38 GNATS OR MOSQUITOES — CHAPTER II by infected insects are desirable, there is no need of such an experiment to estabhsh the biological relation between Mosquito and parasite, for if it is found that the parasites ingested by the Mosquito with the blood of the infected vertebrate undergo developmental changes, it may be taken as proved that the species used is capable of acting as intermediate host, and is therefore dangerous to the vertebrate from which the infected blood was derived. It is well to make use of a fair number of insects in each experiment as all may not bite, and in any case some ten or a dozen will be required for dissection day by day in order to follow out the changes undergone by the parasites within the Mosquito. The method of dissecting out the stomach of the Mosquito is described elsewhere, and it only ren:iains to add that the best medium in which to examine is ordinary "normal saline" solution. If the stomach be very full of blood it is well to puncture it, and shake it to and fro in a watch-glass of saline solution, with the point of a needle, or which is safer, to irrigate it with drops of solution as it lies on the slides.
For permanent preparations Major Boss found nothing so suitable as formol solution, as balsam or glycerin render everything so transparent that all details are lost. Such experiments are, when negative, conclusive only when conducted at the season of the year at which the parasitic disease under investigation is rife, as at any other season it may possibly be merely owing only to unsuitable climatic conditions that the parasites fail to continue their cycle within the Mosquito.
PLATE I.
PLATE I. — To Illdsthate the Anatomy of the Laeva. Fig. 1.— Full-grown larva of Cidex anmdatus (Schrank) : a, respiratory syphon ; h, swimming fan ; c c c c, anal papilla;. Fig. 2. — An antenna, more magnified.
Fig. 3. — Respiratory syphon more enlarged, to show : a, the muscles ; b, the valve-like terminal lobes ; c, the stigma.
Fig. 4. — Segmental respiratory apparatus : a, part of the main longitudinal trunk ; h, lateral branch ; c, cord by which the cast-off lining of the tubes is withdrawn during ecdysis.
Fig. 5. — Left mandible, seen from below.
Fig. 6. — Right maxilla, seen from below: a, internal lobe ; b, external lobe ; c, maxillary palp.
Fig. 7. — Labium, with the lower part of the pharynx, seen from above: a, the labrum itself.
Fig. 8. — One of the natatory compound bristles, much magnified. Fig. 9. — Thorax of larva of C. pipiens, to show : h, the hepatic masses ; i, intestine ; t, main longitudinal trachete.
It will be noticed that in this species the thoracic dilatations of the main tracheae are by no means so marked as in G. anmdatus (Schi-ank), and the whole respiratory system is less developed.
This plate is partly based on Meinert's figures, and partly original.
40 CHAPTEE III.
The Anatomy of the Larva.
From a purely anatomical point of view a good deal has been written on this subject, but on the descriptive side comparatively little has been recorded, so that except in the case of a few of the commonest species, we are quite without adequate descriptions whereby Ciilex larvae may be distinguished among themselves. In by far the greater number of recorded species the larvaB have never been recognised, and still less described. It is obvious, however, that as these insects can be much more easily destroyed in large numbers in the larval stage, by insecticides, or by the filling up of pools, than they can be as flying insects, the accurate description of the larvae of noxious species has become a matter of great importance ; but it is unfortunately one that can, as yet, be hardly said to be commenced.
Observations on this subject, by "breeding out" adult insects from larvae, require much time, and, above all, a settled residence, and the work of the Sanitary Commissioner of an Indian province is of a kind that precludes either requirement, so that my present visit to India has been very unfruitful in results in this direction, and I still can only offer a few general suggestions for future work. As regards the genus Culex, I believe that the proportional length of the breathing tube, as compared with that of some comparatively fixed portion of tlie body, such as the head or thorax, will be found to yield valuable specific characters.
I was at first inclined to think that this proportion varied greatly with the age of the larvae, but found subse-
' THE ANATOMY OF THE LARVA 41 quently that this was a mistake due to the occurrence of several species in the same pool, and that a ratio of this sort remains fairly constant throughout the growth of the larvae. It is easy, for example, to distinguish between the larvae of C. fatigans (Wied), and those of the species I identify as C. impellens (Walker), which constantly are found associated here in the cold weather, the breathing tube of the former being of medium length, while that of the latter is very short ; and I have met with larvae in which the syphon is so short that they may easily be confused with those of Anopheles, the more so as they not unnaturally assume much the same position in the water ; but I was unable to delay my departure in order to ascertain the species to which they belonged, but I believe they were those of a Stegomyia.
In Anopheles, on the other hand, apart from size, the larvae of different species are most difficult to distinguish. Their coloration is singularly alike, and as yet I could not undertake to distinguish those of any individual species, though they differ considerably in the matter of the size of the anal tubercles.
