LIFE HISTOEY AND SEASONAL PREVALENCE 1'29 less for a quarter of an hour or more, would be taken to be such by any one not specially looking for them. As already noted, they exhibit a great preference for the sides of the tank or pool in which they may be, and may also be found along floating sticks, leaves, or other such objects, lying at right angles to, and with their tails touching it, as if moored by the stern. When lying thus at the surface, they have a quaint custom of screwing their heads completely round, so that its ventral surface looks right upwards, and, though otherwise motionless, if closely watched it will be seen that the head is being slowly rotated and swept to and fro, while by the rapid vibrations of the bristles of the " whorl organs," a current of water with its suspended nutritive particles is kept flowing to it.
It is perhaps rather a misnomer to speak thus of the moustache-like bunches of bristles that project from a pair of plates hinged on to the fore corners of the clypeus, as the bristles all work together, and do not bend, one by one in a wave, hke the cilia of the Rotatoria, from a fancied resemblance to which they have got the name. Although capable of being moved separately, the two brushes when in action are brought together, as if grasping at something, and then open out again, and repeating this movement with great rapidity ; according to Nuttall {loc. cit.), three times a second.
He describes, too, how they employ the peculiar bristles on their mandibles to comb off, into the mouth, the nutritive particles that have collected on the brushes. The mouth-organs of Culices are used in exactly the same way, only instead of sweeping the surface, for floating particles, they depend on such as are suspended in the water beneath. "When disturbed the larvae dart backwards into the depths of their puddle, and hide themselves among the loose particles at the bottom, and then, after a few minutes, float slowly back to the surface when the coast is clear. A good deal has been written about these movements, and those of the pupae, but as they are obviously purposive, they naturally vary, and hardly merit detailed description. The nymphs, as may be seen in Fig. 31, float in 9
130 GNATS OR MOSQUITOES — CHAPTER VII much the same position as those of Culex, but perhaps the breathing horns are held, sloped somewhat more forewards, ^des larvae are said, by Dr. Lutz, to He in the water in a sloping position, intermediate between that of Anopheles and Culex, in other words, much as do those of Stegomyia. According to Meinert, Corethra larvae are much less restricted as to the character of the collections of water in which their larvae are reared being often found in deep still water containing but little vegetation, as well as in very foul pools. Their position in the water is even more absolutely horizontal than that of Anopheles, whose position is slightly oblique, and they retain this horizontal posture even during their excursions below the surface. They are very voracious, devouring not only Daphnice and Fig. 31. — Larva and Nymph of Anopheles, to show habitual resting position. The head of the larva is shown looking up, as when feeding. other small crustaceans, but also Dixa larvae and even young fish and molluscous embryos ; nor do they even shrink from cannibalism, smaller specimens of their own species being devoured with gusto if they come within the reach of their jaws. It is only the very young larvae that are so absolutely transparent, as when older they develop a number of patches of pigment, and the contents of the intestinal canal obscure the complete transparency of the body. Still, it is doubtless owing to this transparency that they are the only gna.ts whose larvae live, as a rule, among fishes. All their ecdyses are performed with singular rapidity, even the final emergence of the imago taking only a few minutes to accomplish. Their pupae assume a nearly vertical position in the water, but otherwise do not differ greatly from those of other senera.
LIFE HISTORY AXD SEASONAL PREVALENCE 131 Meinert states that the larvae of Mochlonyx affect the waters of fields and woods, and that they are especially often found in such places as ditches, where the water is clear, but without any particular current. The larvae do not survive the winter, but he considers it probable that the eggs do, as he has found newly-hatched larvae so early in the year as to make it highly improbable that any of the perfect insects could have emerged from their hybernation, even assuming that the adults do hybernate. In a warm room, in captivity, all the changes from egg to imago were gone through in three weeks. Unlike Culex there appears to be only a single generation of larvae developed in the year. The position taken up by the larvae is very like that of Corethra, but they are more in the habit of remaining under water, They are voraciously carnivorous, and appear to be even more arrant cannibals than the larvae of that genus. The pupa also behaves much like that of Corethra , but it is not quite so rapid in its changes of skin. The larvae of the Culicida swarm in spring and summer in the stagnant water of tanks and other domestic collections of water, where they may be found in abundance in Europe from the time the ice is melted. Usually they keep at the surface of the water or a little below it, in an inverted position, the head being lowest, breathing by means of the tube placed at the extremity of the abdomen. They are extremely lively and easily disturbed by any movement of the water, but soon resume their old position. Their body is elongated and has no legs. At first greenish, they soon become greyish and transparent. The larva undergoes several moults, three taking place in the first two or three weeks. In order to get rid of the old skin it places itself horizontally on the surface of the water with the back upwards. As a rule the change takes place through a rent on the thorax, and extends after to the abdominal segments. Like the larva the nymph is capable of swimming but can take no nourishment. When in repose it is contracted into a lenticular form, its abdomen being bent under the thorax and kept closely applied to it. It lies vertically in the water but in a different sense from the larva, as the
