Compound Eye(redirected from Ommotidium)
Also found in: Dictionary, Thesaurus, Medical.
eye, 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 arthropods. In an alternate pathway to the one that transmits visual images, the eye perceives sunlight. This information stimulates the hypothalamus, which passes the information on to the pineal gland. The pineal gland then regulates its production of the sleep-inducing chemical, melatonin, essentially setting the body's circadian clock (see rhythm, biological).
The Human Eye
Anatomy and Function
The human eye is a spheroid structure that rests in a bony cavity (socket, or orbit) on the frontal surface of the skull. The thick wall of the eyeball contains three covering layers: the sclera, the choroid, and the retina. The sclera is the outermost layer of eye tissue; part of it is visible as the “white” of the eye. In the center of the visible sclera and projecting slightly, in the manner of a crystal raised above the surface of a watch, is the cornea, a transparent membrane that acts as the window of the eye. A delicate membrane, the conjunctiva, covers the visible portion of the sclera.
Underneath the sclera is the second layer of tissue, the choroid, composed of a dense pigment and blood vessels that nourish the tissues. Near the center of the visible portion of the eye, the choroid layer forms the ciliary body, which contains the muscles used to change the shape of the lens (that is, to focus). The ciliary body in turn merges with the iris, a diaphragm that regulates the size of the pupil. The iris is the area of the eye where the pigmentation of the choroid layer, usually brown or blue, is visible because it is not covered by the sclera. The pupil is the round opening in the center of the iris; it is dilated and contracted by muscular action of the iris, thus regulating the amount of light that enters the eye. Behind the iris is the lens, a transparent, elastic, but solid ellipsoid body that focuses the light on the retina, the third and innermost layer of tissue.
The retina is a network of nerve cells, notably the rods and cones, and nerve fibers that fan out over the choroid from the optic nerve as it enters the rear of the eyeball from the brain. Unlike the two outer layers of the eye, the retina does not extend to the front of the eyeball. Between the cornea and iris and between the iris and lens are small spaces filled with aqueous humor, a thin, watery fluid. The large spheroid space in back of the lens (the center of the eyeball) is filled with vitreous humor, a jellylike substance.
Accessory structures of the eye are the lacrimal gland and its ducts in the upper lid, which bathe the eye with tears, keeping the cornea moist, clean, and brilliant, and drainage ducts that carry the excess moisture to the interior of the nose. The eye is protected from dust and dirt by the eyelashes, eyelid, and eyebrows. Six muscles extend from the eyesocket to the eyeball, enabling it to move in various directions.
Eyes in Other Animals
The camera type of eye, which forms excellent images, is found in all vertebrates, in cephalopods (such as the squid and octopus), and in some spiders. In each of those groups the camera type of eye evolved independently. In some species, e.g., kestrels, the eye can perceive ultraviolet light, an aid to tracking prey.
Simple eyes, or ocelli, are found in a great variety of invertebrate animals, including flatworms, annelid worms (such as the earthworm), mollusks, crustaceans, and insects. An ocellus has a layer of photosensitive cells that can set up impulses in nerve fibers; the more advanced types also have a rigid lens for concentrating light on this layer. Simple eyes can perceive light and dark, enabling the animal to perceive the location and movement of objects. They form no image, or a very poor one.
The compound eye is found in a large number of arthropods, including various species of insects, crustaceans, centipedes, and millipedes. A compound eye consists of from 12 to over 1,000 tubular units, called ommatidia, each with a rigid lens and photosensitive cells; each omnatidium is surrounded by pigment cells and receives only the light from its own lens. The lenses fit together on the surface of the eye, forming the large, many-faceted structure that can be seen, for example, in the fly. Each ommatidium supplies a small piece of the image perceived by the animal. The compound eye creates a poor image and cannot perceive small or distant objects; however, it is superior to the camera eye in its ability to discriminate brief flashes of light and movement, and in some insects (e.g., bees) it can detect the polarization of light. Because arthropods are so numerous, the compound eye is the commonest type of animal eye.
(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