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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. Dorsal ocelli are a visual pathway that co-exist with the compound eyes in many insects (e.g. Hymenoptera (bees, ants, wasps, sawflies), Diptera (flies), Odonata (dragonflies, damselflies) and Orthoptera (grasshopers, locusts). The number, form, and function of the dorsal ocelli varies markedly throughout insect orders. Despite similar form, the evolutionary origins and functions of the dorsal ocelli appear to be distinct from those of the lateral ocelli (found, for example, in Lepidoptera). Dorsal ocelli are light-sensitive organs found on the dorsal (top-most) surface or frontal surface of the head. They tend to be larger and more strongly expressed in flying insects (particularly bees, wasps, dragonflies and locusts), where they are typically found as a triplet. Two lateral ocelli are directed to the left and right of the head respectively, while a central (median) ocellus is directed frontally. In some terrestrial insects (e.g. some ants and cockroaches), only two lateral ocelli are present, the median ocellus is absent. A dorsal ocellus consists of a lens element (cornea) and a layer of photoreceptors (rod cells). As noted above, ocelli vary widely among insect orders. The ocellar lens may be strongly curved (e.g. bees, locusts, dragonflies) or flat (e.g. cockroaches). The photoreceptor layer may (e.g. locusts) or may not (e.g. blowflies, dragonflies) be separated from the lens by a clear zone (vitreous humour). The number of photoreceptors also varies widely, but may number in the hundreds or thousands for well developed ocelli.
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