This is the head of a blow fly, green bottle fly or calliphorid fly, depending upon your preference. They are especially attracted to carrion and feces (yes, that's a fancy word for poop). The thing sticking out to the left if its "sponging" mouthparts, used to sop up juices from its favorite foods. Larvae, otherwise known as maggots, have been used in modern times to clean up large wounds on people, as they eat the dead and rotting flesh and leave the healthy tissue alone. Maybe I should have saved this for Halloween, but I just couldn't resist. There are live pictures and more information available here http://entnemdept.ifas.ufl.edu/foltz/eny3005/lab1/Diptera/Calliphorid.htm
http://bugguide.net/node/view/7175