A 'polarisation sun-dial' dictates the optimal time of day fordispersal by flying aquatic insects

摘要

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. It was 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 appear to be at least four different types of diurnal flight activity rhythm in aquatic insects, characterised by peak(s): (1) in mid-morning; (2) in the evening; (3) both in mid-morning and the evening; (4) 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. Experimental evidence was found that the proportion (%) P( theta ) of reflecting surfaces detectable polarotactically as 'water' is always maximal at the lowest (dawn and dusk) and highest (noon) angles of solar elevation theta for dark reflectors while P( theta ) is maximal at dawn and dusk (low solar elevations) for bright reflectors under clear or partly cloudy skies. From the temporal coincidence between peaks in the diel flight activity of primary aquatic insects and the polarotactic detectability P( theta ) of water surfaces it is concluded that the optimal times of day for aquatic insects to disperse are the periods of low and high solar elevations theta . The theta -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. This phenomenon is termed the 'polarisation sun-dial' of dispersing aquatic insects