Editing Eye (section)

===Compound eyes===
{{main|Compound eye}}
{{further|Arthropod eye}}
[[File:FLY EYE.jpg|thumb|right|An image of a house fly compound eye surface by using [[scanning electron microscope]]]]
[[File:Insect compound eye diagram.svg|thumb|upright=0.9|Anatomy of the compound eye of an insect]]
[[File:Calliphora vomitoria Portrait.jpg|thumb|Arthropods such as this [[Calliphora vomitoria|blue bottle fly]] have compound eyes.]]

A compound eye may consist of thousands of individual photoreceptor units or ommatidia ([[ommatidium]], singular). The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the [[Polarization (waves)|polarisation]] of light.<ref>{{cite journal
 |url=http://www.suss-microoptics.com/downloads/Publications/Miniaturized_Imaging_Systems.pdf
 |doi=10.1016/S0167-9317(03)00102-3
 |title=Miniaturized imaging systems
 |date=June 2003
 |journal=Microelectronic Engineering
 |volume=67–68
 |issue=1
 |pages=461–472
 |author1=Völkel, R
 |author2=Eisner, M
 |author3=Weible, KJ
 |url-status=usurped
 |archive-url=https://web.archive.org/web/20081001225326/http://www.suss-microoptics.com/downloads/Publications/Miniaturized_Imaging_Systems.pdf
 |archive-date=2008-10-01
}}</ref> Because the individual lenses are so small, the effects of [[diffraction]] impose a limit on the possible resolution that can be obtained (assuming that they do not function as [[phased array]]s). This can only be countered by increasing lens size and number. To see with a resolution comparable to our simple eyes, humans would require very large compound eyes, around {{convert|11|m}} in radius.<ref>{{cite journal|last=Land|first=Michael|title=Visual Acuity in Insects|journal=Annual Review of Entomology|year=1997|volume=42|pages=147–177|url=http://web.neurobio.arizona.edu/gronenberg/nrsc581/eyedesign/visualacuity.pdf|access-date=27 May 2013|doi=10.1146/annurev.ento.42.1.147|pmid=15012311|url-status=dead|archive-url=https://web.archive.org/web/20041123010008/http://web.neurobio.arizona.edu/gronenberg/nrsc581/eyedesign/visualacuity.pdf|archive-date=23 November 2004}}</ref>

Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image.<ref>{{cite journal
| last=Gaten
| first=Edward
| title=Optics and phylogeny: is there an insight? The evolution of superposition eyes in the Decapoda (Crustacea)
| year=1998
| journal=Contributions to Zoology
| volume=67
| issue=4
| pages=223–236
| doi=10.1163/18759866-06704001
| doi-access=free
}}</ref> Compound eyes are common in arthropods, annelids and some bivalved molluscs.<ref>{{Cite journal
| last=Ritchie
| first=Alexander
| title=''Ainiktozoon loganense'' Scourfield, a protochordate from the Silurian of Scotland
| year=1985
| journal=Alcheringa
| volume=9
| page=137
| doi=10.1080/03115518508618961
| issue=2
| bibcode=1985Alch....9..117R
}}</ref> Compound eyes in arthropods grow at their margins by the addition of new ommatidia.<ref name=Mayer2006>
{{Cite journal
| last=Mayer
| first=G.
| year=2006
| title=Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods?
| journal=Arthropod Structure and Development
| volume=35
| issue=4
| pages=231–245
| doi=10.1016/j.asd.2006.06.003
| pmid=18089073
| bibcode=2006ArtSD..35..231M
}}</ref>

====Apposition eyes====
Apposition eyes are the most common form of eyes and are presumably the ancestral form of compound eyes. They are found in all [[arthropod]] groups, although they may have evolved more than once within this phylum. Some [[annelids]] and [[bivalves]] also have apposition eyes. They are also possessed by ''[[Limulus]]'', the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point.<ref name=Land1992/> (Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.)

Apposition eyes work by gathering a number of images, one from each eye, and combining them in the brain, with each eye typically contributing a single point of information. The typical apposition eye has a lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the [[ommatidium]].

====Superposition eyes====
The second type is named the superposition eye. The superposition eye is divided into three types:
* refracting,
* reflecting and
* parabolic superposition

The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to the same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This type of compound eye, for which a minimal size exists below which effective superposition cannot occur,<ref name="Meyer-Rochow 2004">{{cite journal|last1=Meyer-Rochow|first1=VB|last2=Gal|first2=J|title=Dimensional limits for arthropod eyes with superposition optics|journal=Vision Research|date=2004|volume=44|issue=19|pages=2213–2223|doi=10.1016/j.visres.2004.04.009|pmid=15208008|doi-access=free}}</ref> is normally found in nocturnal insects, because it can create images up to 1000 times brighter than equivalent apposition eyes, though at the cost of reduced resolution.<ref>{{cite thesis|type=PhD |last=Greiner |first=Birgit |title=Adaptations for nocturnal vision in insect apposition eyes |publisher=Lund University |date=16 December 2005 |url=http://www4.lu.se/upload/GreinerThesis.pdf |access-date=13 November 2014 |url-status=dead |archive-url=https://web.archive.org/web/20130209164014/http://www4.lu.se/upload/GreinerThesis.pdf |archive-date=9 February 2013 }}</ref> In the parabolic superposition compound eye type, seen in arthropods such as [[mayfly|mayflies]], the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied [[Decapoda|decapod crustaceans]] such as [[shrimp]], [[prawn]]s, [[crayfish]] and [[lobster]]s are alone in having reflecting superposition eyes, which also have a transparent gap but use corner [[mirror]]s instead of lenses.

