Source : Webster's Revised Unabridged Dictionary (1913)

Gadfly \Gad"fly`\, n.; pl. {Gadflies}. [Gad + fly.] (Zo["o]l.)
   Any dipterous insect of the genus {Oestrus}, and allied
   genera of botflies.

   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.

   {Gadfly petrel} (Zo["o]l.), one of several small petrels of
      the genus {Oestrelata}.

Source : WordNet®

gadfly
     n 1: a persistently annoying person [syn: {pest}, {blighter}, {cuss},
           {pesterer}]
     2: any of various large flies that annoy livestock