132 GNATS OR MOSQUITOES — CHAPTER VII humped-np thorax forms the highest part, and it is from the dorsal surface of that part of the body that the respiratory syphons spring, being in the form of two horns or asses' ears. Their upper extremity is cut obliquely, and when the pupa is at rest is always kept above water. The eyes are distinct, and beneath the thorax is found a large mass consisting of the antennae, mouth parts and legs. The abdomen is elongated, segmented, and terminated by two oval plates. If the nymph desires to go beneath the surface of the water it straightens itself and gives a few strokes of the tail, but is soon carried back to the surface, as soon as this has ceased to act, by its own buoyancy. After passing five or ten days in this state, the insect is ready for its last metamorphosis, but this is a most critical period of its life, as if the nymph case upsets during the process of the imago's freeing itself, the insect perishes by drowning, as it is now entirely unable to survive contact with the element in which, up to now,, it has passed its life. The moult that frees the imago from the nymph integuments takes place in the same manner as the preceding ones ; a rent appearing in the upper surface of the thorax through which the gnat protrudes first the head and thorax as much as possible above the aperture. The posterior extremity of the body now contracts a little, and extending itself immediately after, is gradually drawn out in a perpendicular plane. Meanwhile, the old skin of the nymph serves as a sort of boat of which its own body serves as the mast, only a very small portion of the hinder extremity touching it. Next, having drawn from their sheaths the four anterior legs and then the hinder ones, it carries them forward. Soon after it bends towards the water and arranges its limbs, and thus assured of safety, it unfolds its wings and flies off. When first it escapes it is whitish with the thorax greenish, but it very quickly after assumes the proper colours of the adult insect. Both larvae and pupse show a remarkable degree of resisting power to physical, and even chemical agencies, especially those of Culices, though those of Anopheles appear to bear cold better. In Celli's experiments on them, it was found
LIFE HISTORY AND SEASONAL PREVALENCE 133 that freezing for a couple of days, killed all the larvae, while no piipge survived more than twenty-four hours. Desiccation, on the other hand, was better borne by the nymphs, which in a few days, were transformed into very active Mosquitoes, in spite of being placed in dry river sand, so that the drying up of a pool does not stop the development of such insects as have reached this stage, while the larvse were all dead in two days if dried at 20° C, and both stages were killed by two minutes' exposure to a temperature of 40°. Experimenting in India, I found that larvae were usually dead and decomposed before the mud of the pool in which they had lived had dried up by ordinary evaporation. Nearly all species can live only in fresh water, but Bancroft found a species, which has been named marinus, by Mr. Theobald, living in sea water, and Ficalbi collected the larvae of C. nemorosus (salinus) in a salt marsh.
Celli found that Anopheles larvae died within thirteen hours in sea water, and could not live in mixtures stronger than two parts of fresh to one of sea water, which agrees with the results of Ficalbi's recent observations on the salt marshes of Cervia, where he found that though they abounded in neighbouring sweet water pools, they were never present in the actual salt pools, and were only exceptionally found in even slightly brackish water. Stephens and Christophers found that mangrove swamps never contained Anopheles larvae, so that the idea that these saltwater shallows are a cause of malaria is erroneous, though one can well understand that a saturated atmosphere, reeking with the emanations of vegetable putrefraction, may be extremely unhealthy for other reasons. Moreover, although they speak of them as being found in fresh water marshes, it is evident from what they write that, strictly speaking, they are no more to be found in such situations in Africa than they are in India, as they proceed to explain that they found them, not in the main marsh, but in the pools around it. In running water, unless the current be very slow, they cannot exist, and hence they are never found in rivers, except in extremely sluggish streams, such as the Cam, where Theobald mentions having once found them.
134 GNATS OK MOSQUITOES— CHAPTER VII The length of time required to develop from egg to imago varies greatly ; about a month in Italy, according to Celli, in the case of Anopheles, but usually much more rapidly in India.
