Apart from entomologists, there are probably few people who would actively look for horse-flies. For their size, they have one of the most painful bites of any insect and the bite can result in a painful swelling. The family Tabanidae is called deer-flies in the USA and horse-flies in Great Britain. They can cause problems in grazing animals by transmitting various diseases, including anthrax, and reducing milk output in dairy cattle. Blood-sucking is carried out only by the females and their mouth-parts have blade-like appendages that can cut through tough animal skin with ease. However, the flies themselves are not unattractive in appearance. Chrysops relictus is a stoutly-built insect, mostly shiny black but with variable yellow-orange bands around the upper part of the abdomen.
Female horse-flies require blood to help produce eggs whereas males feed on nectar from flowers. Horse-flies lay their eggs in the mud of river banks or in damp earth. The larvae hatch and some are predatory, feeding on the grubs of other mud-dwelling insects. They can paralyse their prey by injecting them with venom, which also pre-digests the grub allowing the horse-fly larva to suck their victim dry. The larvae pupate just below the surface, and over-winter in this state. The adult flies emerge through small holes in the mud in May, and are on the wing until September.
Horse-flies have an extensive world range and are only absent from some isolated island groups such as Hawaii and the colder Arctic regions. They can turn up almost anywhere in Britain, especially in place where there is wet ground and grazing animals.
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