Insect ocelli: ecology, physiology, and morphology of the accessory visual system

Abstract

The peripheral photoreceptor system of adult insects and insect larvae of hemimetabolous insects consists of a pair of compound facet eyes and several simple chamber eyes, the ocelli. The origin of the ocelli is attributed to the simple eyes of crustacean larvae; the ocelli, along with the compound eyes, present the basic plan of the photosensitive system of insects. The evolution of these light-sensitive organs is closely related to flight, allowing to maintain the position of the body in relation to the horizon, they have high sensitivity and fast signal processing, which is critical for small animals easily carried away by air currents. In low light conditions, ocelli increase in size and in some cases also increase light sensitivity through light-reflecting tapetum, loss of polarization sensitivity and color separation. When light intensity is reduced below a critical level, such as in cave dwellers, ocelli disappear. In actively moving diurnal insects, ocelli can acquire polarization sensitivity, features of object vision, and several, mostly two, spectral types of photoreceptors. The high speed of the ocellar visual system is ensured by a small number of synaptic connections projecting to motor circuits.