Deer And Horse Fly

These flies probably received the common name of deer flies and horse flies because they are notorious pests of deer and horses respectively, and of other mammals. They are pests primarily in suburban and rural areas where both their breeding sites are in or near bodies of water and mammal hosts are more abundant. Some species are vectors of disease organisms to humans and other mammals, but in the U.S. most vectored diseases involve livestock. They are found throughout North America, with about 350 species occurring here.

Recognition

Adults medium to large, about 1/4 - 1 1/8" long; without bristles, most stout-bodied. Color ranges from black to pale yellow, usually with stripes on abdomen, often with bright green or purple eyes. Head with compound eyes very large, separated in females but touching in males. Antenna 3-segmented, 3rd segment enlarged basely, with terminal portion elongate and annulized. Wing with calypter large veins and 4-5 posterior cells present.

1. Deer flies smaller, about 1/4-1/2" long; color black or brownish, usually with dark areas on wings, eyes usually with spots, 3rd antennal segment lacks a basal toothlike process; and thin tibiae with apical spurs.
2. Horse flies larger, about 3/8-1 1/8" long; color usually gray or blackish, wings usually lacking dark areas but some species with wings entirely dark, eyes often with horizontal stripes; 3rd antennal segment with a basal toothlike process; and high tibiae lack apical spurs.

Habits

Most tabanid flies are woodland or forest dwellers. Most species feed during full daylight and are most evident on windless, hot, sunny days. In general, larvae of horse flies develop in wet soil close to bodies of water and the larvae of deer flies develop in wet mud often in semi-submerged situations. However some species are aquatic, living in rapidly flowing streams to stagnant ponds and some only in bogs, while others live in relatively dirty soil or in rotten wood.

Adults are strong fliers and are capable of flying over 31 mi. However they generally do not disperse widely, usually less than 2-mi. Moving and dark objects are most often attacked. They often rest on paths and roads, especially in wooded areas. A slight drop in temperature or increase in wind will greatly reduce the numbers flying.