The time required for hatching varies with the species and may be 3-5 days or as long as 30 days at low temperatures. Larvae most often are found just beneath the surface of rapidly flowing water. Since the gill filaments of larvae extract oxygen directly from the water, they do not come to the surface to breath as do mosquito larvae. These cocoons are attached to slightly submerged objects such as rocks, logs, roots and other debris. Pupae also possess respiratory filameters with which they remove dissolved oxygen from the water.
Adults emerge from the pupal case, rise to water's surface, unfold their wings and fly away.
Mating usually occurs shortly after the initial flight. The complete life cycle, from egg to adult, varies from 6-15 weeks and the number of generations per year ranges from one to six, depending on species and climatic conditions. There are numerous references of livestock losses along river basins in the United States, as well as other countries, resulting from black fly attacks. Large numbers of cattle, horses, mules, hogs, turkeys, chickens, sheep, dogs and cats, as well as wild animals have been killed. Death usually occurs as a consequence of an acute toxemia, caused by a vast number of bites, or as a result of anaphylactic shock.