ocelli

Ocelli, singular ocellus, are simple photoreceptive organs found in many invertebrates, especially arthropods such as insects, spiders, and crustaceans. They are distinct from the more complex compound eyes, which form images. Ocelli primarily detect light intensity and changes in illumination, helping organisms regulate circadian rhythms, orientation, flight stability, and startle responses. In some vertebrate groups, the term is used for analogous light-sensing structures, such as photoreceptive organs in the pineal region, though these are often referred to as parietal or third eyes.

Anatomy and function vary across groups but share common features. An ocellus typically consists of a small lens or corneal covering over underlying photoreceptor cells connected to a simple nervous pathway. Unlike compound eyes, ocelli usually do not form detailed images. They may be sensitive to light direction and intensity and, in some species, to polarization of light. The information from ocelli is integrated with other visual and neural cues to influence behaviors such as navigation, timing of daily activities, and stabilization during flight.

Distribution and variation. In insects, there are commonly three ocelli on the top of the head: one median and two lateral. In spiders and other arachnids, ocelli are present in various arrangements and numbers depending on species. Some lineages exhibit reduced or absent ocelli. The presence and arrangement of ocelli can reflect an organism’s ecology and reliance on light cues for survival.

Evolution and terminology. Ocelli are considered primitive, homologous light-sensing organs in many animal groups. The term ocellus comes from Latin meaning “little eye.” In some vertebrates, similar light-sensing structures in the pineal region are sometimes described in terms of a parietal or third eye rather than ocelli.