Polarotaxis in Mayflies

In field experiments, the behavioural responses of Ephoron virgo and Caenis robusta mayflies were studied to lamps emitting horizontally and vertically polarized and unpolarized light. In both species, unpolarized light induced positive phototaxis, horizontally polarized light elicited positive photo- and polarotaxis, horizontally polarized light was much more attractive than unpolarized light, and vertically polarized light was the least attractive if the stimulus intensities and spectra were the same. Vertically polarized light was the most attractive for Caenis robusta if its intensity was about two and five times higher than that of the unpolarized and horizontally polarized stimuli, respectively. This mayfly behaviour facilitates the stability of swarming above water surfaces. Besides the open water surface reflecting horizontally polarized light, the shadow and mirror image of riparian vegetation at the edge of the water surface reflect weakly and non-horizontally (mainly vertically) polarized light. Due to their positive polarotaxis, flying mayflies remain continuously above the water surface, because they keep away from the unpolarized or non-horizontally polarizing edge regions (water surface and coastline) of water bodies.