The two-tailed pasha, Charaxes jasius (Lepidoptera: Nymphalidae) is the largest European butterfly. It has a pair of very large eyes that are suitable for physiological investigations of vision. Its retina was examined with anatomical, electrophysiological and optical methods. We measured the spectral and polarization sensitivity of the photoreceptors, moulded a replica of the cornea, produced serial light microscopic sections of the retina and recorded the eye shine. We have found that each compound eye is assembled of ~14000 ommatidia, which reflect green or red light and contain 9 photoreceptors, arranged similar as in other nymphalid butterflies. The distal photoreceptors R1 in R2 are sensitive to UV (λmax = 340 - 350 nm), blue (B; λmax = 415 - 430 nm) or green (G; λmax = 535 nm) light; the distal photoreceptors R3 and R4 have broadband sensitivity, peaking in the green (G; λmax = 545 nm). The proximal receptors R5-8 have broadband sensitivity, peaking in the green (G; λmax = 545 nm) or orange (OR; λmax = 580 nm). Photoreceptors R1 and R2 have negative-going responses to long-wavelength light. Using selective chromatic adaptation, current injection and receptive field measurements, we have proved that the negative signals originate from photoreceptors R3-9 within the same ommatidium, which project inhibitory synapses to R1 and R2 and together form colour-opponent pairs. We have indirectly measured the spectral sensitivity of the red opponent unit (R; λmax = 620 nm). Its identity is probably R9, the basal photoreceptor, which is only 8 - 18 μm long. Photoreceptors R3-9 are not postsynaptic to adjacent photoreceptors. R1 and R2 receive inhibitory input from either R3 and R4, or R5-8, or with R9, and thus form the opponent pairs UV+G-, UV+OR-, B+G, B+OR- and G+R-. We assume that R3-9 express the same rhodopsin (R540), which has in R5-9 its absorbance peak further long-wavelength shifted due to filtering with distal rhabdomeres and red screening pigments. The red pigments are contained within the ommatidia that reflect red eye shine, which are enriched in the equatorial and ventral retina. The two-tailed pasha is the first Nymphalid butterfly with described opponent pairs of photoreceptors, with a green-sensitive photoreceptor in location R1 or R2, and with a basal red receptor.
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