Ommatidium(redirected from ommatidial)
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the structural and functional unit of a faceted eye in insects, crustaceans, and some myriapods. The om-matidium consists of three sections: a lens with a fixed focal length, the crystalline cone, and the group of light-sensitive receptor cells with nerve outgrowths that combine into nerve fibers. Each lens appears as a facet in the eye. Ommatidia developed in the course of evolution from isolated simple eyes, which eventually were integrated into compound, or faceted, eyes. The number of ommatidia in a compound eye varies, from 100 in a worker ant to 28,000 in a dragonfly.
A faceted eye is specialized to discern movement and does not produce a sharp image or enough information to discern the shape of an object. The field of vision of a compound eye is very broad; for example, in the locust the visual angle of each om-matidium is 20°. Thus, any movement of a predator or prey would be noticed instantly by at least one ommatidium.