The Collaborative International Dictionary of English v.0.48:

Gadfly \Gad"fly`\ (g[a^]d"fl[imac]`), n.; pl. Gadflies. [Gad +
   fly.] (Zool.)
   Any dipterous insect of the genus Oestrus, and allied
   genera of botflies.
   [1913 Webster]

   Note: The sheep gadfly (Oestrus ovis) deposits its young in
         the nostrils of sheep, and the larv[ae] develop in the
         frontal sinuses. The common species which infests
         cattle (Hypoderma bovis) deposits its eggs upon or in
         the skin where the larv[ae] or bots live and produce
         sores called wormels. The gadflies of the horse produce
         the intestinal parasites called bots. See Botfly, and
         Bots. The true horseflies are often erroneously
         called gadflies, and the true gadflies are sometimes
         incorrectly called breeze flies.
         [1913 Webster]

   Gadfly petrel (Zool.), one of several small petrels of the
      genus Oestrelata.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

oestrus \oes"trus\, n. [L., a gadfly; also, frenzy, fr. Gr. ?
   gadfly; hence, sting, fury, insane desire, frenzy.]
   1. (Zool.) A genus of gadflies. The species which deposits
      its larvae in the nasal cavities of sheep is oestrus
      ovis.
      [1913 Webster]

   2. A vehement desire.
      [1913 Webster]

   3. Specifically: (Physiol.) The periodical sexual impulse of
      animals; the period during which female animals are most
      receptive to males; heat; rut.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

Oestrus ovis
    n 1: larvae are parasitic on sheep [syn: sheep botfly, sheep
         gadfly, Oestrus ovis]