A dipterous insect of the family (Estridæ, 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 larvæ 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.
- 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.