Angry Gatekeeper and Hover Flies

Close to the entrance to the Ham Wall RSPB reserve on the Somerset Levels, I found this gatekeeper butterfly feeding, happily minding its own business. Then a hover fly (single pair of wings note) landed and started to feed. The gatekeeper started to twitch its wings, seemingly in response to the fly. Then a second fly landed and the wing movement of the butterfly increased. All three flies continued to feed with the gatekeeper moving as though it was “angry” or  “agitated”. I know “happily”, “angry” and “agitated” are anthropomorphic and probably not applicable to these animals but seemed to apply…