Ocellar system of the insect: comparison of dorsal ocellus and lateral ocellus

The ocellar receptor and neuropil in the dorsal ocellar system of the cockroach and the lateral ocellar system of the swallowtail butterfly larva have been compared. In the dorsal ocellus of the cockroach more than 10,000 retinular cells each with a rhabdomere are piled up one upon another without a specific pattern in the ocellar capsule. Retinular axons synapse exclusively with branches of four thick second order neurons in the lower region of the capsule. Dark granule-containing neurons, originating in the brain, also synapse with the second order neurons in the ocellar neuropil. The ocellar nerve is comprised of four thick second order processes (ca. 6-16 micron) and 20-40 thin processes (ca. 1-5 micron), which include retinular axons, collaterals of thick processes, possible third order processes and unidentified processes. Synapses occur against these processes in the ocellar nerve and in the extreme proximal region of the ocellar neuropil. Six lateral ocelli occur on both sides of head in the butterfly larva. Each ocellus contains seven retinular cells, which are grouped into three distal cells and four proximal cells, or four distal cells, two peripheral cells and one central cell by the position of their rhabdomeres. Retinular axons extend to the brain, and enter the distal optic neuropil. There are six neuropils, each of which contains the seven axons from one ocellus and a dozen or so second order processes. Of seven retinular axons, four occur peripherally and the remaining three occur centrally in the neuropil. All axons synapse with fine branches of interneurons. Second order processes together with at least three central retinular axons extend to the second optic neuropil, but the peripheral axons could not be followed there. The highly organized receptors and the neuropil in the lateral ocellar system suggest a complexly integrated function. An ocellar nerve and proximal ocellar neuropil appear like an extension of the CNS, and direct connections of retinular axons with interneurons in the CNS appear characteristic of the dorsal ocellus. This difference may be related to the distribution of the two ocellar systems: the dorsal ocelli co-exist with the compound eyes of the adult, whereas the lateral ocelli are the only photoreceptor organs of the larval insect.