any member of the insect family Tabanidae (order Diptera), but more specifically any member of the genus Tabanus. These stout flies, as small as a housefly or as large as a bumble bee, are sometimes known as greenheaded monsters; their metallic or iridescent eyes meet dorsally in the male and are separate in the female. Gad fly, a nickname, may refer either to the fly's roving habits or to its mouthparts, which resemble a wedge-shaped miner's tool. Other such names are breeze fly and ear fly. One of the most common species (Tabanus lineola) has bright-green eyes and is known as green head. The genus Chrysops, usually known as deer fly, is slightly smaller than Tabanus and has dark markings on the wings.

Adult horse flies are fast, strong fliers that are usually found around streams, marshes, and wooded areas. They may be carriers of various animal diseases such as anthrax, tularemia, and trypanosomiasis. Females deposit long, flat, black eggs in clusters; the eggs are laid on the grass. Horse flies overwinter in the larval stage, pupate in spring, and emerge as adults by late June.