Synaptic organization of the fleshfly ocellus

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Abstract
The synaptic organization of the fleshfly (Boettcherisca peregrina) ocellus has been studied by transmission electron microscopy. Three types of neuronal element are recognized in the ocellus: (1) 70–100 retinula axons (2) four thick ocellar nerve fibres and (3) several thin ocellar nerve fibres. Synaptic connections accompanied by characteristic presynaptic ribbons have been found between these three elements in five combinations. Four of these are numerous (retinula axons synapsing onto thick ocellar nerve fibres or onto thin ocellar fibres; thin ocellar nerve fibres synapsing onto thick ocellar nerve fibres or onto other thin ocellar fibres). One or two of the thin ocellar nerve fibres are mainly presynaptic elements. The remaining thin fibres are both pre- and postsynaptic. These observations suggest that the thick ocellar nerve fibres are afferent and at least one or two of the thin ocellar nerve fibres are efferent. The function of the remaining thin fibres is not known. The fifth combination is a feedback synapse from thin ocellar nerve fibres onto retinula axons. In addition, neuro-glial synapses have been found between thin ocellar nerve fibres and glial cells. The latter two combinations are less common and may provide alternative neuronal pathways for processing ocellar input.