eyespot

Eyespot, also called an ocellus, is a simple photosensitive structure found in a variety of animals. It typically comprises light-detecting cells arranged behind a pigmented, cup-shaped shielding layer. Eyespots detect light intensity and, in some cases, the direction of light, but they do not form detailed images. They are considered among the most basic visual organs.

In flatworms such as planarians and in many invertebrates, eyespots are paired or clustered photoreceptor structures connected to the central nervous system by nerves. The pigment shield provides directional discrimination, enabling the organism to orient relative to light, which influences behaviors such as phototaxis and habitat selection. In arthropods, ocelli are a form of eyespot that generally detects light intensity and can contribute to circadian regulation and flight.

Eye spots that appear on the wings of some butterflies, moths, and other insects are non-photosensitive pigment-based markings sometimes called wing eyespots. These resemble true eyes and function mainly in predator deterrence by misdirecting attacks toward less vital parts of the creature or by startling potential predators.

Evolution and study: Eyespots illustrate an early stage in the evolution of vision. They show how photosensitive cells can form a light-sensing system without forming detailed images, and how such systems can be elaborated into more complex eyes through changes in photoreceptor cells, pigment arrangement, and neural connections. The development of eyespots across lineages often involves conserved gene networks, including Pax6, underscoring the deep evolutionary roots of visual systems.