Horse Fly

Family Tabanidae — the horse flies and deer flies — contains about 4,500 species worldwide and includes some of the largest and most aggressive blood-feeding flies on Earth. Tabanus atratus, the black horse fly of eastern North America, reaches 28-30 mm and is one of the largest flies in the family. The biting behavior is unique among Diptera: instead of piercing the skin with a stylet (the strategy of mosquitoes, gnats, and tsetse flies), tabanid females have scissor-like mandibles that slice the skin open, after which the fly applies a sponge-like labellum to lap up the pooled blood. The cut is often deep enough to continue bleeding for an hour or more after the fly has departed. Only females blood-feed (the protein is required for egg development); males drink nectar from flowers. The bite is reported as the most painful of any North American fly and is rated severe in the Schmidt-style fly-pain literature. The compound eyes of live tabanids display some of the most dramatic iridescent patterns in nature — bands of green, blue, purple, and gold that pulse across the eye depending on the angle of light. The patterns fade rapidly after death, which is why pinned museum specimens look dull. Tabanids transmit several livestock pathogens (anaplasmosis, anthrax, equine infectious anemia, surra) by mechanical contamination from one host's blood to the next; in tropical Africa some species also transmit Loa loa filarial nematode to humans. Larvae are predatory aquatic mud-dwellers.