Why do dusk-active cockchafers detect polarization in the green? The polarization vision in Melolontha melolontha is tuned to the high polarized intensity of downwelling light under canopies during sunset

In the retina of dusk-active European cockchafers, Melolontha melolontha, the linear polarization of downwelling light (skylight or light from the tree canopy) is detected by photoreceptors in upward-pointing ommatidia with maximal sensitivity at 520 nm in the green portion of the spectrum. To date no attempt has been made to answer the question of why these beetles detect polarization in the green. Here we present an atmospheric optical and receptor-physiological explanation of why longer wavelengths are advantageous for the perception of polarization of downwelling light under canopies illuminated by the setting sun. Our explanation focuses on illumination situations during sunset in canopied optical environments, because cockchafers are active at sunset and fly predominantly under canopies during their swarming, feeding, and mating periods. Using three simple atmospheric optical models, we computed the degree of linear polarization, the linearly polarized intensity of downwelling light, the quantum catch, and quantum catch difference between polarization detectors with orthogonal microvilli under canopies illuminated by the setting sun as functions of wavelength and solar zenith angle. Based upon these computations, we show that the green sensitivity of polarization detectors in M. melolontha is tuned to the high polarized intensity of downwelling light in the green under canopies during sunset, an optimal compromise between simultaneous maximization of the quantum catch and the quantum catch difference. We also briefly discuss how green-sensitive polarization detectors can function efficiently enough during the pre-feeding and egg-laying flights of cockchafers, which always occur prior to sunset and under the sky.