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(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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
In arthropods, the size, shape, color, ommatidium number and surface texture of the compound eye influence many features of the visual field including its dimensions, acuity and sensitivity (Rutowski 2000).
dorsalis 3751.80 776.25 1268.92 [+ or -] 37.84 a [+ or -] 12.55 b [+ or -] 11.20 a F 0.958 4.246 0.659 P 0.390 0.019 0.521 Species Individual square Number of ommatidium area ommatrichia ([micro][m.sup.2]) B.
This can be demonstrated by locating the unique "index" ommatidium, which is the ommatidium with its optic axis horizontal and normal to the body axis of the crab.
Figure 2 is a diagram of a typical ommatidium from the hemispherical regions of the eye outside the midband.
These types vary primarily in the construction of the rhabdom, so only this part of each ommatidium is diagrammed in Figure 3.
From the Hartline-Ratliff formulations of lateral inhibitory interactions in the retina (7), the firing rate of the pth ommatidium ([R.sub.p]) equals its pure excitatory response ([E.sub.P)] minus the sum of inhibitory influences from its neighbors; the inhibition delivered from the jth ommatidium equals the strength of its inhibitory coupling to the pth ommatidium ([Ksub.pj]) multiplied by the response of the jth ommatidium, These relationships can be expressed as:
We recorded the response of a single, optically-isolated ommatidium to a steady 30-s light stimulus and every two seconds applied a brief (100 ms) train of four current pulses to the optic nerve trunk.
A second eccentric cell located in the same ommatidium generated much smaller action potentials ([approximately]2.5 mV) with the same firing rate (see legend).
The distance between the proximal end of the rhabdom and the basement membrane of an ommatidium is relatively short in Petrolisthes and other shallow-water decapods.
The former study, however, illuminated the entire retina, whereas the latter study limited the stimulus to a single ommatidium. Thus, the difference between the TTFs in Figure 1A (solid vs.
The average area occupied by an individual ommatidium was then measured and the number of ommatidia calculated from the total retinal area.
Illumination of a single ommatidium in the retina generally evokes slow depolarizing potentials (0-5 mV) in association with a single train of action potentials (20-60 mV) in the compartment, whereas illumination of the entire retina elicits slow hyperpolarizing potentials (5-15 mV) and a spike train of reduced rate.