Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye. They are found in all arthropod groups, although they may have evolved more than once within this phylum. Some annelids and bivalves also have apposition eyes. They are also possessed by Limulus, the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point. (Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.)
The gene dachshund is involved in the development of the compound eye.
Different opsins are used in the ocelli to the compound eyes.
Both ocelli and compound eyes were probably present in the last common arthropod ancestor, and may be apomorphic with ocelli in other phyla, such as the annelids. Median ocelli are present in chelicerates and mandibulates; lateral ocelli are also present in chelicerates.
It is deemed probable that the compound eye arose as a result of the 'duplication' of individual ocelli. In turn, the disperseal of compound eyes seems to have created large networks of seemingly independent eyes in some arthropods, such as the larvae of certain insects. In some other insects and myriapods, lateral ocelli appear to have arisen by the reduction of lateral compound eyes.
Limulus has two large compound eyes on the sides of its head. An additional simple eye is positioned at the rear of each of these structures. In addition to these obvious structures, it also has two smaller ocelli situated in the middle-front of its carapace, which may superficially be mistaken for nostrils. A further simple eye is located beneath these, on the underside of the carapace. A further pair of simple eyes are positioned just in front of the mouth. The simple eyes are probably important during the embryonic or larval stages of the organism, with the compound eyes and median ocelli becoming the dominant sight organisms during adulthood. These ocelli are less complex, and probably less derived, than those of the mandibulata. Unlike the trilobites', the compound eyes of Limulus are triangular in shape; they also have a generative region at their base, but this elongates with time. Hence the one ommatidium at the apex of the triangle was the original "eye" of the larval organism, with subsequent rows added as the organism grew.
Crustacean can have stalked eyes on peduncles.
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