Definition: A photoreceptor organ in animals. The ocellus receives light signals, but is a simple lens and cannot locate direction.
Many insects have ocelli (plural for ocellus). Some insects, like butterflies and locusts, have dorsal ocelli, which are located on the top of the head. These insects also have compound eyes. Others have lateral ocelli, on the sides of the head. Insects with lateral ocelli include sprintails, fleas, and silverfish.
