gadfly

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A persistent irritating critic; a nuisance.
2. n. One that acts as a provocative stimulus; a goad.
3. n. Any of various flies, especially of the family Tabanidae, that bite or annoy livestock and other animals.

Century Dictionary and Cyclopedia

1. n. 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. They are comparatively large, very active, voracious, and bloodthirsty, with great powers of biting, the mouth-parts being more highly developed than those of any other dipterous insect. They have also great power of flight. The bite is deep and painful, often drawing blood, though not poisonous. In strictness, only the females are gadflies, the males being smaller and quite inoffensive, living on juices of plants. There are more than 1,000 species, of the genera Tabanus, Chrysops, Hæmatopota, and others. One of the commonest gadflies which attack cattle and horses is Tabanus bovinus. See also out under Chrysops.
2. n. 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. These flies sting animals with their ovipositor, and deposit their eggs in the skin.
3. n. Figuratively, one who is constantly going about; a mischievous or annoying gadabout.

Wiktionary

1. n. Any dipterous insect of the genus Oestrus, and allied group of botflies.
2. n. A horsefly: any of various species of fly, of the family Tabanid[ae], noted for buzzing about animals and sucking their blood.
3. n. One who upsets the status quo by posing upsetting or novel questions, or attempts to stimulate innovation by proving an irritant.
4. n. One who merely irritates without making useful suggestions.
5. n. A bloodsucker

GNU Webster's 1913

1. n. Any dipterous insect of the genus Oestrus, and allied genera of botflies.

WordNet 3.0

1. n. a persistently annoying person
2. n. any of various large flies that annoy livestock