Habitat Of Larvae

It is perhaps not too sweeping a statement to assert that mosquitos (Anophelince) will breed in any collection of water if they are unable to find their natural breeding places elsewhere. That which in the early days of the mosquito malarial discovery was described as the "anopheles pool" no doubt exists, but it would be perhaps truer to state that the majority of the Anophelince are found in collections of water of all descriptions,-streams, canals, swamps, marshes, etc.,-which can hardly be designated by that name. Over and over again will the unwary observer be deceived if, after making a superficial inspection, he concludes that there is no possible breeding ground present. Man cannot exist without water, and where there is water, whatever its source, it would be rash to deny the existence of larvae until every nook and cranny had been thoroughly explored. The water may exist in a hidden cistern on the roof of the house, or may occur at the bottom of a forty foot well; yet in both these structures the larvae of Anophelince may be found. In the smallest collections of waters; in the tins filled with water that protect the legs of tables from the attacks of white ants in the tropics; in broken bottles used to adorn a chieftain's walls; in the collections of water in the leaves of tropical plants, such as the pitchers of pitcher plants; in the mouths of old disused cannon; in barrels; kerosene tins; in the smallest "pockets" of water collected in solid rock; in the bottom of old boats; in the foot prints of cattle; in the jars used for storing water against fire; in pig troughs; in fact, in every possible collection of water the larvae of the Anophelince may be found. Again in shallow puddles, "tanks," reservoirs, aboard ship in the bilge water, in lakes, disused quarries, streams, canals, swamps, marshes, deep wells, rivers, in brackish water and even in the sea-in all these places larvae may occur. We must, however, consider the question of the sources of larvae more closely, and point out with what reservations some of the above statements must be made. And, firstly, it must be recognized that a simple naked eye inspection of a source of water is insufficient if there is any floating matter to obscure the view, and even if there is not, minute larvae may escape notice. In all cases the larvae should be "fished" for by dipping into the water with some convenient utensil, such as a white enameled cup or ordinary tin mug. Now it is to a great extent true that the breeding places of the Anophelince and the other Culicidce are different. In a native village in the gold coast examined by Christophers and myself the natives used tubs for storing their drinking water. These tubs contained literally millions of Stegomyia larvae, but never Anophelince larvae. The latter bred in deep wells and in various small pools and larger collections of water a few feet deep, which existed even at the end of the dry season. These pools contained no visible weed, and were constantly exposed to the glare of the tropical sun. Similarly in Sierra Leone the source of Stegomyia larvae was in the collections of water about houses, in domestic utensils, barrels, tins, etc., but it was exceedingly rare to find the Anophelince breed in these, for, as we shall see later, many species exercise great selective power in their choice of a breeding ground, and the Anophelince larvae in Freetown bred in the streams and in their back eddies, in the numerous rock pools existing during the rainy season, and in the collections of waters in badly made drains, etc. So that, broadly speaking, it is true that the Anophelince select other breeding grounds than the other Culicidce, and this may depend upon whether the water provides suitable food for one or the other. On the other hand, this does not preclude their being found together, but it would appear to be commoner for the larvae of the other Culicidce to be found in what may be regarded as a typical breeding place for the Anophelince than to find the reverse; thus it is commoner to find, say, Culex and Anopheles together in a shallow pool than it would be in a tub. It is noteworthy also that when occurring together the proportion is not an even one, but there is frequently a preponderance of one or the other. But perhaps no strict statement could be made on these points. It is, moreover, true, on the whole, that anophelines are not found breeding in the foul, dirty waters affected commonly by Culex and Stegomyia, such as cess pits. The Anophelince prefer, no doubt, clearer water than the others, and there may be a further reason for their not occurring .together more frequently and in a more even proportion, viz., that they would be devoured by the more developed cannibalism of their more powerful relatives. Although, then, the Anophelince and the other Culicidce may be found breeding together, yet we require further observations before we can satisfactorily explain the matter. Again, in certain regions in the tropics, in native huts, the number of Anophelince may far outnumber the others, and this is probably the normal condition unless there is an unwonted supply of artificial breeding grounds, as in the instance noted above. So in many native huts examined by us in Africa and India the Anophelince swarmed in the houses, dotting the thatch like so many minute stalactites, and Culex, etc., might be difficult to find, and likewise the breeding grounds would yield almost entirely Anophelince larvae. What determines the distribution of the culicicles-in fact, why, for instance, in certain of the West Indian islands the Anophelince are absent though Culex, etc., abound-we cannot at present say. There are certain factors, however, which appear to determine the occurrence of larvae in particular waters. Thus in temperate climes they are not found so frequently in shaded pools as in waters exposed to the sun, but here again there is no hard and fast rule, for the larvae of the genus Lophoscelomyia are found in collections of water in split bamboos, where little or no sunlight reaches them. As expressed by Meinert, "elle n'aime pas l'ombre des grands bois mais recherche le soleil et la lumiere." In the tropics one is at first surprised to find that many likely breeding places yield no larvae. Some of the reasons for this are the following: (1) The source of water is too large; thus the larger "tanks" in India are often quite free from larvae, even where there are no fish. The probable reason is that they are too much exposed to the action of winds, though, on the contrary, one finds larvae in smaller pools on which one may see waves constantly breaking on a windy clay. It is possible that the depth of the water has some inhibiting effect, for, generally speaking, deep waters are not selected by mosquitos. (2) The explanation is often a simple one: the pools contain fish, many species of which are extremely destructive of larvae. (3) The pool or reservoir contains a coating of species of Lemna. Many observers have noticed the fact that excessive growth of Lemna is inimical to larvae. The explanation is generally assigned to a mechanical action on the larvae preventing their rising to the surface, but it has not, however, been shown that the imagos have really deposited their eggs in such waters, and often thev amount of Lemna present seems insufficient for it to have a mechanical action. With regard to the kind of water on which the Anophelince will lay eggs, Christophers and myself made some experiments in Calcutta. We could find, however, no selective action in the water chosen by M. rossii for depositing eggs. Pure water, green stagnant water, or foul sewage was indifferently selected. (4) Occasionally, also, larvae have been absent in artificial cement tanks, though present everywhere else in the neighborhood. One was forced to attribute such absence to the effect of wind, in the absence of any more satisfactory explanation. (5) Larvae, too, we found were generally absent in fresh spring water at its source, though this was not always so, for larva? were found by Christophers and myself at elevations of 7000 feet and 4000 feet in spring water, removed from human habitations, though in these cases, from the nature of the mountains in the dry weather, there was no other possible breeding places for miles around. Larva? have been found by various observers in brackish water; thus in Lagos they occur in water containing 0.6 per cent, of salt, but they have been found in water containing as much as 4 per cent, of salt in Algeria (A. maculipennis), and by Bancroft in Australia in the sea (N. annulipes).