122 GNATS OK MOSQUITOES — CHAPTER VII be noted, is exactly the same sort of situation in which they were found by Drs. Stephens and Christophers, in Sierra Leone, and there, both these gentlemen and Koss never found them in really dirty water, so that presumably, both An. costalis and funestus, the species concerned, confine themselves to fairly clean water ; but, as we have seen» several species can adapt themselves to less nice conditions, so that no generalisation as to the sort of water in which Anopheles larvae will or will not be found is possible, and all attempts to formulate such are simply misleading for the genus, and where recorded, should be accepted exclusively for the species to which the observations refer, and to no other.
How ditiicult it is to make any definite statement as to the habits of even a single species, may be judged by the fact that even in Italy, where more attention has been devoted to such points than anywhere else, the larvas of one and the same species are stated to be " paludal " in habit by one authority, while the imagines are said to be domestic by another observer. The fact is that it is impossible to lay down any hard and fast rule, and all that can be done at present is to point out the desirability of always recording the exact species referred to, and in noting the habits of either larvae or adults, so that materials for a more complete statement of the case may accumulate in the future.
The females of, I think I may say, all genera of Mosquitoes lay their eggs in water. During the past year, a large number of observers have been noting these and kindred points, and no one records any fact that in any way confirms Boss's suggestion that they may sometimes deposit them on the dried-up ground where puddles have previously existed. A gravid gnat must deposit her eggs somewhere, and in such surroundings as the interior of a test tube, which is about the most unsuitable vessel I know of wherein to confine gnats with the view to observe points in their life history ; there is nothing really suggestive of usual habits in her being driven to deposit them on the walls of the tube.
LIFE HISTORY AND SEASONAL PREVALENCE 123 The attitude of the females in depositing their eggs is pecuhar. Thanks to the air entangled in the pulvilli, with which the feet are provided, they can rest on the surface of still water, as easily as upon dry ground, and are able to alight upon or take flight from its surface at will. When they have settled themselves in this way on the surface of Fig. 29.— Various forms of Mosquito Bggs.--(l) Egg-boat of Culex, seen from above ; (2) the same, side view (after Sambon) ; (3) separate Culex eggs ; (4) eggs of Panoplites (after Daniels); (5) eggs of Stegomyia; (6) the same more highly magnified (after Theobald) ; (7) groups of Anopheles eggs, as they float on the water (after Sambon) ; (8) egg of Anopheles maculipennis, showing lateral floats, seen from above, x 30 diams. ; (9) the same, viewed laterally, (after Nuttall).
the water chosen for the purpose, they spread out the four anterior legs as a support, while the posterior legs are crossed behind them so as to form an acute angle, and it is into this angle that the eggs are dropped side by side, until an elongated, boat-shaped mass is formed, somewhat raised at each extremity. The hind legs which carry them are extended little by little until the boat is finished, when
124 GNATS OK MOSQUITOES — CHAPTER VII the insect allows it to drop into the water at the mercy of the wind.
In speaking thus of " boat-shaped masses," it must be understood that Ciilices only are referred to, as Ano})heles, Mucidus, PanopUtes, Stegotmjia, and some drop their eggs separately, and in these genera the eggs are provided with separate arrangements to secure flotation. In Culex, the eggs are glued together, as they are laid, into canoe-shaped masses, which, in size and colour look, as they float on the water, not unlike caraway seeds. Each boat consists of from '250 to 300 eggs, which latter are oblong, more pointed at their upper extremity as they lie in the boat, larger and more rounded below, and ending abruptly in a bordered edge, much like that of certain liqueur flasks ; the opening of which may be said to be closed by a thin membrane, by the rupture of which the larvae escapes. The lower or immersed end is furnished with a curious opening, or rather thinning of the egg-shell, and is surrounded with some curious radiated markings. The object of this arrangement is probably to enable the embryo to obtain oxygen from the water in which the little raft floats.
