Degrees of polarization of reflected light eliciting polarotaxis in dragonflies (Odonata), mayflies (Ephemeroptera) and tabanid flies (Tabanidae).

With few exceptions insects whose larvae develop in freshwater possess positive polarotaxis, i.e., are attracted to sources of horizontally polarized light, because they detect water by means of the horizontal polarization of light reflected from the water surface. These insects can be deceived by artificial surfaces (e.g. oil lakes, asphalt roads, black plastic sheets, dark-coloured cars, black gravestones, dark glass surfaces, solar panels) reflecting highly and horizontally polarized light. Apart from the surface characteristics, the extent of such a 'polarized light pollution' depends on the illumination conditions, direction of view, and the threshold p* of polarization sensitivity of a given aquatic insect species. p* means the minimum degree of linear polarization p of reflected light that can elicit positive polarotaxis from a given insect species. Earlier there were no quantitative data on p* in aquatic insects. The aim of this work is to provide such data. Using imaging polarimetry in the red, green and blue parts of the spectrum, in multiple-choice field experiments we measured the threshold p* of ventral polarization sensitivity in mayflies, dragonflies and tabanid flies, the positive polarotaxis of which has been shown earlier. In the blue (450nm) spectral range, for example, we obtained the following thresholds: dragonflies: Enallagma cyathigerum (0%<p*< or =17%), Ischnura elegans (17%< or =p*< or =24%). Mayflies: Baetis rhodani (32%< or =p*< or =55%), Ephemera danica, Epeorus silvicola, Rhithrogena semicolorata (55%< or =p*< or =92%). Tabanids: Tabanus bovinus, Tabanus tergestinus (32%< or =p*< or =55%), Tabanus maculicornis (55%< or =p*< or =92%).