Ocellus

Ocellus (plural: ocelli) is a simple eye or eye-like photoreceptive organ found in various animal taxa. The term originates from the Latin word 'ocellus', meaning 'little eye'. Ocelli differ from more complex eyes in that they do not provide a sharp image but are sensitive to changes in light intensity and direction.

Structure and Function

An ocellus typically consists of a few photoreceptor cells and pigment cells, which detect changes in light intensity and direction. They do not form images but are used for phototaxis (movement in response to light) and circadian rhythms (biological processes that display an endogenous, entrainable oscillation of about 24 hours).

In insects, ocelli are found on the top or sides of the head and are usually distinct from the compound eyes. In some species, the ocelli are capable of detecting ultraviolet light. Insects such as bees and wasps use their ocelli for navigation, orienting themselves in relation to the sun.

In vertebrates, ocelli are often present in immature stages, but they can also be found in adults, particularly in deep-sea fish where they aid in detecting light in the dark ocean depths.

Evolution

The evolutionary origins of the ocellus are not well understood. It is believed that they may have evolved from a more complex eye in an ancestor that lived in a dim or dark environment, where a simple light detector would have been more beneficial than an eye capable of forming images.

In Culture

The term 'ocellus' is also used in a metaphorical sense in literature and art to refer to something small and round that is eye-like or that functions as an eye.

See Also

• Compound eye
• Photoreceptor cell
• Phototaxis
• Circadian rhythm