Ommatidium
The
Description
Ommatidia are typically hexagonal in cross-section and approximately ten times longer than wide. The diameter is largest at the surface, tapering toward the inner end. At the outer surface, there is a cornea, below which is a pseudocone that further focuses the light. The cornea and pseudocone form the outer ten percent of the length of the ommatidium.
The specific organ of ommatidia, or eye units, vary between different organisms. The butterfly compound eye consists of multiple ommatidia, each of which consist of nine photoreceptor cells (numbered from R1–R9), and primary and secondary pigment cells.[5] Nymphalid butterflies have the simplest eye ommatidium structure, consisting of eight photoreceptor cells (R1–R8) and a tiny R9 cell organized into a different tier. [5] These "R cells" tightly pack the ommatidium. The portion of the R cells at the central axis of the ommatidium collectively form a light guide, a transparent tube, called the rhabdom.
Although composed of over 16,000 cells,[6] the Drosophila compound eye is a simple repetitive pattern of 700 to 750 ommatidia,[7] initiated in the larval eye imaginal disc. Each ommatidium consists of 14 neighboring cells: 8 photoreceptor neurons in the core, 4 non-neuronal cone cells and 2 primary pigment cells.[6] A hexagonal lattice of pigment cells insulates the ommatidial core from neighboring ommatidia to optimize coverage of the visual field, which therefore affects the acuity of Drosophila vision.[7]
Rhabdomeres
In true
Since an image from the compound eye is created from the independent picture elements produced by ommatidia, it is important for the ommatidia to react only to that part of the scene directly in front of them. To prevent light entering at an angle from being detected by the ommatidium it entered, or by any of the neighboring ommatidia, six
The size of the ommatidia varies according to species, but ranges from 5 to 50 micrometres. The rhabdoms within them may cross-section at least as small as 1.x micrometres, the category of "small" being assigned in some cross-species studies to those under 2 micrometers.
Mechanism of eye determination
Retinal
See also
- Pseudopupil
- Arthropod eye
- Apposition eye
- Superposition eye
References
- PMID 18089122.
- ^ ISBN 978-0-19-958114-6.
- S2CID 85297324.
- ISBN 978-90-04-09227-3.
- ^ PMID 18490396.
- ^ PMID 19737644.
- ^ PMID 2511048.
- S2CID 10924209.
- PMID 16377567.