From The Collaborative International Dictionary of English v.0.48 [gcide]:
Botfly \Bot"fly`\, n. (Zool.)
A dipterous insect of the family ({Estrid[ae]}, of many
different species, some of which are particularly troublesome
to domestic animals, as the horse, ox, and sheep, on which
they deposit their eggs. A common species is one of the
botflies of the horse (Gastrophilus equi), the larv[ae] of
which (bots) are taken into the stomach of the animal, where
they live several months and pass through their larval
states. In tropical America one species sometimes lives under
the human skin, and another in the stomach. See Gadfly.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
botfly
n 1: stout-bodied hairy dipterous fly whose larvae are parasites
on humans and other mammals