The eggs (figs. 4, 5) are laid upon horse manure. This substance seems
to be its favorite larval food. It will breed also in human excrement,
and because of this habit it is very dangerous to the health of human
beings, carrying as it does the germs of intestinal diseases, such as
typhoid fever and cholera, from the excreta to food supplies. It has
also been found to breed freely in hog manure, in considerable numbers
in chicken dung, and to some extent in cow manure. Indeed, it will lay
its eggs on a great variety of decaying vegetable and animal materials,
but of the flies that infest dwelling houses, both in cities and on
farms, a vast proportion come from horse manure.
[Illustration: FIG. 4.--Eggs of the house fly. About natural size.
(Newstead.)]
It often happens, however, that this fly is very abundant in localities
where little or no horse manure is found, and in such cases it breeds in
other manure, such as chicken manure in backyard poultry lots, or in
slops or fermenting vegetable material, such as spent hops, moist bran,
ensilage, or rotting potatoes. Accumulations of organic material on the
dumping grounds of towns and cities often produce flies in great
numbers.
[Illustration: FIG. 5.--Eggs of the house fly. Highly magnified.
(Newstead.)]
The house fly begins laying eggs in from 2 1/2 to 20 days after
emerging, the time interval depending to a large extent upon
temperature, humidity, and character and abundance of food.
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