How does the water springtail optically locate proper habitats? Spectral sensitivity of photo- and polarotaxis in Podura aquatica

Abstract

Optical water detection by means of horizontally polarized light is widespread among aquatic insects. This ability of these insects usually works in the UV or blue spectral ranges. Recently it was demonstrated that at least one collembolan species, the water springtail (Podura aquatica) also possess positive polarotaxis to horizontally polarized light. These hexapods are positively phototactic, live on the surface of calm waters and usually accumulate in the intimate vicinity of the riparian vegetation. In laboratory experiments we measured the wavelength dependence of phototaxis and polarotaxis of P. aquatica in the 346 nm – 744 nm and 421 nm – 744 nm ranges, respectively. According to our results, the action spectrum of phototaxis is bimodal with two peaks in the blue (λ1=484 nm) and green-yellow (λ2=570 nm) ranges, while polarotaxis operates in the blue spectral range. For the first time we have shown that collembolan polarotaxis functions in the same spectral range as the polarotaxis of many aquatic insects. In this paper we present our experiments and discuss the possible ecological significance of our findings.