e 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. The number
of eggs laid by an individual fly at one time ranges from 120 to 159 and
a single female will usually lay two and sometimes four such batches.
Dunn has recently reported that in Panama a fly may deposit as many as
2,367 eggs in 21 batches, and sometimes an interval of only 36 hours may
occur between the deposition of large batches of eggs. The enormous
numbers in which the insects occur are thus plainly accounted for,
especially when the abundance and universal occurrence of appropriate
larval food is considered. The eggs are deposited below the surface in
the cracks and interstices of the manure, several females usually
depositing in one spot, so that the eggs commonly are found in large
clusters (fig. 4) in selected places near the top of the pile, where a
high degree of heat is maintained by the fermentation below. The second
batch of eggs is laid from 8 to 10 days after the first. The eggs
usually hatch in less than 24 hours. Under the most favorable conditions
of temperature and moisture the egg state may last hardly more than 8
hours. The maggots which issue from the eggs are very small and
transparent. They grow rapidly, completing the growth of the larva stage
in three days under the most favorable conditions, although this stage
usually lasts from 4 to 7 days. The larval period may be prolonged
greatly by low temperature or by dryness or scarcity of the larval food.
As the larvae (fig. 6) attain full size they gradually assume a creamy
white color. A few hours before pupation they become very restless and
migrate from their f