Horse flies and deer flies are true flies in the family Tabanidae in the insect order Diptera. The adults are often large and agile in flight. Only females bite land vertebrates, including humans, to obtain blood. They prefer to fly in sunlight, avoiding dark and shady areas, and are inactive at night. They are found all over the world except for some islands and the polar regions (Hawaii, Greenland, Iceland). Both horse flies and botflies (Oestridae) are sometimes referred to as gadflies. Contrary to popular belief, horse flies can not see infrared light or otherwise detect heat at a distance.

Adult horse flies feed on nectar and plant exudates; males have weak mouthparts, but females have mouthparts strong enough to puncture the skin of large animals. This is for the purpose of obtaining enough protein from blood to produce eggs. The mouthparts of females are formed into a stout stabbing organ with two pairs of sharp cutting blades, and a spongelike part used to lap up the blood that flows from the wound. The larvae are predaceous and grow in semiaquatic habitats.

Female horse flies can transfer blood-borne diseases from one animal to another through their feeding habit. In areas where those diseases occur, they have been known to carry equine infectious anaemia virus, some trypanosomes, the filarial worm Loa loa, anthrax among cattle and sheep, and tularemia. They can reduce growth rates in cattle and lower the milk output of cows if suitable shelters are not provided.

Horse flies have appeared in literature ever since Aeschylus in Ancient Greece mentioned them driving people to "madness" through their persistent pursuit.

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