The true house fly (fig. 3), which is found in nearly all parts of the
world, is a medium-sized fly with four black stripes on the back and a
sharp elbow in one of the veins of the wings. The house fly can not
bite, its mouth parts being spread out at the tip for sucking up liquid
substances.
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. 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 larvæ (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 feeding ground in search of a favorable place in
which to pass the pupa stage. They will often congregate at the edges of
manure piles near the ground or burrow into the soil beneath, or they
may crawl considerable distances away from the pile to pupate in the
ground or in loose material under the edges of stones, boards, etc.
[Illustration: FIG. 6.--Larvæ, or maggots, of the house fly. About
natural size. (Newstead.)]
The pupæ (fig 7), or "sleepers," are more or less barrel shaped and dark
brown in color. In midsummer this stage usually lasts from 3 to 6 days.
The pupa stage is easily affected by temperature changes and may be
prolonged during hibernation for as long as 4 or 5 months. Numerous
rearing experiments in various parts of the country have shown that the
shortest time between the deposition of eggs and the emergence of the
adult fly is 8 days, and 10 and 12 day records were very common.
The adult fly, upon emerging from the puparium, works its way upward
through the soil or manure and upon reaching the air it crawls about
while its wings expand and the body hardens and assumes its normal
coloration. In from 2 1/2 to 20 days, as previously stated, the female
is ready to deposit eggs. As in the case of other periods of its life
history, so the preoviposition period is prolonged considerably by the
lower temperatures of spring and fall. In midsummer, with a
developmental period of from 8 to 10 days from egg to adult, and a
preoviposition period of from 3 to 4 days, a new generation would be
started every 11 to 14 days. Thus the climate of the District of
Columbia allows abundance of time for the development of from 10 to 12
generations every season.
[Illustration: FIG. 7.--Pupæ of the house fly. About natural size.
(Newstead.)]
Flies usually remain near their breeding places if they have plenty of
food, but experiments recently made at Dallas, Tex., show that they may
migrate considerable distances; in fact, house flies, so marked that the
particular individuals could be identified, have been recaptured in
traps as far as 13 miles from the place where they were liberated.