The adult fly is grayish brown and ~12 mm long. The female deposits larvae in and about the nostrils of sheep without alighting. These small, clear-white larvae (initially <2 mm long) migrate into the nasal cavity; many spend at least some time in the paranasal sinuses. The larval period, which is usually shortest in young animals, lasts 1–10 mo. When mature, the larvae leave the nasal passages, drop to the ground, burrow down a few inches, and pupate. The pupal period lasts 3–9 wk, depending on the environmental conditions, after which the fly emerges from the pupal case and pushes its way to the surface. Mating soon occurs, and the female begins to deposit larvae.