oculli

An ocellus, or ocelli in the plural, is a simple photoreceptive organ that detects light intensity and direction without forming a detailed image. The plural oculli appears in some literature, but ocelli is the standard form.

An ocellus typically consists of a small pigment-lined cell cup containing one or a few photoreceptor cells, sometimes with a transparent lens or corneal cover that focuses light onto the receptors. Compared with compound eyes, ocelli have limited resolution and do not contribute to spatial vision. They connect to the nervous system via a single optic nerve tract or a small cluster of neurons.

Functionally, ocelli help regulate circadian rhythms, stabilize flight, and guide movement relative to light sources such as the sun. They provide rapid light-dark detection, which can be important in fast-moving species to maintain horizon orientation or avoid overexposure to bright light.

Ocelli occur in many invertebrate groups, most prominently in arthropods. In insects, adult heads typically bear three ocelli (two lateral and one median) in addition to the two compound eyes; spiders likewise have multiple ocelli arranged on the prosoma and often use them for light sensing. The exact structure and number vary among taxa.

Overall, ocelli are an important example of non-image-forming photoreceptors that complement the visual system by providing ambient light information.