THE ANATOMY OF THE LARVA 45 position without being made to revolve on its aixis under the influence of the strokes of the tail. In addition to these principal tufts, other smaller but also pedunculated tufts spring from the sides of the terga of the segments, the characters of which should be accurately noted in describing species, as it is probable that their form, number and position, may yield good specific characters. The great preponderance of the mesothoracic segment is already well marked, though not to anything like the same extent as in the imago.
Proceeding with the examination of the living larva, the first thing that catches the eye in the thorax are four pairs of what at first sight appear to be pigment spots. A little watching, however, suffices to show that these are not really external markings, but internal organs seen through the transparent carapace, and that their position is constantly altering under the action of the surrounding thoracic muscles, and of the pulsations of the dorsal vessel. Examined with a somewhat higher power, they are seen to be glandular bodies of a sacular form, lined with secretory cells, their cavities being filled with a clear fluid, holding in suspension a quantity of deep brown granular matter. These glands are connected with the upper end of the intestine, and are probably hepatic in function. In the dead, or preserved and mounted larva, they soon become invisible, owing to the discharge of the contained brown matter into the intestine.
These glands are arranged in two sets, an internal and an external, the former of which consists of two glands placed so closely together as often to look like a single mass, and situated nearly in the middle of the thorax, close to the intestinal canal. The other two pairs are placed at a distance from the intestine, in the corners of the proand metathorax respectively. In the middle line the dorsal vessel can be seen pulsating, the action being rather that of a peristaltic wave than a true systole, the heart being here a long, valved tube, corresponding rather to an aorta than to a heart in the usual acceptation of the word. More or less in the middle line, too, its coarser image
46 GNATS OR MOSQUITOES — CHAPTER III often obliterating that of the dehcate, superjacet dorsal vessel, is the stomach, or chylific ventricle, its opacity varying with the fulness of its contents ; and on either side may be seen the dilated thoracic portions of the respiratory trachese, which are easily distinguishable by the spiral thickening of their chitinous lining, while branches of the tracheal system to the various organs can be traced in various directions.
The abdomen is between three and four times as long as the thorax, but narrower, and consists of nine segments, each of which is provided with a large tuft of bristles as well as with numerous hairs, either single or grouped, on the dorsal and ventral surfaces. The cardiac, intestinal, and respiratory tubes can all be traced through the greater part of its length, and the Malphigian tubes, and other intestinal appendages, can all be made out, as the larva takes favourable postures. The nervous system, however, is completely hidden, and owing to its transparency during life, cannot be seen even if the animal be placed in the supine position. In the genera Cidex and Mochlonyx, there springs from the dorsum of the eighth segment a large process, at least as wide and generally about two or three times as long as the remaining segment of the abdomen, and into this the two main respiratory trunks can easily be followed, and are seen to open at its extremity by means of curiously guarded openings. At the root of this breathing horn are a pair of rather short but dense tufts of hairs. The last segment contains the rectum and carries the anal tubercles, the anus being placed almost at the very extremity of the body, but rather towards its ventral aspect. Around the opening are two pairs of delicate, leaf-like expansions, each furnished with a branching twig of trachea, the lower pair being somewhat the larger. They probably act as gills, and subserve respiration during the periods when the larva is completely submerged, which, when the weather is cool, are often somewhat protracted. On either side, too, but originating. a little in front of these anal tubercles, are a pair of large dense tufts of compound hairs, which are employed in swimming much in the same way as a fish's tail, and are so arranged as to form an expansion of similar shape.
THE ANATOMY OF THE LARVA 47 The internal organs are of typically insect plan, and will be most easily understood by considering each system separately.
