Abstract
The compound eye is of typical scotpic organization and is composed of approx. 2000 ommatidia, all of which are of the eucone type. The rhabdom region is rather extensive in M. castanea and amounts to about 2/3 of the retinal width. The distal 28-30% of the rhabdom layer is composed of a single large rhabdomere per ommatidium, made up of microvilli of 1.5 times wider diameter than those of the proximal rhabdomeres. The larger portion of the rhabdom exhibits a characteristic six-lobed pattern in transverse section. The rhabdom occupation area is about 65% at mid-rhabdom level. One rhabdomere-less basal retinula cell is present per ommatidium. It contains spherical, 0.9 μm in diameter, screening pigment granules even at its most distal eng. A tracheal tapetum is lacking despite this species' nocturnal habit. The clear-zone between dioptric structures and light-perceiving elements is widest in the ventral eye region, whereas interommatidial angles are widest in the extreme dorsal part of the eye and narrower form the centre of the eye towards mid-ventral where they once again begin to get larger. This arrangement may allow the beetle to see more detail below and alongside itself than above.
