page_title: Arthropod eye
section_title: Insects and crustaceans
hierarchical_section_title: Arthropod eye / Evolution / Insects and crustaceans
caption_attribution_description: English: This is an electron micrograph of the compound eye of a bed bug.
context_page_description: 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 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.
context_section_description: It generally thought that insects are a clade within the Crustacea, and that the Crustacea are monophyletic. This is consistent with the observation that their eyes develop in a very similar fashion.  While most crustacean and some insect larvae possess only simple median eyes, such as the Bolwig's organ pit-eye of Drosophila and the naupliar eye of most crustaceans, several groups have larvae with simple or compound lateral eyes.  The compound eyes of adults develop in a region of the head separate from the region in which the larval median eye develops.  New ommatidia are added in semicircular rows at the rear of the eye; during the first phase of growth, this leads to individual ommatidia being square, but later in development they become hexagonal.  The hexagonal pattern will become visible only when the carapace of the stage with square eyes is molted.
  Although stalked eyes on peduncles occur in some species of crustaceans and some insects, only some of the Crustacea, such as crabs, bear their eyes on articulated peduncles that permit the eyes to be folded out of the way of trouble.
caption: This is an electron micrograph of the compound eye of a bed bug.
exif: {}