The Collaborative International Dictionary of English v.0.48:

Horsefly \Horse"fly`\, n.; pl. Horseflies.
   1. (Zool.) Any dipterous fly of the family Tabanid[ae],
      that stings horses, and sucks their blood.
      [1913 Webster]

   Note: Of these flies there are numerous species, both in
         Europe and America. They have a large proboscis with
         four sharp lancets for piercing the skin. Called also
         breeze fly. See Illust. under Diptera, and Breeze
         fly.
         [1913 Webster]

   2. (Zool.) The horse tick or forest fly (Hippobosca).
      [1913 Webster]