Synaptic organization of the cabbage looper moth ocellus.

Chemical and electronic synapses are present in the ocellar synaptic region of the moth, Trichoplusia ni. The chemical synapses all appear to be of "conventional" type. Four different chemical synaptic contacts were observed: Receptor cell axons presynaptic to receptor cell axons, receptor cell axons presynaptic to 1st order interneurons, 1st order interneurons presynaptic to receptor cell axons, and 1st order interneurons presynaptic to 1st order interneurons. Two different types of contact made by electronic synapes were observed: Contacts between receptor cell axons and 1st order interneurons, and contacts between 1st order interneurons. The significance of this synaptic arrangement for the generation of "on" and "off" responses in the 1st order interneurons is discussed.