gadfly

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A persistent irritating critic; a nuisance.
• noun One that acts as a provocative stimulus; a goad.
• noun Any of various flies, especially a warble fly, botfly, or horsefly, that bite or annoy livestock and other animals.

from The Century Dictionary.

• noun The popular name of sundry flies which goad or sting domestic animals, as a breeze, breeze-fly, or horse-fly; specifically, a dipterous insect of the family Tabanidæ and suborder Brachycera, representing also a superfamily Hexachætæ.
• noun A common though erroneous name of sundry flies (bot-flies) of the family Œstridæ and genus Œstrus or Hippoderma, belonging to a different series of the great order Diptera from that of gadflies proper.
• noun Figuratively, one who is constantly going about; a mischievous or annoying gadabout.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) Any dipterous insect of the genus Oestrus, and allied genera of botflies.
• noun (Zoöl.) one of several small petrels of the genus Oestrelata.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any dipterous insect of the family Oestridae, commonly known as botflies.
• noun A horsefly: any of various species of fly, of the family Tabanidae, noted for buzzing about animals and sucking their blood.
• noun One who upsets the status quo by posing upsetting or novel questions, or attempts to stimulate innovation by proving an irritant.
• noun pejorative One who merely irritates without making useful suggestions.
• noun slang A bloodsucker

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a persistently annoying person
• noun any of various large flies that annoy livestock