Opponent processing in the retinal mosaic of nymphalid butterflies

The eyes of nymphalid butterflies, investigated with incident illumination, show colourful facet reflection patterns, the eye shine, which is uniform or heterogeneous, dependent on the species. Facet colours suggest that the ommatidia contain different sets of photoreceptors and screening pigments, but how the colours and the cell characteristics are associated has not been clearly established. Here we analyse the retinae of two nymphalids, Apatura ilia, which has a uniform eyeshine, and Charaxes jasius, a species with a heterogeneous eye shine, using single-cell recordings, spectroscopy and optical pupillometry. Apatura has UV, blue and green-sensitive photoreceptors, allocated into three ommatidial types. The UV and blue-sensitive cells are long visual fibres (LVFs), receiving opponent input from the green-sensitive short visual fibres (SVFs). Charaxes has an expanded set of photoreceptors, allocated into three additional, red-reflecting ommatidial types. All red ommatidia contain green-sensitive LVFs, receiving opponent input from red receptors. In both species, the SVFs do not receive any opponent input. The simple retina of Apatura with three ommatidial types and two different colour-opponent channels can support trichromatic vision. Charaxes has six ommatidial types and three different colour-opponent channels. Its expanded receptor set can support tetrachromatic vision.Funding provided by: Air Force Office of Scientific ResearchCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000181Award Number: FA9550-15-1-0068Funding provided by: Javna Agencija za Raziskovalno Dejavnost RSCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100004329Award Number: P3-0333Funding provided by: Ministrstvo za Izobraževanje, Znanost in ŠportCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100005989Award Number: 5442-1/2018/434Electrophysiology, single cell recordings