Forest Preserve District of Cook County, Illinois
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Blow Flies
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.

Blowflies 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.

Flies 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.

Blow 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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