There can be little doubt that mosquitos, as other diptera, dislike wind, though we shall consider later exceptional instances where mosquitos are transported long distances by the wind, yet ordinarily when the wind blows, one is free from their visits. Similarly, heavy rain is to their detriment, while slight rain may stir them to activity after a period of drought, and rain may apparently make Anopheles more numerous, as they seek shelter from it in houses.
That anophelines can fly at a height of 20 feet from the ground as a common occurrence seems probable, as they are found in dwelling rooms at that height; they have also been observed at a height of 30 feet, but the limit cannot at present be stated.
In the tropics they can probably exist for two months or more in the complete absence of breeding grounds during the dry season, though very careful observation is necessary before all possible sources of water can be excluded.
The great source of anophelines in the tropics is the native huts of the aborigines. Here they abound in myriads. They, moreover, actually prefer the strong odor of natives, as observations made by Christophers and myself in Sierra Leone showed. We kept a tent under observation; while this was tenanted by a European a few anophelines only were caught, in the morning; when, however, a negro slept in it, on the first morning 19 were caught, on the second, 62; when again the European occupied it, the numbers rapidly fell. Blue serge cloth, it is well known, anophelines and other mosquitos have a special liking for, and yellow cloth they least like, but this may be rather a matter of color than of smell. Leather goods, old boots, etc., seem to exercise a peculiar attraction.
It was originally supposed that mosquitos, when they had laid their eggs, died. We shall see that this is not so, but that the process can be repeated several times if the mosquitos are properly fed. The observation is, however, probably true of those mosquitos which have hibernated during the winter (in a fecundated state). On the return of spring they sally forth and lay their eggs and die.
Our knowledge on this subject is slight. We have seen already that the breeding grounds of different species may often be different; thus, M. rossii breeds in shallow muddy pools in stagnant water, whereas M. listoni breeds in fresh stream water. Now, the disappearance of M. rossii from certain areas during the dry season is determined by the fact that all these stagnant pools may be completely dried up, and it is not until we eventually reach a village or district having suitable pools that we find this species. So, again, if the streams which held the larvae of M. listoni dry up, then this species becomes scanty or even absent. So, again, the distribution of Mr. nigerrimus (and probably P. superpictus) is determined almost entirely by the formation of large swampy areas of weedy marsh. We have but few records of this seasonal prevalence in the literature of mosquitos. Thus Theobald quotes a statement to the effect that" N. prcetoriensis first appeared about February 10, and gradually became more prevalent, superseding the other common species, P. cinereus." Again, "In the winter, up to June, one only sees C. pipiens. These then disappear, and P. chaudoyei appears."
In temperate climates mosquitos pass the winter in two ways: (1) By hibernation of larvae; (2) by hibernation of adults. The former condition we have already dealt with. Annett and Dutton have described the finding of A. maculi pennis in cold cellars and outhouses during the winter, but not in warmer stables. The hibernating insects are all females, and these are always fertilized. If the females are roused from their sluggishness by bringing them into a warm room, they will feed, and after a time lay eggs. If, however, the temperature be kept low, they do not arouse themselves and do not feed. Grassi found, with regard to the hibernation of A. maculipennis in Italy, that the insects were most numerous in heated rooms, stables, houses, hen roosts, outhouses. They begin to disappear about February, and almost completely in March, but, according to Macdonald, in Spain not until May or June. Probably they have aroused and laid their eggs and died. According to Schoo, in Holland the hibernating Anophelince lay their eggs in February and March, but he states that the infected Anophelince lay no eggs (how this was determined is not stated), and that the latest new infections occur in October and November. Similarly Thayer has found A. crucians and A. maculipennis hibernating in enormous numbers in barns in New Orleans. In Europe the winged insects of A. maculipennis appear about June. The occurrence of isolated cases of new infections with malaria in Italy during winter and spring is attributed to the agency of these hibernating anophelines. In the early months some of these may fly abroad, as early as February on warm days, and, further, as Koch has pointed out, in thatched houses where anophelines exist the temperature is high enough to allow of development of the parasite in the mosquito. One fact, however, has not been established, that these hibernating anophelines actually do contain sporozoites in their glands, and, in fact, according to Schoo, they do not. And Macdonald's observations in Spain were also negative in this respect.* We may mention here incidentally that Macdonald found the percentage of infected anophelines to be 7 per cent, in June and 18 per cent, in July and August.
Though the anophelines, on the whole, are distinctly nocturnal in their habits, yet there are exceptions to this. The time of feeding appears to be chiefly in the early night and at dawn. Some species appear to be particularly active at dawn; thus Christy in Uganda observed that Ce. squamosa and Ce. pharoensis blackened the roof of his tent at dawn, though none were to be seen on the previous evening. So of A. bifurcatus, Blanchard says that "it bites freely at dawn and dusk, but at night practically not at all." In outhouses and shady places A. bifurcatus will also bite during the day time, and there are several observations to show that other species also do so. Thus in Nigeria and Kamerun Anophelince (? species) have been found biting in the day time, and M. rossii also occasionally does so in India, but these occurrences are exceptional, for Christophers and myself, while capturing Anophelince in hundreds of native huts in many parts of Africa and India, have never been bitten under these conditions.
* Suzuki, in Japan, when the temperature of the external air in January was 7° C, caught an anopheline " asleep" in his house. With this he states that he infected himself with malaria .