The time taken by the eggs to hatch is a variable one, dependent very largely upon the temperature and probably upon the genus and even species of mosquito. In Havana, Taylor gives as the duration of the egg stage for S. fasciata twelve to twenty four hours or longer, C. jamaicaensis, twelve hours; C. sollicitans, twelve hours; Ce. argyrotarsis, one and a half days. Again, the eggs of Culex pipiens are stated by Theobald to hatch in twenty hours (temperature not stated). Similarly, Howard, for Culex pungens, gives the time as from one night (probably early morning) to 2 p.m. the same afternoon, but in colder weather the time may extend to one or two days, and Giles states that Culex sp. eggs take two to three days, but the shorter times are nearer the mark for a temperature of 86° to 96°. It is noteworthy that although eggs normally float, yet in certain species they will still hatch out even if submerged.
Occur in the tropics in the water butts around inhabited houses, and their jerky movements downward when disturbed are a familiar sight. These larvae are mostly those of other subfamilies than the Anophelince, though the latter also will breed in tins, such as those protecting the legs of tables from white ants; and, as we shall see later, some even of the Anophelince seem to select the water in tins in preference to the water of streams, pools, or ditches, near at hand. The larvae of the Anophelince are readily told from those of other subfamilies by the position they take up on the surface of water: whereas those of the Culicince, for instance, lie with head dependent and at an angle to the surface of the water, those of the Anophelince lie flat under the surface film of water and indent the latter. The head of the Culicince larvae is relatively large and heavy, and at the hind end they possess a rather long breathing spiracle, the extremity of which comes to the surface. In the Anophelince this tube is absent, but there is a pair of stigmata on the eighth segment. The flat position, moreover, of the Anophelince is maintained by a number of paired structures on either side of several of the segments of the larvae, termed palmate hairs. These are fan shaped, and when the larva is floating, are opened out, suspending the larva under the surface film. The exact attitude of the larvae differs much in the different genera, but data on this point are scarce; likewise it should be noted that the Anophelince do not always float horizontally. When moulting (for larvae cast their skins three or four times), the position is a dependent one, and from its exceptional characters the larva of M. turkhudi always adopts this attitude, but anopheline larvae can be readily distinguished by the absence of a respiratory siphon and by the presence of palmate hairs.
Larvae of other flies may, at first sight, be mistaken for Culicid larvae, especially those of Dixa, Corethra, and Chironomus. The resemblance is, however, only superficial: (1) The Dixa larva generally assumes a u shape. The body consists of 11 segments. On the fourth and fifth segments are two pairs of pseudopods, armed with hooks. These structures are not present in the larvae of the Culicidce; further, there is no siphon tube, but a respiratory cup on the last segment; also some of the posterior segments have dorsal shields. (2) The larvae of Chironomus, or the "blood worm," consist of a head and 12 segments. The first segment has feet armed with hooks. There are also hooks on the hindermost segment. The last two segments bear small, flexible tubes filled with blood. Moreover, there are two kinds of Chironomidce larvae: (a) Those that live on the surface, possessing no tubule and with colorless blood, the pupae of which have a respiratory tuft; (6) those that live deep in the water, with two respiratory tubules, with red blood, and the pupae of which have trumpets. (3) The larvae of Corethra, or phantom larvae, so called from their great transparency, are cannibalistic in their habits and extremely rapid in their almost invisible movements. There are no feet and no respiratory siphon. At the posterior extremity there is a somewhat complex fin structure. The horizontal position is maintained by two pairs of air sacs, one situated on the thorax, and the other pair on the seventh abdominal segment. (4) Larva of Moch lonyx: the air vesicles are greatly enlarged and the trachea remain pervious, as in Culex. It has a respiratory siphon less developed than in Culex. The larvae of the Culicidce, except the Anophelince, have a long respiratory siphon traversed by the tracheal tubes. The length and shape of this tube differ greatly in the different genera; thus it is long and slender in Culex; short and truncated in Stegomyia. The siphon is adorned with hairs and spines, which also differ in the various genera, and, moreover, these differences are of specific as well as generic importance. Further, in the shape of the larval head, in the length of the antennae, and in the characters of their hairs we have a means by which we can certainly differentiate a mosquito (genus) in the larval stage. Thus, in Culex sp. there is, at the junction of the middle and terminal third, a tuft of hairs which is absent in Stegomyia sp., while in the latter there is a small single hair about the middle. The antenna of Stegomyia is spineless, while that of Culex is spiny. Again, the large curved antenna of Tceniorhynchus is characteristic, but so far few larvae have been fully described. The mental plates or lower lip of Meinert are also of generic importance, and the shape of the tooth and its indented margin differ in various genera. These are the main points to which attention should be paid in the description of larvae.