A ‘polarisation sun‐dial’ dictates the optimal time of day for dispersal by flying aquatic insects

Abstract

1. Daily changes in the flight activity of aquatic insects have been investigated in only a few water beetles and bugs. The diel flight periodicity of aquatic insects and the environmental factors governing it are poorly understood. 2. We found that primary aquatic insects belonging to 99 taxa (78 Coleoptera, 21 Heteroptera) fly predominantly in mid-morning, and/or around noon and/or at nightfall. There appears to be at least four different types of diurnal flight activity rhythm in aquatic insects, characterised by peak(s): (i) in mid-morning; (ii) in the evening; (iii) both in mid-morning and the evening; (iv) around noon and again in the evening. These activity maxima are quite general and cannot be explained exclusively by daily fluctuations of air temperature, humidity, wind speed and risks of predation, which are all somewhat stochastic. 3. We found experimental evidence that the proportion (%) P(h) of reflecting surfaces detectable polarotactically as 'water' is always maximal at the lowest (dawn and dusk) and highest (noon) angles of solar elevation (h) for dark reflectors while P(h) is maximal at dawn and dusk (low solar elevations) for bright reflectors under clear or partly cloudy skies. 4. From the temporal coincidence between peaks in the diel flight activity of primary aquatic insects and the polarotactic detectability P(h) of water surfaces we conclude that the optimal times of day for aquatic insects to disperse are the periods of low and high solar elevations h. The h-dependent reflection-polarisation 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. We call this phenomenon the 'polarisation sun-dial' of dispersing aquatic insects.

Bridges as optical barriers and population disruptors for the mayfly Palingenia longicauda: an overlooked threat to freshwater biodiversity?

“…This flight can be observed only if the mayflies are captured and released on the river bank. P. longicauda shows positive polarotaxis (Kriska et al 2007), similarly to other mayfly species (Schwind 1995;Kriska et al 1998;Turcsányi et al 2009) and aquatic insects in general (Schwind 1991;Wildermuth 1998Wildermuth , 2007Bernáth et al 2002;Horváth and Varjú 2004;Csabai et al 2006;Wehner and Labhart 2006;Horváth et al 2008;Lerner et al 2008;Kriska et al 2009). Mayflies receive the polarized light signals reflected from the water surface by their ventral polarization-sensitive eye region (Kriska et al 2007) and identify water by exactly or nearly horizontally polarized light (with angles of polarization 80°\ a \ 100°clockwise from the vertical) with medium degrees of polarization (15°\ d \ 60°) ).…”

Glass buildings on river banks as “polarized light traps” for mass-swarming polarotactic caddis flies

“…Note that male caddis flies should also be polarotactic because they also must find water, where the females occur (Horváth and Varjú 2003). Hence, these insects find water by means of the horizontally polarized water-reflected light, like aquatic insects in general (Schwind 1991(Schwind 1995Horváth and Varjú 2003;Bernáth et al 2004;Csabai et al 2006).…”

When do beetles and bugs fly? A unified scheme for describing seasonal flight behaviour of highly dispersing primary aquatic insects

“…elevation of the sun which determines the polarotactic detectability of water 41 surfaces (Csabai et al 2006), air temperature (Weigelhofer et al 1992), water temperature (Popham 1953, 1957). Generally, the objects of these investigations are restricted to only a few species.…”