Digestive System. — Apart from the gnathites or foot-jaws, which, although of the ordinary masticatory type are rather complex organs, the intestinal canal is of the simplest form, consisting of a narrow oesophagus, which leads into a wide, perfectly straight tube, commonly spoken of as the stomach, which extends to the end of the sixth abdominal segment. Into its thoracic end open the ducts of the hepatic glands, which, when examined in section, are seen to be broad crypts communicating with the • stomach by so wide an opening as to almost appear as diverticulse ; and into its distal, where the tube contracts before the commencement of the rectum, open the five Malphigian tubules, which are slender tubular glands, differing in no way from those of the adult. Lying close to the oesophagus in the head are the two salivary glands, but they are not easy to demonstrate, except in section, and the same remark applies to the rectal glands, which lie beside the rectum in the last segment. The mouth parts consist of an upper lip or labrum, in the form of a convex crenated plate, armed with numerous hairs, and a longer and narrower lower lip, or labium, of somewhat similar structure. Between these are placed the two pairs of foot-jaws, the mandibles above and the maxillse below, feach mandible consists of a somewhat pyriform plate, the wider end of which forms an articulation with the lateral structures of the mouth ; while the narrower end is formed into a rather complex set of dentations divided into two groups, the anterior of which are small and claw-like, and mainly adapted for holding the prey, while the hinder set are better adapted for cutting and crushing. The appendage is further provided with brushes of peculiarly formed hairs, and has, about the middle of the anterior border, two large jointed processes or hairs, which probably are tactile or gustatory in function. The maxilla is a plate of somewhat quadrangular outline, the anterior border being curved and its corners rounded off. It is richly provided with hairs, some of which have a curiously compound
48 GNATS OK MOSQUITOES — CHAPTER III structure, and from its inner and posterior corner there projects a short, three-jointed maxillary palp in the form of a truncated cone, the extremity of which is armed with minute spines. The above description should suftice to enable the dissector to recognise the various parts, but it would be wasted labour to enter into minuter detail in a general description, as the form and arrangement of the various parts differ somewhat in the various species, and the minutiae will best be studied by teazing out the parts from the head of whatever larva happens to be available, with needles, under the simple microscope. Bespiratonj System.-^Thm is very highly developed in these larvae, and presents many peculiarities which differ widely in the different genera ; and hence the remarks below must be understood to apply to Culex only. The two main longitudinal tracheae are in all, however, of such large size that they may be considered rather as elongated air sacs than as mere tubes for the conveyance of air to the tissues.
Doubtless they serve a double function; namely, as receptacles for the storage of air for use during periods of complete immersion, and as hydrostatic organs to secure a proper degree of buoyancy. It is only the two main longitudinal trunks that are so largely developed, the remainder of the tracheal system being of no more than the usual complexity in organisms of this sort. Each of these main trunks commences quite abruptly in the pro thorax, and, rapidly increasing in diameter, attains its largest dimensions in the mesothorax, where, in some species, e.g., in C. nemorosus, there is a marked local dilatation ; in G. pipieiis, however, there is no mesothoracic dilatation, and the tube remains of nearly the same diameter throughout the entire length of the abdomen to its termination at the end of the peculiar dorsal process of the eighth segment. The anastomoses between the two main stems are trifling, there being only two transverse connections placed in the proand mesothorax respectively before the tubes have attained their maximum diameter. The anterior of these two crossbranches is considerably the larger, and immediately in
THE ANATOMY OF THE LARVA 49 front of it, each trunk splits up into a number of branches, which enter the head and are distributed to the various organs contained ia it. The large anterior cross branch gives off no branches, but the hinder one supplies some twigs to the wall of the intestinal canal, and to the hepatic glands.
In the larvae of An. Bossii, however, and presumably of other species of that genus, the communication between the two main trunks is extremely free, as not only are they continued beyond the end of the thoracic dilatation to unite with each other in the middle line in the form of an arch of nearly uniform diameter, but a smaller anterior branch unites with its fellows to form a second concentric arch, of Fig. 14. — Larva of Anopheles Rossii, seen from in front, to show the tracheal arches, and their anastomosing loops. but little less calibre. Moreover, as will be seen in the figure, each arch gives off eight branches, which anastomose with each other to form a series of loops, four on each side, those of the larger arch being directed tangentially, while those of the smaller run forwards ; so that, in addition to the transverse branches described above, there are no less than four channels of communication between the main trunks of opposite sides.
