: any of the swift usually large dipteran flies with bloodsucking females that comprise the family Tabanidae
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.