Singleeye

Singleeye is a term used to describe a single, simple light-sensing organ in some animals, commonly referred to as an ocellus. This contrasts with the multiple lens systems of compound eyes found in many arthropods. In biology, 'single eye' or ocellus typically denotes a basic photoreceptive structure that cannot produce detailed images but can discriminate light intensity and, in some species, light direction and movement.

An ocellus usually consists of a basic lens that focuses light onto a sheet of photoreceptor cells. The surrounding pigment and supporting tissues regulate light entry and sensitivity. Ocelli vary among taxa: some groups have a single ocellus on the head, while others possess multiple ocelli arranged in simple patterns. In many insects, ocelli assist with circadian entrainment and stabilization during flight, complementing the function of compound eyes.

Taxonomic notes: Ocelli are widespread in invertebrates, including insects and crustaceans, and occur in flatworms and some mollusks. They are generally considered separate from compound eyes and are often interpreted as evolutionary precursors or complementary light-sensing structures rather than full visual systems. In humans and most vertebrates, the term 'single eye' is not used for a defined anatomical organ; the expression may appear in cultural, historical, or fictional contexts.

See also: ocellus, simple eye, photoreception, monocular vision.