Polarization vision of aquatic insects

Abstract

In this chapter we show that primary aquatic insects fly predominantly in mid-morning, and/or around noon and/or at nightfall. We describe the different types of their diurnal flight activity rhythm characterised by peaks at low and/or high solar elevations. We present here experimental evidence that the polarization visibility Q(θ) of water surfaces is always maximal at the lowest (dawn and dusk) and highest (noon) angles of solar elevation θ for dark waters, while Q(θ) is maximal at dawn and dusk (low solar elevations) for bright waters both under clear and partly cloudy skies. The θ-dependent reflection-polarization patterns, combined with an appropriate air temperature, clearly explain why polarotactic aquatic insects disperse to new habitats in mid-morning, and/or around noon and/or at dusk. This phenomenon is called the“polarization sundial” of dispersing aquatic insects. We also show that non-biting midges (Chironomidae, Diptera) are positively polarotactic and like many other aquatic insects, their females are attracted to horizontally polarized light. We present here measured thresholds (i.e., the minimum degrees of linear polarization of reflected light that can elicit positive polarotaxis) of the ventral polarization sensitivity in mayflies, dragonflies and tabanid flies. The mayflies Palingenia longicauda swarm exclusively over the river surface; thus, they need not search for water. It could be assumed that this species is not polarotactic. We show here that also P. longicauda has positive polarotaxis, which, however, can be observed only when the animals are displaced from the water and then released above artificial test surfaces. P. longicauda is the first species in which polarotactic water detection was demonstrated albeit it never leaves the water surface, and thus, a polarotactic water detection seems unnecessary for it. The yellow fever mosquito, Aedes aegypti, has been thought to locate its breeding habitats exclusively by chemical cues. We demonstrate here that horizontally polarized light can also attract ovipositing Ae. aegypti females when they are deprived of chemical cues. Aedes aegypti is the first known water-associated species in which polarotaxis exists, but does not play a dominant role in locating water bodies and can be constrained in the presence of chemical cues. Finally, we deal with the negative polarotaxis in the desert locust, Schistocerca gregaria, the ventral eye region of which detects the horizontally polarized water-reflected light, and thus can navigate towards or away from large water surfaces.