Horse-fly
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[Photo] Horsefly, Tabanus sp. Description: horse fly, Diptera family Tabanidae, Tabanus sp. Author: Bruce Marlin Date created: 07/22/2005 Location: Winfield IL USA

Horse flies (family Tabanidae) are among the world's largest flies. These flies are often considered pests because of the painful bites that many, though not all, of the species can inflict on animals and humans. 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 horse flies are also known as March flies.

A type of insect, horse flies are classified as true flies in the order Diptera. 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.

The two best-known types are the common horse flies, genus Tabanus Linnaeus, 1758 and the deerflies, 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 usually also feed on blood which aids in egg development. 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.

Sight
The females' primary sense for locating prey is sight, and they have large compound eyes that serve this purpose well. The flies usually lay waiting in shady areas for prey to happen by. They are attracted to large, dark objects, and to certain animal odors and carbon dioxide. They are also attracted by motion, their eyes being well adapted to its detection. The eyes of horse flies are generally brightly colored, and this coloration is one of the means entomologists use to identify them to species, though the colors rarely persist after death. Sex in most species can be distinguished based on shape of the eyes relative to the frons. Male horse flies are usually holoptic, meaning that their eyes meet and take up the majority of the head. In females, the eyes are separated by a space called the frons.

Bite
The bite from a larger specimen can be singularly 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 its 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.

Habitat
Horse flies are most active in hot weather, mostly in summer and autumn during the daylight hours. Most species also prefer a wet environment, which makes it easier for them to breed. The female lays eggs on vegetation overhanging moist soil. The larvae hatch and drop onto the soil, where they feed on smaller organisms until pupation.

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

Reproduction
Eggs are generally laid on stones close to water or on plant stems or leaves. On hatching, the larvae fall into water or moist earth, feeding voraciously on invertebrates, such as snails and earthworms, and small vertebrates.

Diseases
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.

A common problem in some animals, though, when large flies are abundant, is blood loss. 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.

Citation
"Although the tsetse flies were responsible for transmitting sleeping sickness in most areas, occasionally an epidemic occurred in which the disease might be conveyed to cattle by direct contact with the ordinary horsefly, tanidae. This probably occurred when swarms of these flies surrounded the wretched animals. In one such epidemic some 3000 head of cattle died of trypanosomal disease in northern Rhodesia. Sir David and Lady Bruce returned to England in 1913. David Bruce reported the results achieved by this Sleeping Sickness Commission of the Royal Society in the Croonian Lectures in 1915."

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