compound eye The eye of insects and crustaceans, which consists of numerous visual units, the ommatidia. Each ommatidium consists of an outer cuticle covering a lens, beneath which are 6–8 retinal cells surrounding a light-sensitive rhabdom. Adjacent ommatidia are separated by pigment cells. The eye is convex, with nerve fibres from the retinal cells converging onto the optic nerve. There are two types of compound eye. In apposition eyes, typical of diurnal insects, each ommatidium focuses rays parallel to its long axis so that each gives an image of a minute part of the visual field, producing a detailed mosaic image. In superposition eyes, typical of nocturnal insects, the pigment separating ommatidia migrates to the ends of the cells, so that each ommatidium receives light from a larger part of the visual field and the image may overlap with those received by many neighbouring ommatidia. This produces an image that is bright but lacks sharpness of detail.
com·pound eye • n. an eye consisting of an array of numerous small visual units, as found in insects and crustaceans. Contrasted with simple eye.
compound eye Eye that is made up of ommatidia, whose number may vary from a few dozen in some insects to several thousand in animals with good eyesight, e.g. bees and dragonflies. Many insects and crustaceans, some chelicerates, and a few annelids have compound eyes.
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