Fine Structure of the Compound Eye of Nicagus japonicus Nagel (Coleoptera : Scarabaeoidea, Lucanidae)

The compound eye of Nicagus japonicus Nagel, a lucanid species which is thought to be diurnally active and primitive, has been examined by light and electron microscopy and compared with that of other scarabaeoid beetles. The compound eye lacks a canthus but the anteocular margin of the head possesses a small process in front of the eye. The compound eye is composed of approx. 650 ommatidia of the eucone scotopic type. The crystalline cone is relatively small and tapered and its distal tip never contacts the cornea. Four large Semper cell nuclei occupy the interposed space of 4μm. Although the rhabdom is made up of the seven rhabdomeres of the seven retinula cells per ommatidium other than the basal cell, no distal rhabdomere is formed. However, one of the rhabdomeres is formed by longer, thicker and straighter microvilli of lesser electron density than the other rhabdomeres. The rhabdom occupation ratio, based on transverse sections of the retinula cells, is approx. 24%. Collectively the anatomical features of the Nicagus japonicus compound eye suggest that greater similarity exists with trogid species than with scarabaeid or lucanid species.