gadfly

Any dipterous (“two-winged”) insect or fly of the family Oestridae (commonly known as a bot fly) or Tabanidae (horse fly), noted for irritating animals by buzzing about them, and biting them to suck their blood; a gadbee.

• He that made that ryme in jeſt, little conſidered what a gad-fly may doe in earneſt. It is ſmall wiſedome to contemne the ſmalleſt enemy; the gad-fly is a little creature, but ſome little creatures be ſtingers; […]
• Light fly his ſlumbers, if perchance a flight / Of angry gad-flies faſten on the herd; […]

A person or thing that irritates or instigates.alsoattributivefiguratively

• VVhat gad flye tickles ſo this Macrinus, / That vp-flinging thy tayle, he breakes thus from me.

A person or thing that irritates or instigates.; A person who upsets the status quo by posing novel or upsetting questions, or attempts to stimulate innovation by being an irritant.alsoattributivefiguratively

• There is a function for the gadfly who poses questions that many specialists would like to overlook. Polemics is healthy.
• Dr. [Carl] Hart, 54, the first tenured African-American science professor at Columbia, is a gadfly among drug researchers and a rock star among advocates for decriminalizing drugs.

Synonym of gadabout (“a person who restlessly moves from place to place, seeking amusement or the companionship of others”).alsoattributivefiguratively

• VVhat VVinnie? VVife, I ſay? out of dores at this time! vvhere ſhould I ſeeke the Gad-flye?
• VVhere are thoſe gad-flies going? to ſome Junket novv; […]

A person who takes without giving back; a bloodsucker.alsoattributivederogatory

• He’s a regular gadfly and takes advantage of his friend’s generosity.