The ocelli of arctiid moths: ultrastructure of the retina during light and dark adaptation

The ultrastructure of the dorsal ocelli of two arctiid moths ( Arctia caja ( A. caja ) and Creatonotos transiens ( C. transiens ) was investigated. The two ocelli are positioned laterally on the vertex of the head posterior to the antennae, close to the dorsal margin of the compound eyes. The biconvex corneal lens is located at the apex of a cone-shaped cuticular elevation, which encapsulates the retina. The corneagenous cell layer and the cup-like retina with about 100–130 receptor cells in A. caja (70–90 receptor cells in C. transiens ) are adjoined proximally. The retina is completely enclosed by the perineurium and thus separated from the corneagenous cells and the surrounding hemolymph. Irregularly shaped rhabdomeres, consisting of densely packed microvilli, are present in the distal region of the receptor cells. Up to three cells may form a rhabdom. Thus a loose network of photoreceptive structures over the whole retina results. A unique feature of these arctiid ocelli are photoreceptor vacuoles containing microvilli. The function of these organelles is unknown. The rhabdomeric arrangement within the light and dark adapted retina differs considerably. The ultrastructure of the rhabdomeres indicates an intense membrane turnover. However, changes in adaptation state are not accompanied by dramatic changes in the photoreceptive area of an ocellus.