Gadfly

"I am not going to argue for my own sake, as you may think, but for yours, that you may not sin against the God, or lightly reject his boon by condemning me. For if you kill me you will not easily find another like me, who, if I may use such a ludicrous figure of speech, am a sort of gadfly, given to the state by the God; and the state is like a great and noble steed who is tardy in his motions owing to his very size, and requires to be stirred into life. I am that gadfly which God has given the state and all day long and in all places am always fastening upon you, arousing and persuading and reproaching you. And as you will not easily find another like me, I would advise you to spare me." 

- Socrates, in Plato's Apology