The compound eye of the dung beetle, Bolbocerosoma nigroplagiatum (Coleoptera: Scarabaeidae)

The compound eye of a nocturnal dung beetle, Bolbocerosoma nigroplagiatum, was observed by light and electron microscopy and compared with that of other scarabaeid beetles. The eye is completely divided into two parts by a well developed canthus into relatively narrow dorsal and comparatively wide ventral eyes. The inner surface of the cornea protrudes inwards and forms an exocone, while at the same time the crystalline cone is much reduced Such a condition of a ''duocone'' dioptric apparatus seems to have been found so far only in Omorgus of the Trogidae. The retina is of the scotopic type, and the configuration of the rhabdom is different in dorsal and ventral eyes. The rhabdoms of several rows of the dorsalmost region in the dorsal eye are composed of three rhabdomeres in which parallel and long microvilli are orientated in three directions, and four V-shaped rhabdomeres in transverse section. In the remaining part of the dorsal eye as well as in the entire ventral eye, the rhabdoms exhibit a seven-lobed pattern. One of the rhabdomeres is formed by parallel and linear microvilli which are less electron-dense. A microvillar arrangement such as this suggests a special function e.g. polarization vision.