Horse flies (subfamily Tabaninae) are among the world's largest flies. The females can inflict a painful bite when in search of a blood meal for reproductive purposes.  Males rarely bite, instead feeding on nectar and pollen.

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!