Nature Bulletin No. 76 July 27, 1946
Forest Preserve District of Cook County
Clayton F. Smith, President
Roberts Mann, Superintendent of Conservation
The big flies you sometimes find buzzing loudly and crazily around in
your basement, garage, barn, or even your living room, are not
overgrown house flies. They are blow flies. Of the several species in
this region the most common are the Blue Bottle Fly, Green Bottle Fly,
Black Blow Fly and Screw-Worm Fly.
Adult flies do not grow after they emerge from the pupa case. They lay
small white or ivory-colored oblong eggs which hatch, in a day or two
at ordinary temperatures, into maggots. These feed and grow and shed
their skins several times before they form a brown pupa case shaped
like a medicine capsule. Inside this case a transformation takes place
which results grown fly. In the case of the blow flies the whole process,
from egg to adult fly, takes from 14 to 18 days.
usually breed and feed on the bodies of dead animals, on
decaying meat, fresh meat, or cow dung. The bright metallic-green
screw-worm fly also lays its eggs in wounds and sores, as well as in the
nostrils ears of cattle and humans. Two species are blood-sucking
parasites on nestling birds. Certain blow fly larvae infest the tails of
sheep in this country and that is why our lambs' tails are cut off.
have but one pair of wings, whereas most other insects have two
pairs. They have blood composed of plasma and white corpuscles but
no red They have a heart but instead of lungs they have trachea through
which is piped directly to the muscles and other active organs. They
also have nerves and a brain.
flies have very keen senses for locating dead animals, b by smell.
An animal will not be dead very long, out-of-doors, before there are
several blow flies on it. Their maggots reduce a carcass to bones, claws
and scraps of hide in short order. During World War I a method was
developed for using blow fly maggots to treat gangrenous wounds.
There used to be a "maggot factory" for this purpose in Oak Park.
An interesting thing about blow flies is that in one or two species the
eggs are held in the body of the female until they hatch and then are
dropped as living young.
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Update: June 2012