bot·fly / ˈbätˌflī/ • n. (pl. -flies) a stout hairy-bodied fly with larvae that are internal parasites of mammals, in particular: ∎  a fly (Gasterophilus and other genera, family Gasterophilidae) with larvae (bots) that develop within the guts of horses. ∎  a fly of the warble fly family (Oestridae).

botfly Any of several families of stout, hairy, black-and-white to grey fly. Its larvae are parasites of livestock, small animals and even humans. Usually eggs are laid on the host and the larvae cause damage to the host's skin or internal systems. The botfly that attacks deer is possibly the world's swiftest insect, flying at 80km/h (50mph). Order Diptera; family Oestridae.