Arthropod eye

Insects and all mandibulates have compound eyes, with the exception of some myriapods that have stemmata

Although arachnids (like this spider) do not have compound eyes, xiphosurans and many other extinct chelicerates do

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.^[1] 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.^[1] Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.^{[citation needed]}

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 groups that branched early, such as the velvet worm and horseshoe crab to the advanced eye condition found in insects and other derived arthropods.