Aquatic insect species that leave the water after larval development, such as mayflies, have to deal with extremely different visual environments in their different life stages. Measuring the spectral sensitivity of the compound eyes of the virgin mayfly (Ephoron virgo) resulted in differences between the sensitivity of adults and larvae. Larvae were primarily green-, while adults were mostly UV-sensitive. The sensitivity of adults and larvae were the same in the UV, but in the green spectral range, adults were 3.3 times less sensitive than larvae. Transmittance spectrum measurements of larval skins covering the eye showed that the removal of exuvium during emergence cannot explain the spectral sensitivity change of the eyes. Taking numerous sky spectra from the literature, the ratio of UV and green photons in the skylight was shown to be maximal for θ ≈ − 13° solar elevation, which is in the θmax = -14.7° and θmin = -7.1° typical range of swarming that was established from webcam images of real swarmings. We suggest that spectral sensitivity of both the larval and adult eyes are adapted to the optical environment of the corresponding life stages.

Funding provided by: National Research, Development and Innovation Fund of Hungary*Crossref Funder Registry ID: Award Number: 131738/PD_19Funding provided by: Ministry for Innovation and TechnologyCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100015498Award Number: ÚNKP-21-3Funding provided by: Jan Gershoj*Crossref Funder Registry ID: Award Number: