Wavelength-specific negatively phototactic responses of the burrowing mayfly larvae Ephoron virgo.

Mayflies are typically negatively phototactic during larval development, while their adults possess positive phototaxis. However, no extensive research was made about the wavelength dependence of phototaxis in any mayfly larvae. We measured the repellency rate of Ephoron virgo larvae to light as a function of wavelength in the 368 - 743 nm spectral range. We established that the magnitude of repellence increased with decreasing wavelength and the maximal responses were elicited by 400 nm violet light. This wavelength dependence of phototaxis is similar to the recently reported spectral sensitivity of positive phototaxis of the twilight-swarming E. virgo adults. Negative phototaxis not only facilitates evading predation, avoidance of the blue-violet spectral range could promote the larvae to withdraw towards the river midline in the case of drop in the water level, when the underwater light becomes enriched with shorter wavelengths due to the decreasing depth of overhead river water.