ly.]

LIFE-HISTORY AND HABITS

The eggs of the house-fly may be laid on almost any kind of decaying or
fermenting material. If this is kept moist and a proper temperature
maintained the larvae or maggots (Fig. 47) that hatch from the eggs may
develop. As a rule, however, these requirements are found only under
certain conditions and are ordinarily found only in manure heaps or in
privy vaults or latrines. All observers agree that the female fly
prefers to deposit her eggs in horse manure when this can be found and
when this is piled in heaps in the barn-yard (Fig. 48) or in the field
the heat caused by the decay and fermentation makes ideal conditions for
the development of the larvae. Cow manure may serve as a breeding-place
to a limited extent. The flies are immediately attracted to human
excrement and breed freely in it when opportunity offers. Decaying
vegetables or fruit, fermenting kitchen refuse and other materials
sometimes also serve as breeding-places.

In suitable places in warm weather the eggs will hatch in from eight to
twelve hours and the larvae will become fully developed in from eight to
fourteen days. They then change to pupae (Fig. 50) in which stage they
may remain for another eight to twenty days when the adult flies will
emerge. These figures must necessarily be indefinite because the weather
and other conditions always vary. Under the most favorable conditions of
moisture and temperature it is probably never less than eight days from
egg to adult fly and under unfavorable conditions it may be as long as
six weeks.

The larvae thrive best when the manure is kept quite wet. I have often
found them in almost incredible numbers in stables that had not been
cleaned for some time. The horses standing there at night added fresh
material and kept it just wet enough to make conditions almost ideal
(Fig. 49).

The pupae are usually found where the manure is a little dryer, but it
must not be too dry. When the flies issue from the pupae they push their
way up to the surface where they remain for a short time and allow the
body to harden and the wings to dry before they fly away to other manure
or, as too often happens, to some near-by kitchen or restaurant or
market place.

[Illustration: FIG. 48--Barn-yard filled with manure. Millions of flies
were breeding here and infesting all the near-by houses.]

[Illustration: FIG. 49--Dirty stalls; the manure had not been removed
for some days and the flo