A normal housefly has compound eyes. A compound eye may consist of thousands of individual photoreceptor units or ommatidia.
Simple eye in invertebrates
The visual system is the part of the central nervous system which gives organisms the ability to process visual detail, as well as enabling the formation of several non-image photo response functions. It detects and interprets information from visible light to build a representation of the surrounding environment. The visual system carries out a number of complex tasks, including the reception of light and the formation of monocular representations; the buildup of a binocular perception from a pair of two dimensional projections; the identification and categorization of visual objects; assessing distances to and between objects; and guiding body movements in relation to visual objects. The psychological process of visual information is known as visual perception, a lack of which is called blindness. Non-image forming visual functions, independent of visual perception, include the pupillary light reflex (PLR) and circadian photoentrainment.
A simple eye (sometimes called a pigment pit) refers to a type of eye form or optical arrangement that contains a single lens. A "simple eye" is so called in distinction from a multi-lensed "compound eye", and is not necessarily at all simple in the usual sense of the word. The eyes of humans and large animals, and camera lenses are classed as "simple" because in both cases a single lens collects and focuses light onto the retina or film. Many insects have compound eyes consisting of multiple lenses (up to tens of thousands), each focusing light onto a small number of retinula cells.
The structure of an animal's eye is determined by the environment in which it lives, and the behavioural tasks it must fulfill to survive. Arthropods differ widely in the habitats in which they live, as well as their visual requirements for finding food or conspecifics, and avoiding predators. Consequently, an enormous variety of eye designs are found in arthropods: nature has repeatedly developed novel solutions to overcome visual problems or limitations (for a review of arthropod visual systems see Warrant, 2006).
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.
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.)