horsefly, common name for the large hairy flies of the family Tabanidae. Male horseflies feed on pollen and nectar, but the females suck blood as well and are common pests of animals and sometimes of humans. The bites of many species are very painful. The larger horseflies, e.g., the mourning horsefly and the 1-in. (2.5-cm) black horsefly, belong to the genus Tabanus; the smaller and more common banded horseflies, with black, brown, or yellow bodies and brilliantly colored eyes, are members of the genus Chrysops. The deerflies, which carry the diseases anthrax and tularemia, and in Africa, a filarial worm infestation, belong to this group. Horseflies are most abundant in hot weather. The eggs are laid on plants or stones close to water. The somewhat flattened 1/2-in. (1.3-cm) larvae have fleshy protuberances on each body segment, aiding in locomotion; they live in water or in moist earth and feed on snails and on other insect larvae. Horseflies are classified in the phylum Arthropoda, class Insecta, order Diptera, family Tabanidae. See insect.

Any member of the dipteran genus Tabanus or, more generally, of the family Tabanidae. These stout flies range from as small as a housefly to as large as a bumblebee. Sometimes called greenheaded monsters, horseflies have metallic or iridescent eyes. Adults are fast, strong fliers usually found around streams, marshes, and wooded areas. They may carry animal diseases, including anthrax, tularemia, and trypanosomiasis. The bites of the bloodsucking females can be painful, and a swarm may suck more than 3 oz (about 90 ml) of blood a day from an animal. Males feed on nectar, honeydew, and plant sap. Horseflies of the genus Chrysops, usually called deerflies, are smaller and have dark markings on the wings.

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