How does the water springtail optically locate suitable habitats? Spectral sensitivity of phototaxis and polarotaxis in Podura aquatica.

Optical detection of horizontally polarized light is widespread among aquatic insects. This process usually occurs in the UV or blue spectral ranges. Recently, it was demonstrated that at least one collembolan species, the water springtail (Podura aquatica) also possesses positive polarotaxis to horizontally polarized light. These hexapods are positively phototactic, live on the surface of calm waters and usually accumulate close to the riparian vegetation. In laboratory experiments, we measured the wavelength dependence of phototaxis and polarotaxis of P. aquatica in the 346-744 nm and 421-744 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 show that collembolan polarotaxis functions in the same spectral range as the polarotaxis of many aquatic insects. We present our experiments and discuss the possible ecological significance of our findings.