Information Processing in the Insect Ocellar System: Comparative Approaches t o the Evolution of Visual Processing and Neural Circuits a a This chapter is dedicated to Prof. Hideki Tateda, Shimonoseki City University, Japan.

Publisher Summary This chapter discusses the anatomy, the physiology, and the behavioral roles of insect ocellar systems to provide a search image for the still enigmatic visual system and to theorize on the diversities of ocellar systems. Huge diversities exist in the anatomy, physiology, and behavioral roles among insect ocelli, which prevent a full understanding of the functions of insect ocelli. The chapter also summarizes the diversities of ocellar systems in their structure, neural organization, physiology, and behavioral roles and attempts to interpret them in terms of phylogenetic pathways and adaptations to the environment. This attempt is based on the belief that insect ocellar systems will prove to be a rich field from which to study the evolution of neural circuits. The evolution of neural systems is one of the focal points of neurobiology, but the lack of adequate model systems has hindered the extension of this important research field. The behavioral roles of insect ocelli can be classified into two categories according to their relationship to those of the principal visual organs, the compound eyes. First, the ocelli directly participate in some visual behaviors to complement the compound eyes. Second, the ocelli modulate behaviors mediated by the compound eyes. Many studies have been made on the neural organization of insect ocellar systems using the cobalt filling of the ocellar nerve, the intracellular staining of individual ocellar neurons, and electron microscopy. These studies have revealed both common features and fundamental differences in insect ocellar systems. Electron microscopic studies have shown that the synaptic organizations of ocellar plexus are complicated and differ among different insects.