Nature Bulletin No. 76   July 27, 1946
Forest Preserve District of Cook County
Clayton F. Smith, President
Roberts Mann, Superintendent of Conservation

****:BLOW FLIES

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.