Within the short space of four days, certain ganglia increase enormously in size by the addition of cells, apparently derived directly from the epidermis ; and other ganglia shift their positions bodily and sometimes fuse with others. In the larva each of the first eight abdominal segments has a pair of ganglia ; and yet a pupa, only half escaped from the larval cuticle, has four in the thorax and none in the first segment of the abdomen. During pupal life these four ganglia fuse into one compact mass. During the first two days of pupal life the eighth ganglia migrate into and fuse with those of the sixth segment. In the female the change goes further. A pupa almost ready to burst and give exit to the imago has still the arrangement already described ; but an imago killed immediately after its escape is found to have no ganglia in the seventh or eighth segment, but in the sixth segment are two masses ; the first the pair properly belonging to the segment, lying at its anterior end ; the other a double mass, formed of the seventh and eighth ganglia, lying in the hinder end of the segment. In the male imago the arrangement is the same as in the advanced pupa. In the head the supra-oesophageal ganglion increases enormously in size. The epidermal ("hypodermal") cells, especially those near the borders of the eyes, proliferate freely, and the cells budded off from their inner surfaces migrate inwards and form the new cells of the ganglia. By this process the ganglia, which at the
THE ANATOMY OP THE PUPA 73 commencement of pupal life were comparatively inconspicuous, grow till they almost fill the head, and there are places in the advanced pupa where ganglia and epidermis appear continuous.
Dr. Hurst's paper also contains some very interesting details as to the development of the sense organs, especially of the eyes and antennae ; but for these the reader is referred to the original paper, as their interest is mainly that of the development of these organs in insects in general than that of the Culicidcc in particular.
Beproductive System. — The male generative organs of the adult consist of testes, vasa deferentia, "prostatic glands," copulatory organ, with a common pouch at its base and two pairs of gonapophyses. Of these last, the outer ones are a pair of large forceps for holding the female. Both pairs originate in the larva and are probably the appendages of two segments now fused and indistinguishable. The testes are a pair of cylindrical bodies, already present in the larva, at the sides of the intestine in the sixth segment. They are chambered and the spermatic elements in the hinder chambers are more advanced than those in front. The length of each segment is that of the segment in which they lie. The vas deferens of each side is a direct continuation of the wall of the testis, and is a very narrow tube running directly backwards, quite distinct from its fellow of the opposite side, but the two are closely bound together in their hinder parts and they open behind into the common pouch. The prostatic glands are a pair of elongated glandular tubes, apparently simple, but seen in sections to be double, though the cavities connnunicate behind before opening into the common pouch. This latter is a dilatation of the ejaculatory duct at the base of the copulatory organ, which last is perhaps derived from one of the component somites of the last abdominal segment, and represents its appendages. The hinder part of each vas deferens is, in some Cidicidce, expanded to form a vesicula seminalis of considerable size, but this is not the case in Culex nemoralis.
The female generative organs are a pair of ovaries, the
74 GNATS OR MOSQUITOES — CHAPTER V oviducts uniting behind to form a median oviduct, a median copulatory pouch, and three spermathecse opening into the last. They correspond in size and position to the testes. The median oviduct is formed by the invagination of a region v^hich Dr. Hurst takes to be the ninth sternum, while the anus opens at the posterior end of what he takes to be the eleventh abdominal somite, so that there is no common cloaca. This invagination is already far advanced at the beginning of pupal life, and during it it grows forwards, keeping pace with the forward shifting of the last pair of ganglia, and at all stages lying just behind it till the final ecdysis, when the rapid shifting of the ganglia leaves it behind. Its anterior end is, in the adult, near the front of the seventh segment. In the youngest pupae three flattened invaginations, the future spermathecas, lie on the dorsal wall of this median oviduct. During the pupal period the anterior end of each becomes spherical and acquires a strong chitinous lining. The anterior ends of these organs remain stationary in the eight segment throughout. The bursa copulatrix is a dorsal outgrowth of the invagination which gives rise to the median oviduct, and is a small pouch lying just behind and above the median aperture.
The pupa does not eat. It breathes air through the apertures at the end of its syphons. It floats, thorax upward, by virtue of the large air cavity lying under the hinder part of the thorax and front of the abdomen. This cavity is bounded in front by the legs, at the sides by the wings, and in front by the mouth parts. It extends up at each side of the abdomen, where it is covered by the halteres, and into it opens the patent first abdominal stigmata. The pupa is sensitive to light, and immediately darts backwards when a shadow falls upon it suddenly. The movements, however, though very rapid, are devoid of anything like steering.
The larva has to search for food, but the pupa has simply to get out of the way of danger, and the direction of its flight is of little importance, though since the movement is always backward with reference to the pupa, it is