An ocellus (plural: ocelli) is a type of photoreceptor organ in animals. Also called "simple eyes", ocelli are miniature eyes capable of sensing light but not distinguishing its direction. See also stemmata, which are structurally similar. Ocelli are found in many invertebrates. Insects in particular have two types of ocelli, dorsal ocelli and lateral ocelli.

Dorsal Ocelli

Dorsal ocelli are a visual pathway that co-exist with the compound eyes in many insects (e.g. Hymenoptera (bees, ants, wasps, sawflies), Diptera (flies), Odonata (dragonflies, damselflies) and Orthoptera (grasshopers, locusts, mantises). The number, form, and function of the dorsal ocelli varies markedly throughout insect orders. Despite similar form, the evolutionary origins and functions of the dorsal ocelli appear to be distinct from those of the lateral ocelli (found, for example, in Lepidoptera).

Dorsal ocelli are light-sensitive organs found on the dorsal (top-most) surface or frontal surface of the head. They tend to be larger and more strongly expressed in flying insects (particularly bees, wasps, dragonflies and locusts), where they are typically found as a triplet. Two lateral ocelli are directed to the left and right of the head respectively, while a central (median) ocellus is directed frontally. In some terrestrial insects (e.g. some ants and cockroaches), only two lateral ocelli are present, the median ocellus is absent.

A dorsal ocellus consists of a lens element (cornea) and a layer of photoreceptors (rod cells). As noted above, ocelli vary widely among insect orders. The ocellar lens may be strongly curved (e.g. bees, locusts, dragonflies) or flat (e.g. cockroaches). The photoreceptor layer may (e.g. locusts) or may not (e.g. blowflies, dragonflies) be separated from the lens by a clear zone (vitreous humour). The number of photoreceptors also varies widely, but may number in the hundreds or thousands for well developed ocelli.

Two somewhat unusual features of the ocelli are particularly notable and generally well conserved between insect orders. 1) The refractive power of the lens is not typically sufficient to form an image on the photoreceptor layer. 2) Dorsal ocelli ubiquitously have massive convergence ratios from first- (photoreceptor) to second-order neurons. These two factors have led to the conclusion that the dorsal ocelli are incapable of perceiving form, and are thus solely suitable for light metering functions. Given the large aperture and low f-number of the lens, as well as high convergence ratios and synaptic gains the ocelli are generally considered to be far more sensitive to light than the compound eyes. Additionally, given the relatively simple neural arrangement of the eye (small number of synapses between detector and effector) as well as the extremely large diameter of some ocellar interneurons (often the largest diameter neurons in the animals nervous system) the ocelli are typically considered to be "faster" than the compound eyes [http://www.springerlink.com/content/x0j046w843352w63/] .

One common theory of ocellar function in flying insects holds that they are used to assist in maintaining flight stability. Given their underfocused nature, wide fields of view, and high light collecting ability, the ocelli are superbly adapted for measuring changes in the perceived brightness of the external world as an insect rolls or pitches around its body axis during flight. Corrective flight responses to light have been demonstrated in locusts [http://jeb.biologists.org/cgi/content/abstract/93/1/1] and dragonflies [http://jeb.biologists.org/cgi/reprint/83/1/351] in tethered flight. Other theories of ocellar function have ranged from roles as light adaptors or global excitatory organs, polarization sensors, and circadian entrainers.

Recent studies have shown that the ocelli of some insects (most notably the dragonfly, but also some wasps) are capable of form vision as the ocellar lens forms an image within, or close to the photoreceptor layer (doi:10.1016/j.visres.2007.01.019; doi:10.1016/j.asd.2006.08.012). In dragonflies it has been demonstrated that the receptive fields of both the photoreceptors (doi:10.1085/jgp.20050931)and the second-order neurons (doi:10.1007/s00359-006-0204-8) can be quite restricted. Dragonfly ocelli appear to be especially highly developed and specialised visual organs, which may underlie their exceptional acrobatic abilities.

Lateral Ocelli

Lateral ocelli have a mixture of rod cells and cone cells and are found on the sides of the head, one to six on each side.

Lateral ocelli are the only eyes of the larvae of several orders of insects (fleas, springtails, silverfish, and Strepsiptera).

External links

* John R. Meyer, [http://www.cals.ncsu.edu/course/ent425/tutorial/photo.html Photoreceptors]

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Look at other dictionaries:

  • OCELLUS — nomen floris eximii, Oeillet, de quo sic Franc. Pomey, Ocellus, anne sic dictus, quod in eius aspectu blandissime acquiescat oculus, an quod ipsemet sit hortorum oculus et ornamentum? Nihil certe spectare oculus possit, vel eius formosius formâ;… …   Hofmann J. Lexicon universale

  • Ocellus — O*cel lus, n.; pl. {Ocelli}. [L., dim. of oculus an eye.] (Zo[ o]l.) (a) A little eye; a minute simple eye found in many invertebrates. (b) An eyelike spot of color, as those on the tail of the peacock. [1913 Webster] …   The Collaborative International Dictionary of English

  • Ocellus — (lat.), 1) Äuglein; 2) so v.w. Auge 3) …   Pierer's Universal-Lexikon

  • ocellus — [ō sel′əs] n. pl. ocelli [ō sel′ī΄] [L, dim. of oculus,EYE] 1. the simple eyespot of certain invertebrates, as distinguished from the compound eye of an insect or the camera type eye of vertebrates and cephalopods 2. an eyelike spot, as on a… …   English World dictionary

  • Ocellus — Gesichtspartie eines Triops mit zwei Komplexaugen und einem Ocellus Als Ocellus (lat. „Äuglein“, Mehrzahl Ocelli bzw. Ocellen) werden in der Zoologie die punktförmigen Lichtsinnesorgane (Punktaugen) und Hautaugen verschiedener Tiergruppen… …   Deutsch Wikipedia

  • ocellus — (plural ocelli)) an eye like spot, usually rounded with a lighter border, e.g. the ocellus on the wings of some Rajidae …   Dictionary of ichthyology

  • ocellus — noun (plural ocelli) Etymology: New Latin, from Latin, diminutive of oculus eye more at eye Date: 1819 1. a minute simple eye or eyespot of an invertebrate 2. an eyelike colored spot (as on a peacock feather or the wings of some butterflies) •… …   New Collegiate Dictionary

  • ocellus — n.; pl. ocelli [L. dim. oculus, eye] 1. The simple eyes or eyespots, occurring singly or in small groups, found in many invertebrates. 2. (ARTHROPODA: Crustacea) see nauplius eye. 3. (ARTHROPODA: Insecta) see stemma, ommata. 4. (MOLLUSCA:… …   Dictionary of invertebrate zoology

  • ocellus — /oh sel euhs/, n., pl. ocelli /oh sel uy/. 1. a type of simple eye common to invertebrates, consisting of retinal cells, pigments, and nerve fibers. 2. an eyelike spot, as on a peacock feather. [1810 20; < L: little eye, dim. of oculus EYE; see… …   Universalium

  • ocellus — noun a) A simple eye consisting of a single lens and a small number of sensory cells b) An eyelike marking in the form of a spot or ring of colour, as on the wing of a butterfly or the tail of a peacock See Also: ocellar …   Wiktionary

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