A Gadfly

1. Any dipterous insect of the family Oestridae, commonly known as botflies. Synonyms: botfly, warble fly
2. A horsefly: any of various species of fly, of the family Tabanidae, noted for buzzing about animals and sucking their blood. Synonyms: horsefly, stoat-fly, stout
3. (figuratively) One who upsets the status quo by posing upsetting or novel questions, or attempts to stimulate innovation by proving an irritant. Synonyms: maverick
4. (derogatory) One who merely irritates without making useful suggestions.
5. (derogatory, slang) A bloodsucker; a person who takes without giving back. Synonyms: bloodsucker; scrounger