The horn fly is about one-half the size of an ordinary housefly and possesses a piercing beak. The fly remains on the animal day and night and leaves only to pass to another host or lay eggs. Horn flies feed 20 times a day or more and are marked blood suckers.
The life cycle of the horn fly is relatively short. During the summer months, the flies reproduce in 9 to 12 days, and the adult flies live about seven weeks. The flies are adapted to a warm, moist climate; hot, dry weather or cold weather are unfavorable to their reproduction.
The adult horn fly leaves the animal only for brief periods to lay eggs. The manure apparently loses its attraction for egg-laying flies within 5 to 10 minutes after it has passed from the animal. The eggs are laid beneath the droppings where they hatch in approximately 16 to 20 hours. Low temperatures retard or arrest development of this stage. The larvae burrow into the droppings where they feed and grow, becoming mature in about four days. At this time the larvae migrate to the lower part of the droppings or into the soil to pupate and later emerge as adult flies. The pupal stage lasts about six to eight days and is also dependent on high temperatures and moisture for development.
When the fly emerges from its pupal case, it seeks the nearest animal and begins feeding. The horn fly may mate as early as the second day after emergence and may deposit eggs on the third day. An adult female horn fly is capable of laying 400 eggs during a lifetime.
Source: University of Arkansas Ag Extension