New kind of polarotaxis governed by degree of polarization: attraction of tabanid flies to differently polarizing host animals and water surfaces
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Aquatic insects find their habitat from a remote distance by means of horizontal polarization of light reflected from the water surface. This kind of positive polarotaxis is governed by the horizontal direction of polarization (E-vector). Tabanid flies also detect water by this kind of polarotaxis. The host choice of blood-sucking female tabanids is partly governed by the linear polarization of light reflected from the host’s coat. Since the coat-reflected light is not always horizontally polarized, host finding by female tabanids may be different from the established horizontal E-vector polarotaxis. To reveal the optical cue of the former polarotaxis, we performed choice experiments in the field with tabanid flies using aerial and ground-based visual targets with different degrees and directions of polarization. We observed a new kind of polarotaxis being governed by the degree of polarization rather than the E-vector direction of reflected light. We show here that female and male tabanids use polarotaxis governed by the horizontal E-vector to find water, while polarotaxis based on the degree of polarization serves host finding by female tabanids. As a practical by-product of our studies, we explain the enigmatic attractiveness of shiny black spheres used in canopy traps to catch tabanids.
KeywordsTabanid fly Polarization vision Polarotaxis Degree of polarization Direction of polarization Water detection Host choice Visual ecology
This work was supported by a grant OTKA K-68462 from the Hungarian Science Foundation and grant TabaNOid 232366 (Trap for the Novel Control of Horse-flies on Open-air Fields) funded by the European Commission under the 7th Framework Programme received by G. Horváth and G. Kriska. Gábor Horváth thanks the German Alexander von Humboldt Foundation for an equipment donation. We also thank the valuable comments of three anonymous reviewers.
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