any of a number of dipterous flies (families Cuterebridae, Gasterophilidae, and Oestridae) resembling small bumblebees: the larvae are parasitic in people, horses, sheep, and other mammals
See botfly in American Heritage Dictionary 4
also bot fly(bŏtˈflīˌ)
noun
Any of various stout, two-winged flies, chiefly of the genera Gasterophilus and Oestrus, having larvae that are parasitic on various animals, especially horses and sheep, and sometimes on humans.