Ocellus and ocellar nerves of Periplaneta americana L. (Orthoptera: Dictyoptera)

Abstract

The structure of the cockroach ocellus is described following electronmicroscopical examination. A flat or slightly concave cuticular lens overlies the visual cell groups contained within the upper two-thirds of the ocellar cup. There are no pigment cells or cone cells, no orderly arrangement of visual cell groups nor layering of the latter. Each visual cell group consists of up to 5 visual cells. Visual cell axons develop from visual cells at different depths along the length of the visual cell groups and pass downwards through the reflecting tapetal layer (urate crystals). In the base of the ocellar cup, into which the visual cell axons pass in bundles, are the ocellar giant fibre branches. In this synapsing region both bar and ribbon-type synapses have been observed.The 5 large bipolar fibres in each ocellar nerve enter the brain close to the midline, pass backwards and downwards to arborise in 3 distinct regions of the ipsilateral brain neuropile. Eight cell bodies have been located associated with the 5 large neurones of the ocellar nerve plus 3 of the smaller fibres. Cobalt chloride iontophoresis has been used to visualise the pathways and cell bodies of the ocellar nerve fibres. One fibre appears to traverse the brain to the region where the nervus corporis cardiaci II has its origin. No ocellar fibres enter the ventral nerve cord directly. Some of the implications of these findings are discussed.