Dorsal ocellar system of the American cockroach: II. Structure of the ocellar tract

Abstract

The structure of the cockroach ocellar tract has been examined. Six types of neural processes have been identified in the ocellar tract: long retinular axons, thick second-order processes, dark-vesicle-containing processes, dense-granule-containing processes, thick third-order processes, and through fibers to the ventral nerve cords. Synapses of polarities similar to those found in the ocellar nerve are found in the ocellar tract. These results suggest that the ocellar nerve and ocellar tract may be a single continuous neuropil mass, an extension of the CNS to the ocellar retina. However, many unidentified processes occur in the ocellar tract, and their frequent synapses further suggest that the ocellar tract may play a role in CNS integrative functions in addition to the integration of ocellar information. Synapses from thick second-order processes to the thick third-order process are assumed to form a main ocellar pathway, though there are many other third-order processes such as through fibers. Efferent properties of the dark-vesicle-containing processes are confirmed. In addition, two pairs of giant monopolar neurons (40–65 μm) occur in the pars intercerebralis, and their descending axons maintain close proximity to the ocellar tract as they extend to the ventral nerve cords.