Subject: In 'maggot therapy' why do maggots only eat decayed-not healthy flesh?

Since the time of Napolean it has been known that maggots that infest 
the wounds of soldiers actually had a beneficial effect on the wound.
Today in certain bad wounds--especially with diabetics - maggot 
therapy is being used again. Certain hospitals are using sterilized 
maggots to eat out decaying wounds. Maggots will only eat decaying 
flesh and not healthy tissue. How do the maggots know to only eat 
decayed flesh and not the healthy flesh? What is it about the healthy 
flesh that the maggots don't like or how do they tell the difference? 
Also how do they get all the maggots out of the wound when they have 
done their job? Do they turn into flies and fly away? 

Re: In 'maggot therapy' why do maggots only eat decayed-not healthy flesh?