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 Chrysopss
. Often considered pests for the bites
that many inflict, they are among the world's largest true flies
. They can be very 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
In Australia, they are known as March flies
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:
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.
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.
The females' primary sense for locating prey is sight, and they have large compound eyes
that serve this purpose well. The flies usually lie waiting in shady areas for prey to happen by. They are 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.
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.
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.
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.
Eggs are laid on stones close to water on plant stems or leaves until they hatch. On hatching, the larvae
fall into water or moist earth, feeding voraciously on invertebrates, such as snails and earthworms, and small vertebrates.
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.
"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."