Structural reactions to polarized light of microvilli in photoreceptor cells of the moth Spodoptera

Extreme polarisation sensitivity in the retina of the corn borer moth Ostrinia

The visual system of the European corn borer (Ostrinia nubilalis) was analysed with microscopy and electrophysiological methods (electroretinograms and single-cell recordings). Ostrinia nubilalis has a pair of mainly ultraviolet-sensitive ocelli and a pair of compound eyes, maximally sensitive to green light. The ommatidia contain a tiered, fused rhabdom, consisting of the rhabdomeres of 9-12 photoreceptor cells with sensitivity peak wavelengths at 356, 413, 480 and 530 nm. The photoreceptors in a large dorsal rim area have straight rhabdomeres and high polarisation sensitivity (PS_1,2=3.4, 14). Elsewhere, in the main retina, the majority of photoreceptors have non-aligned microvilli and negligible PS, but each ommatidium contains one or two blue-sensitive distal photoreceptors with straight microvilli parallel to the dorsoventral axis, yielding extremely high PS (PS_1,2,3=56, 63, 316). Rhabdoms containing distal cells with potentially high PS have evolved at least twice: in moths (Crambidae, Noctuidae, Saturniidae), as well as in dung beetles (Scarabaeidae). The distal photoreceptors with high PS, sensitive to vertically polarised light, represent a monopolatic system, which is unsuitable for the proper analysis of electric field vector (e-vector) orientation. However, the distal photoreceptors might be used in conjunction with polarisation-insensitive photoreceptors to detect objects that reflect polarised light with stereotyped orientation.