Insects in the order Diptera, family Tabanidae, are commonly called horse flies, and sometimes also forest flies or deer flies. The former, however, can also refer to Hippoboscidae, and the latter refers specifically to the horse-fly genus Chrysops. They range in length from 30 to 60 mm. Often considered pests for the bites that many inflict, they are among the world's largest true flies. They are known to be extremely noisy during flight. They are also important pollinators of flowers, especially in South Africa. Tabanids occur worldwide, being absent only at extreme northern and southern latitudes. Flies of this type are among those known sometimes as gadflies, zimbs or clegs. In Australia, they are known as "march flies"; elsewhere this term refers to the unrelated dipteran family Bibionidae.

There are approximately 3,000 species of horse flies known worldwide, 350 of which are found in North America. At least three subfamilies are recognised:

• Chrysopsinae
• Pangoniinae
• Tabaninae
• the genus Zophina is of uncertain placement, though it has been classified among the Pangoniinae.

Two well-known types are the common horse flies, genus Tabanus Linnaeus, 1758 and the deer flies, genus Chrysops Meigen, 1802 also known as banded horse flies because of their coloring. Both these genera give their names to subfamilies. The "Blue Tail Fly" in the eponymous song was probably a tabanid common to the southeastern United States.

Diet

Adult horse flies feed on nectar and sometimes pollen. Females require a blood meal for reproduction. Males lack the necessary mouth parts (mandibles) for blood feeding. Most female horse flies feed on mammal blood, but some species are known to feed on birds, amphibians or reptiles. Immature or larval horse flies are fossorial predators of other invertebrates in moist environments.

Bite Behaviour

The bite from a larger specimen is extremely painful, especially considering the light, agile, and airborne nature of the fly. Unlike insects which surreptitiously puncture the skin with needle-like organs, horse flies have mandibles like tiny serrated scimitars, which they use to rip and/or slice flesh apart. This causes the blood to seep out as the horsefly licks it up. They may even carve a chunk completely out of the victim, to be digested at leisure.

The horsefly's modus operandi is less secretive than that of its mosquito counterparts, although it still aims to escape before pain signals reach their mark's sphere of awareness. Moreover, the pain of a horsefly bite may mean that the victim is more concerned with assessing and repairing the wound, than finding and swatting the interloper. The bites become extremely itchy, sometimes causing a large swelling afterwards if not treated quickly, and they may bite more than once. The horsefly is able to bite through a light sweater.

Predators

Aside from generalized predators such as birds, there are also specialist predators such as the Horse guard wasp, a type of Sand wasp that preferentially attacks horse flies.

Reproduction

Eggs are laid on stones close to water on plant stems or leaves until they hatch and in some cases have been known to lay their eggs in humans just under their skin. On hatching, the larvae fall into water or moist earth, feeding voraciously on invertebrates, such as snails and earthworms.

Diseases

Blood-borne diseases in particular are a problem. Tabanids are very good vectors of equine infectious anaemia Virus, as well as some Trypanosome species. Some horsefly species are known to transmit disease and/or parasites. Species in the genus Chrysops are biological vectors of Loa loa, transmitting this filarial worm between humans. They have also been known to transmit Anthrax among cattle and sheep.

Blood loss is a common problem in some animals, when large flies are abundant. Some animals have been known to lose up to 300 ml of blood in a single day, which can severely weaken or even kill them.

See also

Look up horsefly in Wiktionary, the free dictionary.

• Deerfly
• Use of DNA in forensic entomology

Gallery

A Tabanid from the Western Ghats

Tabanus glaucops France

Tabanus eggeri France

Tabanus unifasciatus France

References