ng more species, and
systematists everywhere have been describing new ones until now the
total number of described species is probably over five hundred, more
than sixty of which occur in the United States.

This shows only one phase of the great interest that has been taken in
the mosquitoes since the discovery of their importance as carriers of
disease. Not only have they been studied from a systematic standpoint
but an endless amount of work has been done and is being done in
studying their development, habits, and structure until now, if one
could gather together all that has been written about mosquitoes in the
last ten or twelve years he would have a considerable library.

[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 larvae (_Theobaldia
incidens_); two larvae feeding on bottom, others at surface to breathe.]

[Illustration: FIG. 54--Mosquito larvae (_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.

THE EGGS

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.

THE LARVAE

When the larvae are ready to issue they burst open the lower end of the
eggs and the young wrigglers escape into the water. The larvae 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 en