Origin
No fossil organisms have been identified as similar to the last common ancestor of arthropods; hence the eyes possessed by the first arthropod remains a matter of conjecture. The largest clue into their appearance comes from the onychophorans: a stem group lineage that diverged soon before the first true arthropods. The eyes of these creatures are attached to the brain using nerves which enter into the centre of the brain, and there is only one area of the brain devoted to vision. This is similar to the wiring of the median ocelli (small simple eyes) possessed by many arthropods; the eyes also follow a similar pathway through the early development of organisms. This suggests that onychophoran eyes are derived from simple ocelli, and the absence of other eye structures implies that the ancestral arthropod lacked compound eyes, and only used median ocelli to sense light and dark. However, a conflicting view notes that compound eyes appeared in many early arthropods, including the trilobites and eurypterids, suggesting that the compound eye may have developed after the onychophoran and arthropod lineages split, but before the radiation of arthropods. This view is supported if a stem-arthropod position is supported for compound-eye bearing Cambrian organisms such as the Anomalocaridids. An alternative, however, is that compound eyes evolved multiple times among the arthropods.
There were probably only a single pair of ocelli in the arthropod concestor; Cambrian lobopod fossils display a single pair, and while many arthropods today have three, four, or even six, the lack of common pathway suggests that a pair is the most probable ancestral state. The crustaceans and insects mainly have three ocelli, suggesting that such a formation was present in their concestor.
It is deemed probable that the compound eye arose as a result of the 'duplication' of individual ocelli. In turn, the dispersal 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.
Read more about this topic: Arthropod Eye, Evolution
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