Ommatidium


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Related to Ommatidium: Rhabdomere

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.

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).
521 Species Individual square Number of ommatidium area ommatrichia ([micro][m.
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.
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.
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 shortening of both cornea and cone, relative to the overall length of one ommatidium, and the loss of the orderly arrangement of microvilli in the rhabdom also point toward an adaptation to minimize photon loss and maximize photon capture (Laughlin et al.
These results suggest that constant inhibition from surrounding receptors changed the temporal response properties of the recorded ommatidium.
We teased away a nerve fiber of an ommatidium located at the center of the eye and pulled it into the microsuction electrode to record its response (6).
The rhabdomeral segments (R-segments) of the photoreceptors of each ommatidium cannot be individually distinguished as they contribute to the large volume of rhabdom.