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:

Breeze \Breeze\, Breeze fly \Breeze" fly`\, n. [OE. brese, AS.
   bri['o]sa; perh. akin to OHG. brimissa, G. breme, bremse, D.
   brems, which are akin to G. brummen to growl, buzz, grumble,
   L. fremere to murmur; cf. G. brausen, Sw. brusa, Dan. bruse,
   to roar, rush.] (Zool.)
   A fly of various species, of the family Tabanid[ae], noted
   for buzzing about animals, and tormenting them by sucking
   their blood; -- called also horsefly, and gadfly. They
   are among the largest of two-winged or dipterous insects. The
   name is also given to different species of botflies. [Written
   also breese and brize.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "gadfly":
   abettor, actuator, animator, cajoler, coax, coaxer, encourager,
   energizer, firer, gad, galvanizer, goad, impeller, inducer,
   inspirer, lash, mover, moving spirit, oxgoad, persuader, pleader,
   prick, prime mover, prod, prompter, rowel, spark, spark plug,
   sparker, spur, stimulator, sting, tempter, wheedler, whip,
   whiplash