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Ommatidium |
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ommatidium [‚äm·ə′tid·ē·əm]
(invertebrate zoology) The structural unit of a compound eye, composed of a cornea, a crystalline cone, and a receptor element connected to the optic nerve. Ommatidium 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. Want to thank TFD for its existence? Tell a friend about us, add a link to this page, add the site to iGoogle, or visit the webmaster's page for free fun content. |
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No references found | Ommatidia develop a greenish-yellow fringe, producing a "halo" effect around eye. 318), it has been obvious for many years that such structures need not be exceptionally small and need not be extremely like ommatidia to behave like ommatidia. The resulting columns of hardened and optically altered material inside the hemisphere act as ommatidia because they guide light along a precise line from each lens, Lee says. |
ommatidia |
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