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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.



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 ?
exotica appears to be correlated with the area of the compound eye (rather than simply with the size of the animal), we calculated the product of the square root of the total number of ommatidia and the average ommatidial diameter, plotting the values against the maximum length of the eye.
We see confirmation for this notion in the ommatidial organization of the eye of the smaller specimens, which seemingly favors spatial resolution.
Type II ommatidia are located in two of the four most dorsal ommatidial rows of the midband.
3), found in the remaining two ommatidial rows of the midbands of most mantis shrimp species.
Since four rows of the midband include ommatidial Types II and III, eight varieties of receptor tiers exist in their main rhabdoms.
Bursey, 1975; Gaten, 1994) - a distal retinula cell (R8) with four cytoplasmic lobes occupies the tier between the crystalline cone and the seven regular retinular cells, but in Paralomis an ommatidial retinula is composed of only seven regular cells (1-7) and lacks the distal eighth cell.
The estimated membrane surface of an ommatidial rhabdom of Paralomis (231 x [10.
3[[micro]meter]-wide vesicles, penetrate between the retinula cells of individual ommatidial units, thus apparently increasing their effectiveness in reflecting light towards the more distally placed rhabdom.