Horse-flies or horseflies are true flies in the family Tabanidae in the insect order Diptera.
Adult horse-flies feed on nectar and plant exudates; the males have weak mouthparts and only the females bite animals, including humans, to obtain 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.
Female horse-flies can transfer blood-born diseases from one animal to another through their feeding habit. In areas where diseases occur, they have been known to carry equine infections anaemia virus, some trypanosomes, the filaria worm Loa loa, anthrax among cattle and sheep, and tularemia.
As well as making life outdoors uncomfortable for humans, they can reduce growth rates in cattle and lower the milk output of cows if suitable shelters are not provided.
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.