A simple eye (sometimes called a pigment pit) refers to a type of eye design or optical arrangement that contains a single lens. "Simple" in this case does not refer to the number of cells present in the eye or the visual acuity of the eye. Indeed, the optical properties and anatomy of simple eyes is often incredibly complex. Human eyes and camera lenses are classed as "simple", because in both cases a single lens collects and focuses light onto the retina (eye), film (analog cameras), or Image sensor (digital cameras). This is most easily contrasted with the compound eye of insects, where each eye consists of multiple lenses (up to tens of thousands), each focusing light onto a small number of retinula cells.
The structure of an animal's eye is determined by the environment in which it lives, and the behavioural tasks the animal must fulfill in order to survive. Arthropods differ widely in the habitats in which they live, as well as their visual requirements for finding food or conspecifics, and avoiding predators. Consequently, an enormous variety of eye designs are found in arthropods: nature has repeatedly developed novel solutions to overcome visual problems or limitations (for a review of arthropod visual systems see Warrant, 2006).
Many snails and slugs (gastropods mollusks) also have ocelli, either at the tip of the tentacles or at the base of the tentacles. However some other gastropods, such as the Strombidae, have much more sophisticated eyes.
Spiders, which do not have compound eyes, have many, usually four, pairs of simple eyes, with each pair adapted for a specific task or tasks. There are principle, absent in those with only three pairs, and secondary eyes. Only the principle eyes have a retina that can be moved. The secondary eyes have a reflector at the back of the eyes. The light sensitive part of the receptor cells is next to this so they get direct and reflected light. In hunting or jumping spiders for example, a forward-facing pair possesses the best resolution (and even telescopic components) in order to see the (often small) prey at a large distance. Night hunting spiders' eyes are very sensitive in low light levels with a large aperture, f0.58.
Dorsal ocelli are light-sensitive organs found on the dorsal (top-most) surface or frontal surface of the head of many insects (e.g. Hymenoptera (bees, ants, wasps, sawflies), Diptera (flies), Odonata (dragonflies, damselflies) and Orthoptera (grasshoppers, locusts)). The ocelli co-exist with the compound eyes, thus most insects possess two anatomically separate and functionally different visual pathways.
The number, form, and function of the dorsal ocelli varies markedly throughout insect orders. 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. Note that the unfortunately labelled "lateral ocelli" here refers to the sideways-facing position of the ocelli, which are of the dorsal type. They should not be confused with the lateral ocelli of some insect larvae (see stemmata).
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. #The refractive power of the lens is not typically sufficient to form an image on the photoreceptor layer. #Dorsal ocelli ubiquitously have massive convergence ratios from first-order (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 animal's nervous system) the ocelli are typically considered to be "faster" than the compound eyes.
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 and dragonflies 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. In dragonflies it has been demonstrated that the receptive fields of both the photoreceptors and the second-order neurons can be quite restricted. Further research has demonstrated that these eyes not only resolve spatial details of the world, they also perceive motion. Second-order neurons in the dragonfly median ocellus respond more strongly to upwards moving bars and gratings than to downwards moving bars and gratings. However this effect is only present when ultraviolet light is used in the stimulus; when ultraviolet light is absent, no directional response is observed. Dragonfly ocelli are especially highly developed and specialised visual organs, which may support the exceptional acrobatic abilities of these animals.
Research on the ocelli is of high interest to designers of small unmanned aerial vehicles. Designers of these craft face many of the same challenges that insects face in maintaining stability in a three-dimensional world. Engineers are increasingly taking inspiration from insects in order to overcome these challenges.
Whilst (in ''Drosophila'' at least) the genes eyeless and daschund are both expressed in the compound eye but not the simple eye, there are no reported ''developmental'' genes uniquely expressed in the simple eye.
Egfr (Epidermal growth factor receptor) promotes the expression of orthodenticle (and possibly Eyes absent ('Eya')) and as such is essential for simple eye formation.
Category:Eye Category:Arthropod anatomy
ar:عين بسيطة de:Ocellus es:Ojo simple fr:Ocelle (œil) it:Ocello he:עינית (בעלי חיים) nl:Ocellus no:Medianøye pl:Przyoczko pt:Ocelo fi:Pistesilmä sv:PunktögaThis text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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