from The American Heritage® Dictionary of the English Language, 4th Edition
- n. One of the structural elements, resembling a single simplified eye, that make up the compound eye of insects and other arthropods.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. One of the conical substructures which make up the eyes of invertebrates.
from the GNU version of the Collaborative International Dictionary of English
- n. One of the single eyes forming the compound eyes of crustaceans, insects, and other invertebrates; one of the eyes of an ommateum.
from The Century Dictionary and Cyclopedia
- n. A radial element or segment of the compound eye of an arthropod.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. any of the numerous small cone-shaped eyes that make up the compound eyes of some arthropods
Since insects have compound eyes (with straight tubes or light guides radiating out from the centre of the eye like the spines of a hedgehog), this might amount in practice to something as simple as keeping the light in one particular tube or ommatidium.
We do not know the stages through which the eye has passed to its present perfected state, but, since the number of simple eyes (facets) has become very much greater in the male than in the female, we may assume that their increase is due to a gradual duplication of the determinants of the ommatidium in the germ-plasm, as I have already indicated in regard to sense-organs in general.
In Bombus impatiens a similar pattern is likely, since UV opsin is found in zero, one or two receptors per ommatidium, and these cells always correspond to the R1 or R5 photoreceptors
In the honeybee each ommatidium contains six green photoreceptors, which give rise to the short visual fibres.
Each of the crab's ommatidium is like one of your camera's pixels - but instead of millions there is just 8,000.
Their compound eyes have up to 30,000 facets, each of which is a separate light-sensing organ or ommatidium, arranged to give nearly a 360° field of vision, important for taking prey on the wing, as has done the female shown above.
Nephrops norvegicus), and increases in sensitivity up to 1000 above the more common apposition type eye (where light remains within a single facet/ommatidium).