From each of the main trunks in the thorax there issue also three rather large branches, one for each seg- 4
50 GNATS OR MOSQUITOES — CHAPTER III inent, which divide up into branches that pass to the contained muscles and viscera. Between these lateral thoracic branches I have not been able to make out any longitudinal communication ; but like the lateral abdominal branches they extend as pervious tubes nearly to the skin of the larva, and are continued to the latter as an impervious chitinous cord, surrounded by true dermic tissue, to actually blend with the external chitinous covering of the larva. In the abdominal region the arrangement is essentially the same, only here there is a continuous longitudinal anastomosis between the lateral branches, one of which issues from the main trunk of all but the last segment, and after a short course divides into three main branches — an anterior to anastomose with the posterior branch of the lateral trachea of the segment in front of it, a posterior to communicate with that behind it, and a continuation, which after distributing branches of air supply is continued to the skin, first as a pervious tube and then as an impervious cord, exactly as in the thorax. These cords represent the rudiments of the future segmental stigmatic trunks of the imago, and it is by their agency that the discarded lining of the portion of the tracheal system belonging to the segment is withdrawn from the body whenever the larva undergoes its periodical change of skin. From these three principal segmental branches there issue branchlets which carry air to the various muscles and viscera of the segment. These lateral stigmatic cords can best be studied in the abdominal region ; and if a cast skin, such as will be found floating in abundance on the surface of any vessel in which larvae are kept, be carefully examined, it will be seen that the main longitudinal air sac has broken up into segmental lengths, and that each piece is attached by its cord to the skin of the <".orresponding segment ; for it must be remembered that not only the outer skin but the whole of the lining of the tracheal system is thrown off at each ecdysis. The structure of the peculiar dorsal horn of the eighth segment remains to be described. Each of the two main stems passes up through it separately, to end in a pair of stigmata at its extremity, whose openings are considerably smaller
THE ANATOMY OF THE LARVA 51 than the diameter of the tube, being placed in the centre of a small circular plate supported by chitinous rays. The mechanism by which these openings are protected from the entry of water is rather complex and consists of a sort of valve, formed by five flap-like lobes at the end of the process. These flaps consist of two pairs and a small unpaired lobe, the larger of the two pairs being placed opposite the small unpaired one. Between the two air tubes and around them are a number of muscular strands, which originate in the eighth segment and extend through the process to be inserted into the bases of the flaps, so that, when they contract, the stigmata are drawn down somewhat within the process, and the valves close over them.
Vascular System. — This consists of a delicate, long, wide tube, which extends along the dorsal aspect of the body, from the hinder part of the head to the very extremity of the abdomen. Its structure is so delicate that, except in the living larva, where its shape and position can be followed by its movements, it is extremely difficult to make out. It consists essentially of a long, thin-walled tube, with valvular constrictions opposite the incisurae, communicating freely with the perivisceral spaces by means of cribriform apertures, which, however, are anything but easy to demonstrate. There is no system of peripheral vessels at all comparable with those of the higher animals, the function of the organ being to keep in motion the perivisceral fluid, which is thus kept flowing over the tracheae, which lie free in the perivisceral spaces between the various organs and tissue elements. It acts, in fact, rather as a churn than as a pump. In front of the thorax it extends as a narrower vessel into the head, and this portion is sometimes spoken of as the " aorta," but I look upon this term as a misnomer, as, save in diameter, the extension differs in no way from the rest of the dorsal vessel, is quite unprovided with branches, and appears at its anterior end to lose itself in the general interstitial lymph spaces of the head. The circulating fluid is colourless and contains but few cellular elements, those that are present being of the nature of
52 GNATS OR MOSQUITOES — CHAPTER III leucocytes, so that while fulfilHng the functions of both, the fluid is rather of the nature of lymph than blood. Nervous System. — The demonstration of the nervous system can only be made by dissection, as its elements are too delicate to be distinguished from the overlying intestine, even in the supine position, and for this purpose specimens hardened in alcohol are best. Even thus it is not altogether easy to follow out any length of the chain, and for those who will be satisfied to examine a closely-allied larva, it is recommended to dissect the larva of one of the Epherneridce, in which the nerve cord can be separated with singular ease. These larvae are very common in grassy pools and small running streams, and may be recognised by their long caudal bristles, which give them the appearance of possessing a long forked tail. When one has become accustomed to what to expect to see by practice on these easier subjects, it will be found easy enough to teaze out the nerve cord in Culex larvae. In the head, however, this is scarcely practicable by the method of teazing, and sectionising must be resorted to. Taking the young larva as a type, a pair of ganglia can be demonstrated for each segment, and on emerging from the thorax the lateral cords separate to pass backwards across the oesophagus, to combine behind it in the large superoesophageal ganglion or brain. From this mass filaments are given off to the eyes and antennae, and from it, as well as from the ganglia of the segmental chain, fibres pass to the corresponding muscles and to the periphery, sensory filaments having been actually traced into the bases of the hairs. At the time of pupation the location of the gangha undergoes changes of startling rapidity. Mr. F. V. Theobald notes that in a few minutes prior to the escape of the pupa from the larval skin the first abdominal ganglia come to lie in the posterior part of the thorax, and during pupal life the changes are equally rapid. In four days the fore-brain increases tenfold in bulk, the first abdominal ganglia fuse with the three thoracic pairs, and about the same period the eighth pair shift forward and fuse with the seventh ganglia, and in the $ , but not in the c? , the double mass so formed