Functional diversity of neural organization in insect ocellar systems

Most insects have simple eyes (ocelli), in addition to compound eyes. Although the ocelli can detect only changes in light intensity averaged over their large visual field, they do play various roles in the behavior of insects, as they have advantages over compound eyes in terms of photic sensitivity and the speed of signal transmission. I summarize here our present knowledge on the neural organization of ocellar systems of a number of insects, and propose that they can be classified into three types: (1) the "cockroach type" ocellar system where signals from the photoreceptors first converge onto a small number of second-order neurons, and then diverge to a large number of third-order neurons which project into a number of target neuropils of the brain; (2) the "bee type" ocellar system where photoreceptor signals are passed to various target neuropils by a large number of second-order neurons; and (3) the "locust type" ocellar system where ocellar signals are transmitted to various target neuropils by both second- and third-order neurons. I propose that: (1) the "cockroach type" ocellar system can be characterized as a sensitive type where higher sensitivity is attained because of the higher ratio of convergence of photoreceptor signals onto second-order neurons; (2) the "bee type" ocellar system is a fast type where a high speed of signal transmission is attained at the possible cost of sensitivity; and (3) the "locust type" ocellar system is an intermediate type where both speed and sensitivity are emphasized. A possible phylogenetic relationship between the three types of ocellar systems is discussed.