The Silver-washed fritillary (Argynnis paphia, Linnaeus 1758) is a woodland butterfly from the family Nymphalidae. Using electrophysiological, anatomical and optical methods, we examined and compared the retina of both sexes. We measured the spectral and polarization sensitivity of photoreceptors with microelectrodes and measured the absorbance of screening and filtering pigments in semi-thin sections. Each ommatidium has 9 photoreceptors (R1–9). Receptors R1 and 2 are sensitive to UV (λmax = 335 nm) or blue (λmax = 435 nm); in males also to green (λmax = 535 nm) light. R3–8 have maximum sensitivity in the green (λmax = 545 nm), in males also in the orange (λmax = 575 nm). The basal cell R9 has a sensitivity peak red shifted to 615 nm. The sensitivity of R9 could only be measured by selective adaptation of green-sensitive photoreceptors with red opponency. Namely, the photoreceptors form mutual inhibitory synapses and thereby narrow their spectral sensitivity, which improves the resolution of color vision. In both sexes the opponent receptor pairs UV+G– and B+G– occur, while males have three additional pairs B+G+, B+Y– and G+R–. In the ventral part of the male retina, we found cells that most likely co-express blue and green opsin and form the opponent pair B+G+R–. In histological preparations, we showed that red screening pigments (RSP) are found in the distal part of the retina of all ommatidia in both sexes. In males however, RSP in some ommatidia that are located in the proximal retina spectrally filter the light incident in the proximal retina and thus shift the R9 sensitivity peak to the red part of the spectrum. Females have trichromatic color vision, but in males we assume that the system of RSP and functional receptor R9 creates an additional red channel and thus the physiological basis for tetrachromatic vision.
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