Nightmare Maggot

This creature has an inverted life cycle. Technically, it is a fly, although the dominant stage in their life cycle is the larva. A huge, carnivorous, toothy parasitic and predatorial larva.
The fly is the size of a baseball, and when it stings, it is very painfull, it injects its eggs about two inches deep through a needle like sting that sticks in the body of the victim, and drops dead immediately.

The eggs will incubate for 2 days, and the larva will go out and starts a parasitic stage in which it will start growing and chewing the host from the inside out. The host will die in about 1 month, and the larva will continue eating the carcass from the inside, until eventually making the corps burst, emerging as a 2 m long, and half a meter thick chitinous and carnivorous maggot, that moves fast as a snake, capable of hunting prey for years, biting it to death with its thousand small yet razor sharp teeth.

Eventually, the maggot will dig a hole and start shrinking quickly, until transforming again into this baseball sized fly, that will last alive for some weeks, in which it has to mate and lay eggs. The male flies die after mating, and the female fly eats its body.
The eggs can be removed through surgery, but once they hatch, it's too late.