Insect Ocelli: Ecology, Physiology, and Morphology of the Accessory Visual System

The peripheral photoreceptor system of adult insects and larvae of incompletely metamorphosed insects consists of a pair of compound eyes and several simple chamber-type eyes, or ocelli. The origin of ocelli is linked with the simple eyes of crustacean larvae; ocelli, along with compound eyes, form the basic plan of the photosensory system of insects. The development of these light-sensitive organs is closely related to flight, allowing body position to be maintained relative to the horizon. They are highly sensitive and fast-acting, which is critical for small individuals which are easily carried away by air currents. Insect species which have transitioned to life in low-light conditions have ocelli of greater size and can also have enhanced photosensitivity due to a reflective tapetum and loss of polarization sensitivity and color discrimination. When light intensity decreases below a critical level, for example, when living in caves, ocelli disappear. Ocelli in actively moving daytime insects can acquire sensitivity to polarization, features of object vision, and several (usually two) spectral photoreceptor types. The high speed of the ocellar visual system is ensured by its small number of synaptic relays and direct connections with effector neural circuits.