A number of native species occur in North America, particularly in the more humid states where the moist conditions the larvae need are available year-round.
Horse fly females are blood feeders, while the males feed on plant juices. The mouthparts of the female are scissors-like, and they slash open the skin, cause the blood to flow with their saliva, and lap up the blood. They are not incriminated as vectors of any specific diseases in North America, but are extremely annoying, have painful bites, and can be serious threats to the health of livestock or horses when they feed in large numbers. The larvae of many species are predators, living in moist soils, under wet leaf litter, or even in running water. They may live in the mud at the bottom of ponds or ditches, feeding on other organisms that come nearby.
Horseflies are some of the largest flies in North America, with adults of some species growing to over an inch long, and with a wingspan of 2.5 inches. They are heavy bodied with the abdomen tapering to a narrow end, and colors range from black to brown, sometimes with stripes or spots on them. Quite often their eyes are rainbow colored with green, pink and reddish hues. The antennae are very distinctive, being stout and elongate, and on horseflies the large third segment has a tooth-like projection at its base.
Characteristicts Important to Control:
Control is difficult to achieve since the larvae can live in so many varied, inaccessible sites. These flies rarely enter structures but can be serious pests outdoors, where they react to movement to signal a potential food source. Repellents can be somewhat effective, and wearing long sleeves and long pants when in areas likely to have these flies will reduce their ability to bite.