Butterfly catchers was a derogatory term used to describe the more scientific-minded captains in the early Federation Starfleet. The term was coined at a time when the direction of Starfleet wasn't yet established, and there was some strife between those captains who had been military veterans of the Earth-Romulan War, and those who were scientists. The war veterans felt that the scientists didn't have what it takes to command a starship, as they knew nothing of space combat. The two groups were eventually able to reconcile and work together as a unified whole. (ST novel: Starfleet: Year One)