Horse-fly

Horse-flies or horseflies (for other names, see common names) are true flies in the family Tabanidae in the insect order Diptera. They are often large and agile in flight, and the females bite animals, including humans, 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. Adult horse-flies feed on nectar and plant exudates; the males have weak mouthparts and only the females bite animals to obtain 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 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 appeared in literature since Aeschylus in Ancient Greece mentioned them driving people to madness through their persistent pursuit. Apart from the common name 'horse-flies', broad categories of biting, bloodsucking Tabanidae are known by a large number of common names. 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 a spike. The most common name is 'cleg', 'gleg' or 'clag', which comes 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. Chrysops species are 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. In North America they are known as 'breeze-flies', and in Australia some are known as 'March flies', a name used in other Anglophonic countries to refer to the non-bloodsucking Bibionidae. 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 but in males they are almost touching; they are often patterned and brightly coloured in life but appear dull in preserved specimens. The terminal segment of the antennae is pointed and is annulated, appearing to be made up of several tapering rings. There are no hairs or arista arising from the antennae. Both head and thorax are clad in short hairs, but no bristles are on the body. The membranous forewings are clear, uniformly shaded 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 or empodium in addition to two claws that enable them to grip surfaces. Species recognition is based on details of head structures (antennae, frons, and maxillae), the wing venation and the 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). The larvae are long and cylindrical with small heads and 12 body segments. They have rings of tubercles (warty outgrowths) known as pseudopods around the segments, and also bands of short setae (bristles). The posterior tip of each larva has a breathing siphon and a bulbous area known as Graber's organ. The outlines of the adult insect's head and wings are visible through the pupa, which has seven moveable abdominal segments, all except the front one of which bears a band of setae. The posterior end of the pupa bears a group of spine-like tubercles.