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First published online January 27, 2004
Journal of Experimental Biology 207, 755-765 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00810
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Why is it worth flying at dusk for aquatic insects? Polarotactic water detection is easiest at low solar elevations

Balázs Bernáth1,2, József Gál3 and Gábor Horváth1,*

1 Biooptics Laboratory, Department of Biological Physics, Eötvös University, H-1117 Budapest, Pázmány sétány 1, Hungary,
2 Plant Protection Institute of the Hungarian Academy of Sciences, Department of Zoology, H-1525 Budapest, P. O. B. 102, Hungary
3 International University Bremen IUB, School of Engineering and Science, P. O. B. 750561, D-28725 Bremen-Grohn, Germany

* Author for correspondence (e-mail: gh{at}arago.elte.hu)

Accepted 20 November 2003

Using 180° field-of-view imaging polarimetry, we measured the reflection-polarization patterns of two artificial surfaces (water-dummies) in the red, green and blue spectral ranges under clear and partly cloudy skies at different solar elevations. The dummies consisted of a horizontal glass pane with a matt black or matt light grey cloth underneath, imitating a dark or bright water body, respectively. Assuming that polarotactic water insects interpret a surface as representing water if the degree of linear polarization of reflected light is higher than a threshold and the deviation of the direction of polarization from the horizontal is lower than a threshold, we calculated the proportion, P, of the artificial surfaces detected polarotactically as water. We found that at sunrise and sunset P is maximal for both water-dummies and their reflection-polarizational characteristics are most similar. From this, we conclude that polarotactic water detection is easiest at low solar elevations, because the risk that a polarotactic insect will be unable to recognize the surface of a dark or bright water body is minimal. This partly explains why many aquatic insect species usually fly en masse at dusk. The daily change in the reflection-polarization pattern of water surfaces is an important visual ecological factor that may contribute to the preference of the twilight period for habitat searching by polarotactic water insects. Air temperature at sunrise is generally low, so dusk is the optimal period for polarotactic aquatic insects to seek new habitats.

Key words: dusk-flying water insects, aquatic habitat recognition, polarotaxis, reflection polarization, polarization sensitivity, 180° field-of-view imaging polarimetry


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© The Company of Biologists Ltd 2004