The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

botfly
    n 1: stout-bodied hairy dipterous fly whose larvae are parasites
         on humans and other mammals