Compound eye ultrastructures in six species of ecologically diverse stag-beetles (Coleoptera, Scarabaeoidea, Lucanidae)

Abstract

The structural organizations of the compound eyes of six Japanese stag-beetles, i.e. Aesalus asiaticus (abbr. Aa) and Platycerus acuticollis (Pa) of the subfamily Aesalinae, Figulus boninensis (Fb) of the Figulinae, Nipponodorcus rubrofemoratus (Nr) of the Dorcinae, Lucanus maculifemoratus (Lm) and L. gamunus (Lg), both of Lucanidae, were examined by light and electron microscopy. The results of this study formed the basis for a comparison between eye structures on the one hand and beetle ecology, activity, and phylogeny on the other. In Pa, instead of a true cone, some electron dense material was seen in each cone cell; this suggests that the typical "eucone condition" had been replaced by an acone type of eye or a transitional condition between the eucone and acone types. The eyes of all other species were of the eucone type. The retinae of both Aa and Pa exhibited typically photopic organization, but those of Nr and Lm were clearly representatives of the scotopic type. The retinae of Fb and Lg were intermediate in nature. The retinula cells numbered eight in Aa, Pa, and most of the ommatidia of Fb; in the other species they varied in number from eight to 11. Consequently, the rhabdom itself exhibited variations depending on the number of the retinula cells. With 88%, the ratio of rhabdom to cytoplasmic area in transverse sections of the retinula cells, was largest in Lm and Nr, while in Aa and Pa it measured a mere 5-6%. The differences appear to parallel the degree to which the species are active in day or at night.