Horse flies and deer flies are true flies in the family Tabanidae in the insect order Diptera. The adults are often large and agile in flight. Females parasitize land vertebrates, including humans, biting them to obtain blood. They prefer to fly in sunlight, avoiding dark and shady areas, and are inactive at night. They are found all over the world except for some islands and the polar regions (Hawaii, Greenland, Iceland). Both horse flies and botflies (Oestridae) are sometimes referred to as gadflies. Contrary to popular belief, horse flies cannot see infrared light or otherwise detect heat at a distance. Adult horse flies feed on nectar and plant exudates; males have weak mouthparts, but females have mouthparts strong enough to puncture the skin of large animals. This is for the purpose of obtaining enough protein from blood to produce eggs. The mouthparts of females are formed into a stout stabbing organ with two pairs of sharp cutting blades, and a spongelike part used to lap up the blood that flows from the wound. The larvae are predaceous and grow in semiaquatic habitats. Female horse flies can transfer blood-borne diseases from one animal to another through their feeding habit. In areas where those diseases occur, they have been known to carry equine infectious anaemia virus, some trypanosomes, the filarial worm Loa loa, anthrax among cattle and sheep, and tularemia. They can reduce growth rates in cattle and lower the milk output of cows if suitable shelters are not provided. Horse flies have long appeared in literature. The father of Greek tragedy, Aeschylus, depicts a gadfly (Ancient Greek: μύωψ, mýops) driving Io to madness in Prometheus Bound through its relentless pursuit. The philosopher Socrates later adopted the gadfly as an ironic mascot for his persistent questioning of those who spoke with him.
Common names
Tabanidae are known by a large number of common names. The subfamily Chrysopsinae is known as deer flies, perhaps because of their abundance on moorland where deer roam, and buffalo-flies, moose-flies and elephant-flies emanate from other parts of the world where these animals are found. The term "horse fly" refers primarily to Tabaninae that are typically larger and stouter, and that lack the banded wings deer flies have. Other common names include tabanids, gadflies, green-headed flies, and green flies. The word "Tabanus" was first recorded by Pliny the Younger and has survived as the generic name. In general, country folk did not distinguish between the various biting insects that irritated their cattle and called them all "gad-flies", from the word "gad" meaning "spike". Other common names include "cleg[g]", "gleg" or "clag", which come from Old Norse and may have originated from the Vikings. Other names such as "stouts" refer to the wide bodies of the insects and "dun-flies" to their sombre colouring. In Australia and the UK they are also known as March flies, a name used in other Anglophonic countries to refer to the non-bloodsucking Bibionidae.
Description Adult tabanids are large flies with prominent compound eyes, short antennae composed of three segments, and wide bodies. In females, the eyes are widely separated; in males, however, they are almost touching. The eyes are often patterned and brightly coloured in living tabanids but appear dull in preserved specimens. The terminal segment of the antennae is pointed and annulated, appearing to be composed of several tapering rings. There are no hairs or arista stemming from the antennae. Both the head and thorax are clad in short hairs, but no bristles are on the body. The membranous forewings are clear, either shaded uniformly grey or brown, or patterned in some species; they have a basal lobe (or calypter) that covers the modified knob-like hindwings or halteres. The tips of the legs have two lobes on the sides (pulvilli) and a central lobe (empodium) in addition to two claws that enable them to grip to surfaces. Species recognition is based on details of head structures (antennae, frons, and maxillae), wing venation, and body patterning; minute variations of surface structure cause subtle alterations of the overlying hairs, which alters the appearance of the body.
Tabanid species range from medium-sized to very large, robust insects. Most have a body length between 5 and 25 mm (0.2 and 1.0 in), with the largest having a wingspan of 60 mm (2.4 in). Deer flies in the genus Chrysops are up to 10 mm (0.4 in) long, have yellow to black bodies and striped abdomens, and membranous wings with dark patches. Horse flies (genus Tabanus) are larger, up to 25 mm (1 in) in length and are mostly dark brown or black, with dark eyes, often with a metallic sheen. Yellow flies (genus Diachlorus) are similar in shape to deer flies, but have yellowish bodies and the eyes are purplish-black with a green sheen. Some species in the subfamily Pangoniinae have an exceptionally long proboscis (tubular mouthpart).
Vision
Like in many insects, the compound eyes of tabanids are capable of detecting the polarization of light. In female tabanids, it is a determining factor in their host targeting. The optical units (ommatidia) making up the compound eye of flies of the suborder brachycera contain a cluster of 8 photoreceptor cells (retinula cells; R); R1–6 on the cluster's rim are for achromatic vision and motion detection, while R7 and R8 in the cluster's center detect color and polarity of incoming light. Despite the clear differences between the outer R1-6 and the inner R7 and R8, the two groups do overlap to an extent in their function. There are two kinds of ommatidia, termed pale and yellow, that differ only in the wavelength sensitivity of R8. R1–R6 share a sensitivity to light from ultraviolet into the green wavelength range. The sensitivity of R7 peaks in ultraviolet, while R8 peaks in blue and green for pale and yellow respectively. With the peak sensitivity of the color-seeing retinula cells being in the blue and green wavelength ranges, they are not able to detect significant amounts of light in the infrared and red wavelengths. They are therefore not able to see the heat of objects by the infrared light they radiate.
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