These flies probably received the common name of deer flies and horse flies because they are notorious pests of deer and horses respectfully 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.
Adults are medium to large in size, about ¼-1 1/8” long, without bristles and most stout-bodied. Their colour ranges from black to pale yellow, usually with stripes on the abdomen, often with bright green or purple eyes. Deer flies are typically smaller while Horse flies are larger. Eggs are laid in masses of 100- 1000 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. 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.
Most tanabid 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. Adults are strong fliers and capable of flying over 50 km. However, they generally do not disperse widely, usually less than 3 km. Moving and dark objects are most often attacked. They often rest on paths and roads, especially in wooded areas.