compound eye(redirected from Ommotidium)
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compound eye:see eyeeye,
organ of vision and light perception. In humans the eye is of the camera type, with an iris diaphragm and variable focusing, or accommodation. Other types of eye are the simple eye, found in many invertebrates, and the compound eye, found in insects and many other
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(also faceted eye), the principal paired organ of vision in insects, crustaceans, and some other invertebrates. Compound eyes are made up of special structural units, ommatidia, whose corneal lenses are in the shape of convex hexahedrons. In insects they are immobile and located at the sides of the head, often occupying almost the entire surface, for example, in dragonflies, flies, and bees. In crustaceans they sometimes are set on movable stalks. The compound eyes most thoroughly studied are those of insects and larvae of species with incomplete metamorphosis; the eyes of such insects are composed of hundreds or even thousands of ommatidia.
Depending on the anatomical characteristics and optical properties of the ommatidia, three types of compound eyes are distinguished: apposition, superposition, and neurosuperposition. In apposition compound eyes, which are usually characteristic of diurnal insects, adjacent ommatidia are isolated from one another by an opaque pigment, and the receptors perceive only perpendicular light rays falling along the axis of a given ommatidium.
In superposition compound eyes, which are characteristic of nocturnal and crepuscular insects and many crustaceans, the ommatidia may be isolated as a result of the pigment’s ability to shift. When there is insufficient light, the rays that fall at an oblique angle are superposed and pass through several facets. Thus the sensitivity of the eye increases with weak illumination.
Neurosuperposition compound eyes accumulate signals from sense cells in various ommatidia, which receive light from the same point in space. In some insects, such as praying mantises and mayflies, one part of the eye may be of the apposition type, while the other is of the superposition type.
In all types of compound eyes the rhabdomeres of the sense cells, which contain a photopigment similar to rhodopsin, serve as the photosensitive element. The absorption of quanta of light by the photopigment is the first link in the chain of processes that results in the generation of a nerve signal by the sense cell.
Because of the projection of the retina on the optic ganglia of the brain and, in part, the characteristics of compound-eye optics, the ommatidium’s raster plays a more important role in the perception of the environment than individual sense cells. Because the optical axes of the ommatidia diverge at extremely acute angles (l°–6°), the compound eye is unable to distinguish minute details. However, its high adaptability and high contrast sensitivity (1–5 percent) permit some insects to distinguish the flashing of light at frequencies as high as 250–300 hertz (Hz) (the human limit is approximately 50 Hz). Compound eyes enable many invertebrates to distinguish color, perceive ultraviolet rays, and determine the direction of the plane of linearly polarized light.
REFERENCESMazokhin-Porshniakov, G. A. Zrenie nasekomykh. Moscow, 1965.
Prosser, L., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. Chapter 12. (Translated from English.)
G. A. MAZOKHIN-PORSHNIAKOV