The Horse Fly

Common Name:
Horse and Deer Flies
Scientific Name:
Mostly Chrysops spp. and Tabanus spp.


These flies probably received the common name of deer flies (Chrysops spp.) and horse flies (Tabanus spp.) 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 of 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.


Adults medium to large, about 1/4- 1/8" (6-30 mm) 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 (dichoptic) in females but touching (holoptic) in males. Antenna 3- segmented, 3rd segment enlarged basally, with terminal portion elongate and annualized (subdivided by rings into 2-8 sections). Wing with calypter (lobe at base on rear margin) large, veins R4 and R5 divergent, enclosing wing tip, 4 or 5 posterior cells present. In addition -

  1. Deer flies (Chtysops spp.) smaller, about 1/4-1/2" (6-12 mm) long; color black or brownish, usually with dark areas on wings, eyes usually with spots, 3rd antennae segment lacks a basal toothlike process; and hind tibiae with apical spurs.

  2. Horse flies (Tabanus spp.) larger, about 3/8-1 1/8" (10-30 mm) 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 hind tibiae lack apical spurs.


  1. Snipe flies (Rhagionidae) have antennae with 3rd segment more or less rounded, bearing a long slender terminal style (prolongation), and wings with calypters (lobes at base on rear margin) small or vartigial.

  2. Soldier flies (Stratiomyidae) have antennae with 3rd segment rounded or elongate and wing with branches of radius (R4+R5) crowded towards front, with R5 ending in front of wing tip.

  3. Other flies of public health importance with 3-segmented antennae have fewer than 4 posterior cells in wings.


  1. Chrysops called Osten Sacken. Length about 1/4-3/8" (7-9 mm); color black with mid-dorsal yellow triangles on abdomen and large pale yellow spots on sides near base, wings with characteristic brown markings; widespread in United States.

  2. Chrysops carbonarius Walker. Length about 1/4-3/8" (7-9 mm); color black with yellowish gray and greenish gray markings, wings with characteristic black markings; widespread in United States.

  3. Chrysops discalis Williston. Length about 3/8" (8-10.5 mm); color of female gray to yellowish gray with black spots on abdomen, male mostly black with yellowish gray spots on abdomen, wings with characteristic brown pattern; distributed in western U.S.

  4. Chtysops flavidus Wiedemann. Length about 3/8-1/2" (8-12 mm); color brown to black with light tan markings, wings with characteristic black markings; found throughout eastern U.S.

  5. Black horse fly, Tabanus atratus Fabricius. Length about 3/4-1 " (20-25 mm); color entirely black including wings; found throughout United States.

  6. Greenhead, Tabanus americanus Forester. Length about 7/8-1 1/8" (22-28 mm); color with eyes bright green, thorax and abdomen reddish brown, wings with markings only along front edge; found throughout eastern U.S. west to Mississippi Rivpr and eastern Texas, also southern Canada.

  7. Striped horse fly, Tabanus lineola Fabricius. Length about 1/2-5/8" (12-15 mm); color with thorax striped brown and gray, abdomen brown or black above with 3 gray stripes; found throughout eastern U.S. and in Texas.

  8. Western black horse fly, Tabanus punctifer Osten Sacken. Length about 3/4-7/8" (18-22 mm); color black, male with thorax white on rear and side margins, female with thorax all pale and hairy, wings lightly smoky with single dark spot in outer portion; found in Pacific Northwest, Montana, Wyoming, Colorado, California, and southwestern U.S.


Eggs are laid in masses of 100-1,000 eggs covered with a jellylike material on leaves, rocks, or debris overhanging water or on moist areas; deer flies often lay single layer masses, horse flies lay 3-4 layer masses. Upon hatching the larvae fall into the water or onto moist soil. In general, the larvae of horse flies are carnivorous and cannibalistic, feeding on insect larvae, snails, earthworms, etc. whereas, larvae of deer flies feed on decaying organic matter (detritovores). They go through 4-9 instars. Larvae move to the drier fringe areas of their habitat to pupate, aquatic larvae to decaying vegetation at water's edge and soil dwellers to the drier edges of their habitat. Developmental time (egg to adult) may require 3 months to 3 years. Adults live about 3-4 weeks, and produce 5-6 batches of eggs. Most species overwinter as larvae. The mouthparts are for tearing and lapping, not piercing. Both sexes feed on nectar, but females require a blood meal for egg laying. In the U.S. only the deer fly, Chrysops discalis, is a vector of disease to humans. In this case it is the bacterium Francisella tularensis (McCoy & Chapin) which causes tularemia that is transmitted from rabbits, hares, and other rodents. After biting an infected animal, they can transmit the disease for about 5 days. Apparently the transmission is mechanical. Tabanids also mechanically transmit a number of diseases to domesticated animals such as anthrax, California encephalitis, western equine encephalitis, etc.


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 (Tabanus spp.) develop in wet soil close to bodies of water and the larvae of dear flies (Chtysops spp.) 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 dry soil or in rotten wood.

Adults are strong fliers and are capable of flying over 31 mi (50 km). However they generally do not disperse widely, usually less than 2 mi (3 km). 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.

The females can enter structures in considerable numbers and then congregate at the windows since they are attracted to light. Both sexes of many species occasionally come to lights at night.