All Culicidæ are aquatic in their larval and pupal stages. Almost all small collections of water, both natural and artificial, may form breeding grounds for these pests. Some even breed in pitcher plants and many in bromelias. The favourite resorts for the larvæ of Anophelina are small natural collections of water, such as puddles, ditches and small pools around swamps; certain species (A. maculipennis, etc.) live in rain barrels as well. They may also occur in the sluggish water at the edges of rivers or even in mid river, where the flow is checked by masses of water weeds (Myzomyia funesta, etc.). The Stegomyias prefer artificial collections of water, but also occur in natural pools. The yellow fever species (S. fasciata) prefers small collections, such as in barrels, pots, jars, etc. Culex occur in all manner of places—rain barrels, tanks, cisterns, ponds and ditches. Some of the South American species of Culex, Wyeomyia, Joblotia, etc., breed in the collections of water at the base of bromelia leaves.[387] Very few Culicid larvæ live in salt water except in Australia, where Dr. Bancroft has found them in salt water of specific gravity 1·040 (Mucidus alternans and Culex annulirostris). Other salt water mosquitoes are known in America. The food of the larvæ is very varied; the majority appear to feed upon confervæ, small crustacea and insects; some are cannibals, readily devouring others of their own kind. The larger larvæ of Megarhinus, Psorophora, Toxorhynchites and Mucidus are extremely ravenous and devour one another.
Fig. 395.—a, eggs of Culex; b1 b2, eggs of Anopheles; c, egg of Stegomyia; d, egg of Tæniorhynchus; e, egg of Psorophora.
There are two main types of larvæ, the Anopheline and Culicine; in the former there is no respiratory siphon, in the latter the siphon is long or moderately long. The head offers certain marked peculiarities which are of specific value; this especially applies to the Anophelina, in which the frontal hairs are of great service in distinguishing the larvæ,[388] whilst in Culex the number and position of the spines at the base of and on the siphon are characteristic. The position assumed by the larvæ in the water also varies in the different groups; most of the Anophelines lie horizontally, most of the Culicina and Ædeomyina hang head downwards. The pupæ also vary, but not to the same extent; the chief differences to be noticed are in the form of the two respiratory trumpets.
Fig. 396.—Diagram showing the structure of a typical mosquito. (Theobald.)
The eggs, which may be laid separately (Anopheles maculipennis, Stegomyia fasciata, Joblotia nivipes, etc.), or in rafts (Culex pipiens, C. fatigans) or in chains (Pseudotæniorhynchus fasciolatus), present a great variety of forms. The most peculiar are shown in fig. 395 (Tæniorhynchus, Culex, Stegomyia, Anopheles, Psorophora).