In Mr. Aitken's paper, already quoted, he says : " I was anxious to ascertain next the length of the larva life, but found that it varied indefinitely with the conditions. Given warmth and plenty of food a larva will come to maturity in eight days, or perhaps less, but I have had one for more than a fortnight, and then it died before becoming a pupa. The time spent in the pupa state in all my specimens was more than twenty-four, but less than forty-eight hours. So I think we may put down the time which it takes to produce a Mosquito at something between ten days and a fortnight, from the laying of the egg ; and water, treated with kerosine oil once a fortnight, should be perfectly safe. It is very difficult to ascertain the normal life of the adult Mosquito, because we cannot keep them in natural conditions. Some of mine lived for ten days, but, as I have said, they would not lay their eggs, and that alone could not but affect the length of life. . . . To my astonishment I found y4?iOj9/<e/e.s larvse swarming in a dirty drain filled with rotting straw, which gave the water the colour of beer. In May, Colonel Weir sent me two or three bottles of water from other places in Bandora, with Anopheles larvae in them. The breaking of the monsoon, of course, upsets the haunts of all Mosquitoes for a time, but on the '2nd of July I went out to Chinchpoogly and explored the quarries at the foot of the hill, where Dr. Christy told me he had found larvae soine time before. I found larvae in many of the pools and one specimen in a little puddle, not more than four inches deep, which must, I am sure, have been as dry as bone ten days before, for we had a long period of hot, sunny weather, as you will remember, in the middle of June."
This agrees with such scanty observations as I was personally able to make in India, but, owing to the peripatetic nature of my duties, breeding-out observations were rarely practicable.
In England, and also in the somewhat similar tempera-
LIFE HISTORY AND SEASONAL PREVALENCE 135 tare of Indian hill stations, Gulicidoi larvae develop even more slowly than in Italy.
In Northern Europe all larvge and pupse that may not be ripe for their remaining metamorphoses, perish during the winter, but in Italy, according to Celli, the larvse of Anopheles " hybernate " (CM., p. 78 and 166) and some, at any rate, of the adult insects that appear early in the spring are furnished by larvae that have passed the winter in that stage. I am nearly sure that the same is the case in Northern India, though in a tank in my garden at Shahjahanpur, where I had found them present in swarms on my arrival, and which I had always kept supplied with water, so as to keep them under observation, they suddenly disappeared about January 11th, without any apparent reason, as the severest part of the "cold weather" was already past. Unfortunately I was unable to find any other pool, in which to continue observations, as a long spell of drought during the last three months of 1900, had completely dried all such pools as had remained under natural conditions. I have on one or two occasions noticed the larvae, though abundant a few days before in all stages, disappear from a pool in the same unaccountable manner, and strongly suspect that this must have been due to the outbreak among them of some epidemic disease.
Although they were thus capable of maintaining themselves as larvae, no imagines were being produced, and for six whole weeks not a single pupa could be found, though carefully sought for ; nor, so far as I could judge, did the individual larvae grow to any extent, as all sizes were present from the first, and the large ones did not appear to become more numerous.
The lowest temperature ever observed in this tank was 56' F. in the early morning, but the margin of temperature inimical to pupiation must be a very narrow one as, after a fall of rain accompanied by cloudy nights, during which the night temperature could not fall to the usual minimum, the higher temperature sufficed to bring about the appearance of a few pupae.
Now the rain had not perceptibly altered the depth of
136 GNATS OR MOSQUITOES — CHAPTER VII the tank, which was indeed filled periodically from the well — and so fluctuated more from this cause than it could do from any shower, — so that the occurrence has evidently no connection with the rain pei- se. Brought into the house, these pupae gave issue to their imagines, but I cannot say if this is also the case with those in the open — at any rate, none appeared in the bungalow.
Celli's observation may, however, be considered to practically settle the question, as it is little hkely that the larvae are unable to survive the mild cold weather of Upper India, if they can do so in the comparatively severe winter of Italy, and hence it may be accepted that in subtropical and the warmer parts of temperate climates, the survival of larvae is one method whereby the permanence of the species is secured, not only in Anopheles, but also in other gnats, as the same thing was undoubtedly the case with both C. fatigans, Wied., and G. impellens, Walk., although, in their case, the imagines were more or less in evidence through the entire period, and were, I believe, even breeding in a leisurable fashion whenever the weather turned a trifle warmer. During the short Danish summer, Meinert finds that Anopheles can find time for the rearing of but a couple of broods, and it is natural that in a limited period such as this, there should be more or less definite seasons of appearance of larvae, and that the flying insects should appear in periodical swarms ; but in Italy the process goes on more or less irregularly throughout the much longer summer, and the same is the case in Northern India.
In Italy, Celli says that the hybernating larvae pupate and turn into Mosquitoes in April, and the first generations of new larvee are found in the first half of May. With us, in Oudh, as we have seen, the new Mosquitoes put in an earlier appearance, though they rarely can find many opportunities to deposit their eggs till the advent of the rains, as even their own winter quarters must usually be dried up during the fierce heat of the months that intervene. Just as the process commences earlier with us, so it continues into the later months of the year, so that active Anopheles Mosquitoes are to be met with till quite the end