====Parabolic superposition====
This eye type functions by refracting light, then using a parabolic mirror to focus the image; it combines features of superposition and apposition eyes.<ref name=Cronin2008/>

====Other====
Another kind of compound eye, found in males of Order [[Strepsiptera]], employs a series of simple eyes—eyes having one opening that provides light for an entire image-forming retina. Several of these ''eyelets'' together form the strepsipteran compound eye, which is similar to the 'schizochroal' compound eyes of some [[trilobites]].<ref>{{Cite journal|last1=Horváth|first1=Gábor|last2=Clarkson|first2=Euan N.K.|year=1997|title=Survey of modern counterparts of schizochroal trilobite eyes: Structural and functional similarities and differences|journal=Historical Biology|volume=12|issue=3–4|doi=10.1080/08912969709386565|pages=229–263|bibcode=1997HBio...12..229H }}</ref> Because each eyelet is a simple eye, it produces an inverted image; those images are combined in the brain to form one unified image. Because the aperture of an eyelet is larger than the facets of a compound eye, this arrangement allows vision under low light levels.<ref name=Land1992/>

Good fliers such as flies or honey bees, or prey-catching insects such as [[praying mantis]] or [[dragonfly|dragonflies]], have specialised zones of [[ommatidium|ommatidia]] organised into a [[Fovea centralis|fovea]] area which gives acute vision. In the acute zone, the eyes are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution. The black spot that can be seen on the compound eyes of such insects, which always seems to look directly at the observer, is called a [[pseudopupil]]. This occurs because the [[ommatidia]] which one observes "head-on" (along their [[optical axis|optical axes]]) absorb the [[incident light]], while those to one side reflect it.<ref name="Zeil">{{cite journal |author1=Jochen Zeil |author2=Maha M. Al-Mutairi |year=1996 |title=Variations in the optical properties of the compound eyes of ''Uca lactea annulipes'' |journal=[[The Journal of Experimental Biology]] |volume=199 |issue=7 |pages=1569–1577 |doi=10.1242/jeb.199.7.1569 |url=http://jeb.biologists.org/cgi/reprint/199/7/1569.pdf |pmid=9319471 |access-date=2008-09-15 |archive-date=2009-02-25 |archive-url=https://web.archive.org/web/20090225084203/http://jeb.biologists.org/cgi/reprint/199/7/1569.pdf |url-status=live }}</ref>

There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the [[mysid]] shrimp, ''Dioptromysis paucispinosa''. The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialised retina. The resulting eye is a mixture of a simple eye within a compound eye.

Another version is a compound eye often referred to as "pseudofaceted", as seen in ''[[Scutigera]]''.<ref name="Müller 2003">{{cite journal|last1=Müller|first1=CHG|last2=Rosenberg|first2=J|last3=Richter|first3=S|last4=Meyer-Rochow|first4=VB|title=The compound eye of Scutigera coleoptrata (Linnaeus, 1758) (Chilopoda; Notostigmophora): an ultrastructural re-investigation that adds support to the Mandibulata concept|journal=Zoomorphology|date=2003|volume=122|issue=4|pages=191–209|doi=10.1007/s00435-003-0085-0|s2cid=6466405}}</ref> This type of eye consists of a cluster of numerous [[ommatidia]] on each side of the head, organised in a way that resembles a true compound eye.

The body of ''[[Ophiocoma wendtii]]'', a type of [[brittle star]], is covered with ommatidia, turning its whole skin into a compound eye. The same is true of many [[chiton]]s. The tube feet of sea urchins contain photoreceptor proteins, which together act as a compound eye; they lack screening pigments, but can detect the directionality of light by the shadow cast by its opaque body.<ref name="Ullrich-Luter2011">{{Cite journal | last1=Ullrich-Luter | first1=E.M. | last2=Dupont | first2=S. | last3=Arboleda | first3=E. | last4=Hausen | first4=H. | last5=Arnone | first5=M.I. | title=Unique system of photoreceptors in sea urchin tube feet | doi=10.1073/pnas.1018495108 | journal=Proceedings of the National Academy of Sciences | volume=108 | issue=20 | pages=8367–8372 | year=2011 | pmid= 21536888| pmc=3100952| bibcode=2011PNAS..108.8367U | doi-access=free }}</ref>

====Nutrients====
The '''ciliary body''' is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. The inner layer is transparent and covers the vitreous body, and is continuous from the neural tissue of the retina. The outer layer is highly pigmented, continuous with the retinal pigment epithelium, and constitutes the cells of the dilator muscle.

The '''vitreous''' is the transparent, colourless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye.<ref name=Ali&Klyne1985>{{harvnb|Ali|Klyne|1985|page=8}}</ref> It is produced by certain retinal cells. It is of rather similar composition to the cornea, but contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field, as well as the hyalocytes of Balazs of the surface of the vitreous, which reprocess the [[hyaluronic acid]]), no blood vessels, and 98–99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibres with the [[mucopolysaccharide]] hyaluronic acid, and also a wide array of proteins in micro amounts. Amazingly, with so little solid matter, it tautly holds the eye.