Ocellus

Overview
An ocellus (plural: ocelli) is a type of photoreceptor organ in animals. Also called "simple eyes", ocelli are miniature eyes capable of sensing light but not distinguishing its direction. See also stemmata, which are structurally similar. Ocelli are found in many invertebrates. Insects in particular have two types of ocelli, dorsal ocelli and lateral ocelli.

Photoreceptors
In insects, ocelli are involved (along with compound eyes) in the horizon-detecting response, decreasing the latency between horizon changes and the insect's response.

Dorsal Ocelli
Dorsal ocelli are light-sensitive organs on the dorsal surface or on the top of the head. There are generally three, forming a triangle.

They consist of a few dozen rod cells covered with a corneal lens. In some species, such as locusts, dorsal ocelli may contain several hundreds of retinula cells. Insects with dorsal ocelli also have compound eyes.

Lateral Ocelli
Lateral ocelli have a mixture of rod cells and cone cells and are found on the sides of the head, one to six on each side.

Lateral ocelli are the only eyes of the larvae of several orders of insects (fleas, springtails, silverfish, and Strepsiptera).