Horse-flies or horseflies 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 bot flies (Oestridae) are
sometimes referred to as gadflies.
Adult horse-flies feed on
nectar and plant exudates; the males have weak mouth-parts and only the
females bite animals to obtain enough protein from blood to
produce eggs. The mouth-partsof
females are formed into a stout stabbing organ with two pairs of sharp cutting
blades, and a sponge-like part used to lap up the blood that flows from the
wound. The larvae are predaceous and grow in semi-aquatic 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. Shakespeare uses the theme
of the maddening gadfly in his plays King Lear and Antony and Cleopatra.
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