Horse fly (Tabanus spp.) mouthparts. The dark, middle projection is the labium, used by the fly to lap up fluids. The much-feared scissor-like cutting parts are enclosed within the labium and not visible here. The top protrusion, though dangerous looking, is one of two antennae. Alan R Walker
Biting flies are feared around the world for their painful bites, inflicted on both animals and humans. Like houseflies, which have no piercing mouthparts, biting flies can spread disease-causing pathogens that hitchhike on the flies’ legs, abdomen, or mouthparts, transmitting disease from ill to susceptible animals. This effect is reinforced by their feeding habit of severing capillaries near the skin surface to drink the blood that pools there.
Given these characteristics, it is surprising that biting flies are not more important pathogen spreaders than they actually are. Many of the dozens of diseases they are suspected of spreading are based on having identified those pathogens somewhere on a fly in the past. But documented transmission by biting flies between animals has been rare, for several reasons.