The flies lay eggs.
The eggs become maggots, which are eaters of decayed flesh.
Maggots are now part of a treatment for people with non-healing/slow-healing ulcerations and wounds. Maggots only eat decayed flesh, leaving healthy tissue untouched. Of course, the maggots used are not picked off a pile of garbage. they are bred in sterile conditions and carefully counted during the procedure and accounted for after the treatment.
When the dead tissue is eaten, healthy tissue has a better chance of forming. Less chance of gangrene occuring.
Pretty cool stuff, actually. Who'd'a thought.
http://www.google.com/search?q=maggo...utf-8&oe=utf-8