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. Females parasitize land vertebrates, including humans, biting them 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.
