ocellus

An ocellus is a simple, light-detecting organ found in a range of animals, particularly many invertebrates. The word comes from the Latin ocellus, meaning “little eye.” Ocelli are not capable of forming detailed images; instead, they provide information about light intensity and sometimes light direction.

Structure and function

Most ocelli consist of a pigment-containing light sensor layer that responds to illumination. Some ocelli have additional features such as a protective cornea or a lens, but they generally lack the complex optics of true image-forming eyes. The primary role of ocelli is to guide behavior through light sensing: regulating circadian rhythms, aiding in horizon detection, and helping stabilize flight or movement in response to changes in ambient light.

Distribution across taxa

Ocelli occur in many invertebrate groups, especially insects, where most species possess three ocelli arranged on the top of the head between or above the lateral simple and compound eyes. In spiders, some species have simple eyes (ocelli) arranged in various patterns. Planarians and other flatworms possess simple eye-like structures that function as light spots. In some vertebrate lineages, a parietal or “third” eye—known as the pineal or parietal ocellus in reptiles such as the tuatara and some lizards—functions as a light sensor related to circadian biology, though it is not a conventional image-forming eye.

Relation to other eyes

Ocelli are distinct from compound eyes, which comprise numerous ommatidia that collectively form images. Ocelli provide coarse light sensitivity and occasional directional information, while compound eyes deliver high-resolution visual input.

Evolution and diversity

Ocellular photoreception appears to have evolved multiple times across different lineages. Ocelli can be reduced or modified in species with other dominant eye systems, reflecting their flexible role in light perception and behavioral regulation.