The egg-boats must needs float on the surface of the water, as the embryos perish if they become submerged. Only their neck comes in contact with the water. When just hatched the eggs are entirely white, but they soon become shaded green, and in less than half a day they become grey. Usually they are laid between five and six in the morning, but in England Mr. Theobald finds that the evening is also chosen for the purpose, and the larvae escape in two or three days, but the interval varies a great deal, according to the temperature, and the particular species. Those of Mucidus apparently take a long time, as Major Close, I.M.S., at Moradabad, kept some, laid by a confined female for a month without the larvae appearing. As, however, the experiment was conducted in a test tube, and it is possible the embryos may have perished for lack of oxygen, this observation must be taken with reservation. As soon as the larvae of Culex have escaped from the eggs, the egg-
LIFE H [STORY AND SEASONAL PREVALENCE 125 boats break up, and the empty shells form very pretty microscopic objects, their surface being closely set with minute spines, the form of vi^hich varies in different species, and so might possibly be made to serve as a means of identifying the species to which they belong. C. pipiens ova measure 0"9 mm. in length, C. nigritulus 0 6, and according to Howard those of C. pungens (= C. fatigmis, Wiedemann) 0*7 in length and 0"16 in diameter. Anopheles eggs are quite different in form and are deposited separately. Sometimes, they lie on the surface of the water in stars or groups of other forms, but as they are in no way connected together, I regard this as quite accidental, and of no moment. As regards the form of the egg, the following excellent description, extracted from Dr. G. H. F. Nuttall and A. E. Shipley's paper " On the Structures and Biology of Anopheles " {maculipennis) in the Journal of Hygiene, I., p. 49, may be quoted: — " When first deposited the eggs are white, but they soon darken. Each ovum measures 0'7 to I'O mm. in length and is at its greatest breadth about 0"16 broad. The egg is boat-shaped and one end is sHghtly deeper and fuller than the other. The surface which, were the egg a boat, would be the upper, is flattened but slightly convex. It is marked by minute reticulations (fig. 2). The under surface of the boat is characterised by much larger and more regular reticulations, which divide the surface into fairly equal hexagonal areas. The rim {a) of the boat is thickened and very regularly ribbed. Along the centre of each side, extending over a space of rather more than one-third the total length, this rim is much thickened, the ribbing is more marked and the whole forms a very conspicuous and characteristic feature of the egg. This thickening recalls the rounded float which runs along the edge of a life-boat (fig. 29 ; 7, 8, 9). It serves the same purpose, being composed of air chambers and is used to keep the boat-shaped egg with its flat surface uppermost. Howard refers to the membrane we are about to describe as the ' clasping membrane,' notes the reticulated surface exhibited by the eggs, as also the presence of 5 — 7 minute dark circular spots at the
126 GNATS OR MOSQUITOES — CHAPTER VII ends. His measurement of the egg is given as only 0"57 mm. As in other insects the egg doubtless varies in size. " The colour of the egg soon after it is laid is greyish black. If the eggs are subject to much attrition a delicate membrane splits off which gives the surface of the intact egg its reticulated appearance. Stripped of this membrane, which desquamates in irregular whitish fragments, the egg appears with a glistening black surface comparable to that of patent leather. One end of the egg is slightly blunter and more rounded than the other, and this contains the head end of the embryo. It is an interesting point that when the egg, as frequently happens, is drawn by capillary action a little way up from the water on to a leaf or some other half-submerged object the head or blunt end always points downwards, and thus should the hatching take place whilst the egg is in this position the larva emerges into the water, and not into the air."
I doubt if these ova can resist more than a very temporary dessication, but it is very difficult to judge of this point by laboratory experiments, as not unfrequently, they refuse to hatch out at all. The ova of some Stegomyice, however, which resemble them a good deal, being laid separately, and having also similar lateral floats, are certainly much more resistant, and may apparently be uninjured by being left high and dry for a month. Carroll, Agramonte, and Lazear {Philadelphia Med. Journ., October 27, 1900, p. 291), state that in experimenting on conveyance of yellow fever by Mosquitoes (Stegomyia, fasciata, Fabr.), they obtained a supply of this species from Dr. Finlay. Thirty days previously, vide Finlay, ova had been deposited by a female just at the edge of the water in a small basin, whose contents had been allowed to shghtly evaporate, so that these ova, at the time of their visit, were entirely above contact with the water. Notwithstanding this long interval, they were promptly converted into the larval stage, after a short period, by simply raising the level of the water in the basin. The separately-laid eggs of Panoplites have a very peculiar shape, like a long, narrow pear, one end being prolonged into a sort of neck exactly like that of a Florence flask.
LIFE HISTORY AND SEASONAL PREVALENCE 127 According to Meinert, the ova of Gorethra are laid in flat, round, jelly-like masses, each containing from 100 to 150 eggs, but Miall describes them as arranged in spiral lines, so as to form a gelatinous sheet. We have no information as to the eggs of the other genera, and as a matter of fact, our knowledge on this point is very scanty, and further observations are much required. The insects pair towards evening. The males assemble in large numbers, flying hither and thither without travelling far, and the females appear among them in smaller numbers, so that the moment one appears she is clasped by a male, allowing themselves to float in the air, or flying together. The coupHng only lasts a few moments, and when it is completed the pair separate, and the fertilised female proceeds to deposit her eggs.
As already remarked, the time taken by the ova to hatch out depends upon the species and on the circumstances of its environment, but, in most, may be said to be about three or four days. Even when newly escaped, they can be easily distinguished by the naked eye, if favourably lighted, as black dots with a transparent, wriggling tail. Being mainly air-breathing animals, though it is possible that the anal papillae may aid in respiration, by acting as gills, they cannot remain under water for any length of time, and all species pass the greater part of their time at the surface, lying so that the respiratory opening is open to the air. The larvaB of Culex, especially those species which possess a long respiratory syphon, pose themselves in a nearly vertical position, as if hung from the surface of the water, as is well shown in the following illustration, which was photographed from living larvae and pupae by my friend, Mr. Koyle, of Kosa, N.W.P.
The larvae of Stegomyia, whose breathing tubes are very short, lie much more obliquely, their position approaching that of Anopheles, but this cannot be looked upon as a generic peculiarity, as those of C. pulchriventer , which have an equally short syphon, take up the same position, which is presumably simply necessitated by the shortness of the tube. The species figured by Christie in his useful little
128 GNATS OR MOSQUITOES — CHAPTER YII book, "Mosquitoes and Malaria," is probably a Stegomyia (pi. v., figs. 9, 10).
The larvae oi Anopheles on the other hand, lie absolutely horizontally, with the respiratory orifices, which are almost Fig. SO.^Larvse and pupto of C. fatigans, Wied., photographed from life, about twice natural size.
flush with the rest of the dorsum, just emerged, and the head barely submerged.
Lying in this position, they look just like scraps of strawblackened by long immersion, and as they often lie motion-
PLATE V.