[Illustration: FIG. 50--Pupa of house-fly with the end broken to allow the fly to issue.]
[Illustration: FIG. 51--Head of stable-fly showing sharp piercing beak.]
[Illustration: FIG. 52--Mass of mosquito eggs (_Theobaldia incidens_).]
[Illustration: FIG. 53--Mosquito eggs and larvæ (_Theobaldia incidens_); two larvæ feeding on bottom, others at surface to breathe.]
[Illustration: FIG. 54--Mosquito larvæ (_T. incidens_), dorsal view.]
Those who are particularly interested in the group will find some of these books and papers easily accessible, so there may be given here only a brief summary of the more important facts in regard to the structure and habits of the mosquitoes in order that we may more readily understand the part that they play in the transmission of diseases and see the reasonableness of the recommendations in regard to fighting them.
Mosquito eggs are laid in water or in places where water is apt to accumulate, otherwise they will not hatch. Some species lay their eggs in little masses (Fig. 52) that float on the surface of the water, looking like small particles of soot. Others lay their eggs singly, some floating about on the surface, others sinking to the bottom where they remain until the young issue. Some of the eggs may remain over winter, but usually those laid in the summer hatch in thirty-six to forty-eight hours or longer according to the temperature.
When the larvæ are ready to issue they burst open the lower end of the eggs and the young wrigglers escape into the water. The larvæ are fitted for aquatic life only, so mosquitoes cannot breed in moist or damp places unless there is at least a small amount of standing water there. A very little will do, but there must be enough to cover the larvæ or they perish.
The head of the larvæ of most species is wide and flattened. The eyes are situated at the sides, and just in front of them is a pair of short antennæ which vary with the different species.
The mouth-parts too vary greatly according to the feeding habits. Some mosquito larvæ are predaceous, feeding on the young of other species or on other insects. These of course have their mouth-parts fitted for seizing and holding their prey. Most of the wrigglers, however, feed on algæ, diatoms, Protozoa and other minute plant or animal forms which are swept into the mouth by curious little brush-like organs whose movements keep a stream of water flowing toward the mouth.