Ocellus is a type of simple eye or an optical arrangement that contains a single lens. It is named after the Celtic god Ocelus, who was venerated as a protector. It is not necessarily simple in the usual sense of the word. Human eyes and camera lenses are classified as simple, because in both cases a single lens collects and focuses light onto the retina or film.
Dorsal ocelli are light-sensitive organs, which coexist with compound eyes. They are larger and more strongly expressed on flying insects like dragonflies, bees, wasps and locust where they are typically found as a triplet. One theory is that their function is to assist in maintaining flight stability.
Spiders have several pairs of simple eyes with each pair adapted for a specific task. The forward-facing pair possess the best resolution with telescopic components to see prey at a large distance.
We are constantly learning new tricks from the natural world. Research on the ocellus is of high interest to designers of small drones since these crafts face many of the same challenges that insects face in maintaining stability in a 3-D world.