In about a fortnight the larva is fully grown and becomes a pupa. The pupa (fig. 393), which moves in jerky movements, remains in the water, but partakes of no food. In shape it somewhat resembles a tadpole, that is to say, it consists of a bulky anterior portion, on the surface of which the head, with its appendages, is recognizable, and a more slender segmented abdomen. Above, on the thorax, there are two small trumpet-shaped breathing tubes for the conveyance of air to the tracheal system. After three or four days the perfect mosquito hatches out, remains a short time on the surface of the water until its chitinous integument is hardened, and then flies away.
The females that are fertilized in the autumn hibernate in sheltered spots in the open air, or in houses, cellars, under stairs, in stables, barns, etc., and are the progenitors of the first generation of the following year.
In accordance with the climate of a country, or the kind of weather of a year, the conditions in regard to the manner of life and the duration of the development of the mosquito vary. At all events, the life-history of the mosquito elucidates many points relating to malaria which were hitherto not understood.
[The length of the egg, larval and pupal life varies so much that it is not possible to give an account of any value here. Frequently the eggs may incubate in two days, whilst I have had Stegomyia fasciata eggs from Cuba that have hatched out under abnormal circumstances more than two months after they were laid (“Mono. Culicid.,” iii, p. 6). Some larvæ, as Anopheles bifurcatus, live for months during the winter. Some mosquitoes therefore hibernate as larvæ. The larvæ and pupæ of the different genera present very marked characters, mainly in regard to the structure of the siphons. Specific differences may be found in the frontal hairs of Anopheline larvæ and in the number and arrangement of a group of spines at the base of the siphon in Culicines.—F. V. T.]
Culicidæ or Mosquitoes.
The importance of these insects to man is very great. They not only produce painful bites, which may become inflamed and give rise to a considerable amount of œdema, but they are more important on account of the part they play in the distribution of various diseases. Culicidæ may not only carry disease germs, but act as intermediate hosts for certain parasites, such as some of the Anophelina for malarial parasites, Culex for Filariæ, and Stegomyia for yellow fever, etc.; the last-named is in any case the distributor of that fatal disease. It is therefore very necessary to know the life-history, habits and characters of these pests.
Mosquitoes exist in almost all parts of the world from the Arctic circle to the tropics; temperate regions suffer from them less than the two extremes, but even there they form not only a source of great annoyance but of danger as malaria and possibly now and again yellow fever carriers. A few years ago comparatively few species were known, now some 800 odd have been described. Their number will probably not stop far short of 1,000, in spite of the fact that many have been described under different names, yet really the same species. Some are purely domestic, others entirely sylvan; the former, as we might expect, often have a very wide distribution, having been taken from place to place in boats and trains. The more rapid transport becomes, the greater becomes the possibility of this wide distribution of many species increasing, and the spread of other species from their natural home to foreign parts by sea and then by trains further inland.
Fig. 394.—Heads of Culex and Anopheles: (1) Culex male; (2) Culex female; (3) Anopheles male; (4) Anopheles female. (After Daniels.)