In desert mine sites - although not exclusive to - when a fly manages to slip in and become trapped behind a lense of your safety glasses, it beats and buzzes around - between eye and lense - searching an escape in much the same way as a frightened bird might panic into walls and windows after accidentally finding its way into a room; the sensation of the fly in this state, although harmeless, still has an unsettling element of fear that goes with it, a little like feeling a spider crawling around inside your shirt sleeve.