Morphology of higher-order ocellar interneurons in the cockroach brain

Most species of insects have two or three ocelli, in addition to a pair of compound eyes. In the cockroach ocellus, a large number of photoreceptors converge upon four second-order neurons, which exit the ocellus and project into the ocellar tract neuropil of the brain, where they form synapses with at least 15 third-order neurons. Third-order neurons project into a variety of neuropils in the brain, including the posterior slope, a premotor center from which descending neurons originate. I examined the morphology and ocellar response of neurons in the posterior slope of the cockroach, using intracellular recording and stainings. Most ocellar neurons of the posterior slope exhibited transient depolarizations at the cessation of ocellar illumination, which seem to reflect sign-conserving synaptic input from third-order neurons. Ocellar neurons of the posterior slope project into various areas of the brain, including 1) the central complex, a higher center implicated in higher locomotory control, 2) the pedunculus of the mushroom body, an associative center, 3) the lamina (the first neuropil of the culus of the mushroom body, an associative center, 3) the lamina (the first neuropil of the optic lobe), 4) the antennal lobe (olfactory center), 5) the tritocerebrum (mechanosensory center), and 6) the subesophageal and thoracic motor centers. These results suggest that the posterior slope is a high-order ocellar center from which ocellar signals are transmitted to various target neuropils of the brain, as well as a premotor center to form descending motor commands.