The Compound Eye of the Dung Beetle Geotrupes auratus (Coleoptera : Scarabaeidae)

Abstract

The compound eyes of the dung beetles, Georupes auratus are completely divided into dorsal and ventral eye parts by a well developed canthus. The cone sheath is relatively thick and its proximal extension, known as the cone tract, penetrates the intercellular space in the centre of a group of retinula cells without splitting into four separate components. The cone tract derives its compactness and tight connection with retinular cell bodies from extensive desmosomes. At the dorsalmost edge of the dorsal eye, the rhabdom is relatively small like that of photopic eyes. The microvilli are organized in a diagonal pattern. Over the greater part of the dorsal eye, the rhabdom is formed by seven rhabdomeres which in transverse section show interdigitation. The ventral eye, on the other hand, consists of overall uniform ommatidia, with the rhabdom exhibiting a ¥-shaped pattern in transverse section at mid region. Unlike the situation in scotopic eyes, neither cellular transformation nor migration of retinula cell nuclei are found upon light-dark-adaptation since the apical ends of the retinula cells are tightly connected with cone tract. The term "pseudoscopic eye" is proposed for such an eye. Fine-structurally, only small vesicles appeared at the base of the microvilli during dark-adaptation.