Quite recently Grassi {G.S.Z., p. 81) has shown that the bristles with which these larvae are provided, in various situations, differ sufficiently in the various species to be made use of to afford definite indications of the species to which a larva belongs. These bristles are often of very complex structure, and this is especially the case with those on the dorsum of the thorax and head. Confining his attention to those of the latter region, he finds that the characters of two special bristles, viz., the sub-median pair, in the middle of the fore border of the head, and those placed at its external angles, suffice to easily distinguish the four European species of Anopheles larvae known to him, and on comparing his figures with the corresponding bristles of the larvae of An. Eossii, I find that in that species too these bristles again differ from any of those. It will be observed that the difference between the bristles of An. Eossii, as compared with those of An. superpictns, is very marked, and is alone quite sufficient to set at rest any
42 GNATS OR MOSQUITOES — CHAPTER III doubts as to the absolute distinctness of the species in question.
In addition to the above, there are a variety of other compound bristles, notably those of the swimming fans, which might be made use of in distinguishing species, but, with the above exception, we can go no further than the recognition of genera, and of some even of these we are quite ignorant of the characters of the larvae. Mamlifiainis. ^t/unalus Fig. 13. — Tbe median aud angular bristles of the fore margin of the clypeus of five species of AnopJielcs. Copied from Grassi, with the exception of the figure of the bristles of An. Bossii. (Original.) Of the larvae of Megarhina, Pso7'ophorc( , and Mucidus we know nothing. Those of Uranotrc?iia are said by Dr. Lutz to have a very short syphon, and to look much like those of Anopheles, resting in the water m an oblique position. In Mr. Theobald's new genus Stegomyia, S. pseudotceniatus and S.fasciatus certainly have very short syphons, and the same was the case in a larva I met with in Allahabad in August, and which, though I was unable to breed out, was almost certainly that of S. gicbernatoris ; so that probably the larvae of this genus are characterised by the possession of short breathing tubes. The same, however, is the case with certain Culices, notably of C. pulcliriventer, the syphon of which is no larger than one of its anal tubercles. The larvae of all the Culicidce are aquatic, and are encephalous, i.e., have a well-developed head. When just hatched, the larvae are of just sufficient size to be easily
THE ANATOMY OF THE LARVA 43 seen by the naked eye, and are at this stage so transparent, that almost all details of their anatomy can be made out in the living larva placed under the microscope in a little water. If all superfluous water be carefully removed from beneath the cover, by means of a strip of absorbent paper, so as to slightly press upon without crushing the larva, its naturally vivacious movements will be sufficiently restrained to admit of its being observed at leisure. The larvae are easily recognised by their vivacious wrigghng movements, as minute worm-like bodies, with a disproportionately large head with a pair of prominent black eyes, and at first sight what appears to be a vertically bifurcated tail. Being quite unprovided with legs, they swim by means of the strokes of a tail armed with large expansions of bristles. When more closely examined, it is seen that, like the adult insect, it consists of three well-defined regions, the head, thorax, and abdomen. Commencing with the head, it is seen to be not quite so wide as the thorax, bat considerably wider than the abdomen, and forms a truncated cone, wider than it is long, and separated from the thorax by a distinct sulcus. It is ornamented on the dorsum with several patches and lines of pigment, the arrangement of which, however, is not very constant ; and at the broadest part laterally are placed the two large eyes. It is further ornamented ni various places with tufts of strong bristles, the structure of which is often very complex.
From two slight prominences a little in front of the eyes spring the antennae, and projecting from the middle of the anterior border of the head may be seen a complex arrangement of bristles springing from the upper lip and mouth parts.
The greater part of the upper surface of the head is formed by the dorsal surface of the third metamere. In front of this is the clypeus or dorsal plate of the second metamere, a short but broad plate, with a shallow, curved indentation in front, while laterally it is armed with a pair of incurved tufts of bristles which form what Meinert (De Encephale Mygelarver) speaks of as a whorl, or rotatory
44 GNATS OR MOSQUITOES — CHAPTER III organ ; as he believes that it is by the vibrations of these bristles that nutritive particles are directed into the mouth. Anterior again to the clypeus is a small, rounded, median prominence, the labrum or dorsal plate of the first metamere.
The eyes are large and placed laterally, forming a sector of about 150°, and close behind them may be distinguished a minute ocellus, generally of oval form. The antennae spring from the antero-external corners of the third metamere, and form a pair of short, curved horns, fairly freely movable on their basal articulation with the head, but elsewhere rather rigid. The greater part of the organ consists of a rather stout basal joint, which is provided at the inner side of its distal extremity with a tuft of strong, compound bristles.
The next joint is less than half the length of the basal, and is distally armed with a few long, stiff bristles, while, like the basal joint, it is beset throughout its length with stiff, short spines. The last portion of the antenna or flagellum is very minute, although there are indications that it is in reality composed of three very short articulations. Besides the flagellum there are also attached to the end of the second joint two peculiarly-formed processes or jointed hairs, which are almost certainly sense organs of some sort, and are most probably olfactory organs. In general form the thorax forms a sort of six-sided box, and is somewhat larger than the head in all dimensions. Although no sutures can be distinguished on its surface, its division into its three component segments is sufficiently indicated in its outline, and by three pairs of lateral tufts of bristles, which are longer and stouter than those of any other region of the body. The component hairs of these tufts are all compound, each being clothed throughout its length with filaments of considerable proportional length ; and each tuft springs from a nipple-shaped tubercle, which appears to be capable of a certain amount of voluntary movement, though, from its position, it is obviously not the rudiment of a leg. These tufts appear to act by way of lateral keels, whereby the larva is maintained in any