Arthropod eye

The arthropods ancestrally possessed compound eyes, but the type and origin of this eye varies between groups, and some taxa have secondarily developed simple eyes. The organ's development through the lineage can be estimated by comparing stem groups such as the onychophora and Limulus to the crown group condition.

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.)

Limulus

Limulus, the horseshoe crab, has traditionally been used in investigations into the eye, because it has relatively large ommatidia with large nerve fibres (making them easy to experiment on). It also falls in the stem group of the chelicerates; its eyes are believed to represent the ancestral condition because they have changed so little over evolutionary time. Indeed the horseshoe crabs are often considered to be living fossils. Most other living chelicerates have lost their lateral compound eyes, evolving simple eyes in their place.

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.

Eyes and functions

A large number of arthropods have two types of eye: large compound eyes, and smaller ocelli. The two eye types are used in concert, because each has its own advantage. Insects can function perfectly well with either type of eye surgically removed, but the two types combine to give better performance. Ocelli can detect lower light levels, and have a faster response time, while compound eyes are better at detecting edges and objects.