The botfly is a family of Oestroidea. It is one of several families of hairy flies whose larvae live as parasites within the bodies of mammals. There are approximately 150 known species worldwide.
Only one bot fly species attacks humans, the Dermatobia hominis.
Botflies deposit eggs in a host body, or sometimes use an intermediate vector: common houseflies for example. The smaller fly is firmly held by the botfly female and rotated to a position where the botfly attaches some 30 eggs to the body under the wings. Larvae from these eggs, stimulated by the warmth of a large mammal host, drop onto its skin and burrow underneath.
Eggs can be deposited in animal skin directly, or the larvae drop from the egg: the body heat of the animal induces hatching upon contact. Some forms of botfly also reside in the digestive tract when consumed by a licking action.
Mature larvae drop from the host and complete the pupal stage in soil. They do not kill the host animal, and thus are true parasites. (Wiki)