Noun

1. Any dipterous insect of the genus Oestrus, and allied group of botflies.
2. A horsefly: any of various species of fly, of the family Tabanid[ae], noted for buzzing about animals and sucking their blood.
3. One who upsets the status quo by posing upsetting or novel questions, or attempt to stimulate innovation by proving an irritant.
4. A bloodsucker

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