Linearly polarized light as a guiding cue for water detection and host finding in tabanid flies

Abstract

In this chapter we show that tabanid flies are attracted to horizontally polarized light stimulating their ventral eye region. Female and male tabanids use this polarotaxis governed by the horizontal E-vector to find water, while another type of polarotaxis based on the degree of polarization serves host finding by female tabanids. We show that female tabanids are less attracted to bright than dark hosts, the reason for which is partly that dark hosts reflect light with higher degrees of polarization than bright hosts. We also demonstrate that the use of a striped fur pattern has the advantage that such coat patterns attract far fewer tabanids than either homogeneous black, brown, grey or white equivalents. The attractiveness of striped patterns to tabanids is also reduced if only polarization modulations (parallel stripes with alternating orthogonal directions of polarization) occur in homogeneous grey surfaces. The attractiveness to tabanids decreases with decreasing stripe width, and stripes below a certain width threshold are unattractive at all to tabanids. Further, the stripe widths of zebra coats fall in a range where the striped pattern is most unattractive to tabanids. Tabanids are strongly attracted by CO2 and ammonia emitted by their hosts. We show here that the poor visual attractivity of stripes to tabanids is not overcome by olfactory attractiveness. Finally, we show that dark spots on a bright coat surface also disrupt the visual attractiveness to tabanids. The smaller and the more numerous the spots, the less attractive the host is to tabanids. The attractiveness of spotty patterns to tabanids is also reduced if the target exhibits spottiness only in the angle of polarization pattern, while being homogeneous grey with a constant high degree of polarization. This could be one of the possible evolutionary benefits that explains why spotty coat patterns are so widespread in mammals, especially in ungulates, many species of which are tabanid hosts.