On the eighth segment are situated the openings of the tracheal system. There is in the Anophelince no projecting tube, as in other Culicidce, but the openings are situated on the surface of a specialized area. This area is approximately quadrilateral in shape, and is bounded laterally by chitinous plates armed with teeth. No differences in the disposition, number, etc., of these teeth have so far been described in different species. The stigmata lie in the anterolateral corners of this quadrilateral area, and are protected by a hinged flap which bounds this area anteriorly. This flap, when the larva is breathing, is folded forward so that its dorsal surface looks anteriorly, and the stigmata are uncovered and are directly in communication with the air. When the larva proceeds to sink, the flap is folded back and the continuity with the atmosphere is broken. The variations in this most interesting structure have been so far not at all studied, and no doubt many differences exist in different species.
Lastly, on the ninth abdominal segment we have the opening of the anus. This segment is cylindric in shape and terminates posteriorly in a somewhat flattened surface, on the center of which opens the anus. Arising above the anus are four long branched hairs; they project posteriorly and overhang the anus. Arising below the anus, on a curved chitinous base on either side, is a large fan of branched hairs which, in the normal position, hangs ventrally downward and resembles a rudder. Further, arising from around the anus itself we have the four transparent, finger like processes called the anal papillae, in which tracheal tubes are seen ramifying; two of these are ventral, two dorsal, symmetrically placed. In these structures and in the various hairs of these regions there does not appear to be much variation in different species.
The movements of the larva? of the Anophelince are, as we have already said, characteristic when disturbed; they may "skate" along the surface horizontally, progressing with backward jerks, or, if they seek the bottom, they rapidly dart below with forcible wriggles. They lie at the bottom quite motionless, and may be poked about before they will give any signs of life. Undoubtedly this lifeless position is assumed by them for protective purposes. In a few minutes they usually rise, but their immersion may be prolonged to one quarter of an hour. The motion to the surface is effected by a series of backward jerks until the surface film is penetrated, when often at once the head rotates, the mouth faces dorsally, and the brushes begin their vigorous sweeping movements.
Here again no absolute value can be given, the duration being determined by at least two main factors: (1) Food; (2) temperature. That food is a very important factor is shown by laboratory experiments, where if the larvae are kept in a glass dish filled with tap water only, little or no growth is effected; while again temperature is of great importance, as shown by the " hibernation " of larvae through the winter in temperate climes or through the "cold" weather of the tropics. The larval stage of A. maculipennis was found by Nuttall and Shipley to last eighteen to twenty one days, at a temperature, at first, of 16° to 19° C, and later of 23° to 26° C. Similarly Howard gives the larval stage of A. maculipennis in America as sixteen days, while Grassi, for the same species in Italy during the summer, gives the duration as twenty to twenty two days. Similarly for Ce. argyrotarsis Lutz gives the time of total development as three weeks at a temperature of 25° C. That the duration of the larval stage in the tropics is less than in temperate climes is shown by the examples, already quoted, of two anophelines, viz., Cellia argyrotarsis, twelve days; M. rossii (temperature of water, 96° to 102° F.), eleven days.
Celli and Casagrandi have made experiments on this point and find that anophelines can resist desiccation at 20° C. for two days; at 35° C. for one day; and at 40° C. for two minutes only, while also the pupae cannot survive longer this degree of temperature. It is interesting also to note that larvae were found to exist on moist earth for four days. Christophers finds that larvae stranded on mud which had dried so far as to lose its glistening surface did not come to life on the addition of fresh water.
During this time the larva undergoes one or more moults. Not infrequently during this process the position of the larva is not quite horizontal, but is slightly inclined toward the surface, but at an angle not so great as that of Culex. The larval stage of the Anophelince may, however, be prolonged much beyond these limits. Thus in the case of A. bifurcatus some larvae caught by Nuttall and Shipley on September 14, 1900, were kept through the winter; only one survived, and that pupated on March 19, 1901 (the imago taking eleven days longer to hatch), the larval stage thus lasting seven months; and there are a number of observations which show that this is one method, viz., the hibernation of larvae, by which mosquitos survive during the winter.
James, in Lahore, India, found that in permanent breeding places larvae of M. culicifacies could be found in a hibernating condition throughout the winter (temperature, 55° F. about); their movements were sluggish and they grew extremely slowly, if at all. None were seen to develop into pupge, but it is not safe to conclude that this applies to all species, for it was not so at Lahore, where in the case of Ce. pulcherrima and N. fuliginosus, eggs, young larvae, and pupae were found in new breeding places formed during the winter. That larvae are capable of withstanding low temperatures is shown by Galli-Valerio's and NarbeFs observations. They found the larvae of A. bifurcatus, but not of A. maculipennis, below the ice in Switzerland. Others have actually found larvae of Culex pungens frozen within the ice. When thawed out, the larvae were alive and eventually developed into imagos. But it must not be inferred from these observations that this is the sole method by which mosquitos tide over the winter. For instance, Grassi observed that A. bifurcatus hibernated chiefly in the larval form, finding the larvae in midwinter in Italy, but the imagos rarely, but that this was not the case with A. maculipennis: no larvae of this species could be found (in Holland they first appear about the middle of May), while the imagos were plentiful in outhouses, huts, etc. So, also, Nuttall and Shipley found that of a collection of larvae of A. bifurcatus and A. maculipennis kept in the laboratory the larvae of the latter all died during the winter, whereas the former lived. So, also, Theobald observed a number of larvae of A. bifurcatus during November in England, and found that by the end of December only 2 per cent, had pupated. It is not likely that any seasonal cessation from pupation occurs in the tropics, except where there is a definite "cold weather." We shall see later that the imagos can exist for some time during the "dry season," when no opportunity for breeding occurs.