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.

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 ever since Aeschylus in Ancient Greece mentioned them driving people to "madness" through their persistent pursuit.

[6] The term "horse-fly" refers primarily to Tabaninae that are typically larger and stouter, and that lack the banded wings deer flies have.

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".

[10] Adult tabanids are large flies with prominent compound eyes, short antennae composed of three segments, and wide bodies.

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.

[11] 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.

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.

[14] Horseflies can lay claim to being the fastest flying insects; the male Hybomitra hinei wrighti has been recorded reaching speeds of up to 145 kilometres per hour (90 mph) when pursuing a female.

[15] Tabanids are found worldwide, except for the polar regions, but they are absent from some islands such as Greenland, Iceland,[3] and Hawaii.

[11] They mostly occur in warm areas with suitable moist locations for breeding, but also occupy a wide range of habitats from deserts to alpine meadows.

[19] With a necessity for high-protein food for egg production, the diet of early tabanomorphs was probably predatory, and from this, the bloodsucking habit may have evolved.

Tabanid identification is based mostly on adult morphological characters of the head, wing venation, and sometimes the last abdominal segment.

[23]A 2015 study by Morita et al. using nucleotide data, aimed to clarify the phylogeny of the Tabanidae and supports three subfamilies.

[30] Adult tabanids feed on nectar and plant exudates, and some are important pollinators of certain specialised flowers;[19] several South African and Asian species in the Pangoniinae have spectacularly long probosces adapted for the extraction of nectar from flowers with long, narrow corolla tubes, such as Lapeirousia,[31] and certain Pelargonium.

[5] The flies seem to be attracted to a potential victim by its movement, warmth, and surface texture, and by the carbon dioxide it breathes out.

When the insect lands on an animal, it grips the surface with its clawed feet, the labium is retracted, the head is thrust downwards and the stylets slice into the flesh.

[37] Bites can be painful for a day or more; fly saliva may provoke allergic reactions such as hives and difficulty with breathing.

[33] Tabanid bites can make life outdoors unpleasant for humans, and can reduce milk output in cattle.

[40] More recent research by the same lead author shows that the stripes were no less attractive to tabanids, but they merely touched—and could not make a controlled landing to bite.

The larvae were discovered to tunnel in a spiral motion while the mud was still wet and plastic, forming a partitioned cylinder in the center of which the larva settled to pupate after closing the entrance; this adaption protects the pupae against mudcracks when the mud dries up, as a spreading crack would change direction when it hit the wall of the cylinder.

Males usually appear first, but when both sexes have emerged, mating takes place, courtship starting in the air and finishing on the ground.

[5][12] Eggs are often attacked by tiny parasitic wasps, and the larvae are consumed by birds, as well as being paratised by tachinid flies, fungi, and nematodes.

Anecdotal reports of bites leading to fatal anaphylaxis in humans have been made, an extremely rare occurrence.

[56] Cattle can be treated with pour-on pyrethroids which may repel the flies, and fitting them with insecticide-impregnated eartags or collars has had some success in killing the insects.

[57] The National Health Service of the United Kingdom recommends that the site of the bite should be washed and a cold compress applied.

"[60] William Shakespeare, inspired by Aeschylus, has Tom o' Bedlam in King Lear, "Whom the foul fiend hath led through fire and through flame, through ford and whirlpool, o'er bog and quagmire", driven mad by the constant pursuit.

[61] The physician and naturalist Thomas Muffet wrote that the horse-fly "carries before him a very hard, stiff, and well-compacted sting, with which he strikes through the Oxe his hide; he is in fashion like a great Fly, and forces the beasts for fear of him only to stand up to the belly in water, or else to betake themselves to wood sides, cool shades, and places where the wind blows through.

"[39] The "Blue Tail Fly" in the eponymous song was probably the mourning horsefly (Tabanus atratus), a tabanid with a blue-black abdomen common to the southeastern United States.