The fine structure of the ocelli of Schistocerca gregaria. The neural organisation of the synaptic plexus.

A study of the organisation of the locust dorsal ocellus shows that the structure is designed to provide the maximum possible effective aperture. The condenser-like cuticular lens and the dispersal of the rhabdome over a large proportion of the circumferential area of the retinula cells increases the light gathering power of the eye. The synaptic plexus of the ocellus has two major features: (i) the retinula cells are repeatedly and reciprocally connected by synapses and junctions, and (ii) there is an extensive lateral and feedback network between the receptors and interneurons. A unified structure is described for a synapse that presents differing profiles dependent upon the angle of section. A distinct morphological class of junction is described between retinula cells. The synaptic arrangements of morphologically identical retinula cells vary from cell to cell and the synaptic plexus is not organised with a high degree of spatial precision. The overall synaptic configurations are discussed in terms of the varied response characteristics of units in the ocellar nerve.