Ocellus, one of the simple eyes found in many of the Arthropoda. In this phylum the eyes occur in two distinct types. There is the compound eye, which contains a large number of distinct structures, each of which has all the elements of an eye; each element in this eye has its own lens, visual rods, retinal cells, etc.; in one eye there may be thousands of these structures grouped into a mass. In the second place, there is the ocellus, which consists of one such structure placed by itself. In some of the simplest insects, such as the Collembola (q.v.), and in Limulus (q.v.), it is the only optical organ present. In others, such as the larvae of the May-flies (Ephemeridae) and many Arachnida, both types of eye are present; others, such as the larvae of the Dermaptera or Earwigs, have only the compound eye.