Spectral organization of ommatidia in flower-visiting insects.

This article reviews recent advances of studies on the spectral organization of the compound eye in lepidopteran and hymenopteran insects. The compound eyes consist of ommatidia, which contain a set of photoreceptor cells. The common feature is that the ommatidia can be divided into three types, according to the combination of spectral classes of photoreceptors. Honeybees and nymphalid butterflies provide the simplest case with three photoreceptor classes having peak sensitivities in the ultraviolet (UV), blue (B), or green (G) wavelength region. These receptors populate the ommatidia in fixed combinations. In type I ommatidia, the main eight photoreceptors consist of one UV, one B, and six G receptors. Type II has two UV and six G receptors, and type III has two B and six G receptors. The organization is basically retained in all insect species studied so far, although some butterflies have more than six classes of spectral receptors, including those sensitive in the violet and red wavelength regions. To acquire these additional receptors, two distinct strategies are applied: the multiple opsin strategy, taken by the Japanese yellow swallowtail butterfly Papilio xuthus, and the filter strategy, used by the small white butterfly Pieris rapae.