How Can Dragonflies Discern Bright and Dark Waters from a Distance? The Degree of Linear Polarization of Reflected Light as a Possible Cue for Dragonfly Habitat Selection

Abstract

While monitoring the dragonfly fauna of dark ponds in peatland and bright ponds in gravel pits, Bernáth et al. (2002) observed that certain species prefer exclusively bright water bodies, while other species prefer only dark waters, and some species are ubiquitous, choosing dark and bright waters with equal frequencies (Table 20.1). It is a well-known optical phenomenon that two water bodies, being bright and dark to the human eye viewing downwards perpendicularly to their surface, cannot be distinguished from each other from a distance. Then the angle of view with respect to the water surface is very narrow (“grazing” angle of view), and the amounts of reflected light are practically equal for both dark and bright waters. How can dragonflies distinguish a bright from a dark pond before they get sufficiently close to perceive brightness differences?