Horse flies (subfamily Tabaninae) are among the world's largest flies, and inhabit all continents save Antarctica. The females can inflict a painful bite when in search of a blood meal for reproductive purposes.  (All adult horse flies feed on nectar and pollen.) Horse flies can be very noisy in flight, but by my experience not more so than some of the large robber flies (Asilidae) or flower flies (Syrphidae).

Horse Fly Foot Anatomy: Tabanid tibia, tarsi 1-5, claws and three pads. Flies in the family Tabanidae have three foot pads instead of two, as in most other Diptera (fig.1).

Faceted eye structure casts rainbows

Figure 1. Horse fly foot anatomy

Horse flies and deer flies bother me most often when I am walking; they are attracted to dark moving objects, and they will continually buzz about my head and land on my hair. I have been bitten by deer flies, but never a horse fly. Deer fly bites are terrible - and you can swat the shit out of them and they will just fly away as if nothing happened!

• Stout flies with large squamae (scales above the halteres, also called calypters)
• Feet with 3 pads (as opposed to 2)
• 3rd antennal segment elongated, clearly made up of several fused parts
• 3rd antennal segment with a prominent tooth at base in some groups
• Wing veins R4 and R5 fork to form a large 'Y' across the wing tip.

1. Bugguide.net, Stonemyia tranquilla