On the ultrastructure and identity of the eyes of Cyphophthalmi based on a study of Stylocellus sp. (Opiliones, Stylocellidae)

Abstract

The laterally positioned eyes of stylocellid mite harvestmen are simple ocelli composed of a uniconvex cuticular lens, a lentigen layer, a retina, and a layer of pigment cells. Basal laminae separate the lentigen layer (preretinal membrane) and the pigment layer (postretinal membrane) from the retina. The retina is composed of retinular cells and glial cells. The optic nerve comprises mostly afferent axons formed by the retinular cells, which are accompanied by glial cells. Likely there are also few efferent axons. The retinular cells are characterized by their peculiar nuclei, numerous granules and indications of high membrane turnover. A prominent central region of the eye shows numerous, but poorly ordered interdigitations of long microvilli-like processes presenting a poorly developed closed rhabdom. A smaller region with microvilli forming a rather disordered, open rhabdom is located opposite to the lens. The retina is proximally and laterally surrounded by pigment cells containing, in addition to the usual dense granules, some crystalline inclusions, which may act as a tapetum. Hence, the retina seems to be composed of a proximal part and a distal part with two differently organized simple rhabdoms. The eyes of Stylocellus thus show basically the same organization as the median (primary) eyes of other Opiliones and are likely laterally displaced median (primary) eyes. The occurrence of these primary eyes in Stylocellidae (and Pettalidae) strengthens the idea that the presence of median eyes is thus a plesiomorphic character of these cyphophthalmid harvestmen.