Most insects have larger compound eyes and smaller simple eyes called ocelli. “Ocelli” literally means, “Little eyes”. The lenses of the ocelli are made of cuticle secreted by epidermal cells of the insect. Some insects, such as dragonflies have well developed ocelli with a cuticular lens that focuses light on the light receptors. Dragonfly ocelli may have thousands of receptors that may be used to detect patterns, especially the location of the horizon.
In other insects the ocelli may contain far fewer receptors (hundreds instead of thousands) with lenses of marginal focusing quality that may not focus light on the receptors. At the high end of ocelli function, Dragonflies may process a relatively complete “view” of the horizon. In other insects, the ocelli seem to be important for flight, but have less capability.
Eyes such as ocelli, that contain a single lens, are called “simple eyes”. Other animals with “simple eyes” include spiders, millipedes, mollusks and vertebrates. The human eye contains a single lens and is thus considered a “simple eye”. The single lens of a simple eye can be contrasted with the multiple lenses of the insect compound eye. A compound eye is composed of multiple similar units, each unit with a cuticular lens and another type of lens called a crystalline cone. Only crustaceans and insects have compound eyes.