M. illinoiensis 6 6 were most actively engaged in territory patrolling during noon. They were observed to avoid areas on the water surface that were shaded. Areas on the water surface that were not avoided by <3 3 were artificially shaded upon which such areas were avoided. It is concluded that <J S of M. illinoiensis may use the shade-sun boundary on the water surface as a cue of its territory boundary.