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- Not to be confused with the celtic god Ocelus
So called 'simple', or 'camera' type eyes are an eye design similar to that found in humans and utilised in cameras. Namely, a single lens collects light and focusses this onto the retina, film (analog cameras), or CCD (digital cameras). This is most easily contrasted with the compound eye, where each eye consists of multiple (up to tens of thousands) of lenses which each focus light onto a small number of retinula cells.
An amazing variety of eye designs exist in arthropods, depending on the environment in which they live (for a review see 1). Simple eyes in arthropods can be broken into three categories:
Traditional simple eyes, which have good focussing and resolution, and are used for several purposes. For example, spiders, which do not have compound eyes, have many pairs of simple eyes, with each pair adapted for a specific task or tasks. 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.
Ocelli (singular ocellus) are a type of eye present in many insects in addition to the compound eyes. Two evolutionary distinct ocellus types exist: dorsal ocelli, found in most insects, and lateral ocelli, which are found in the larvae of some insect orders2. They are structurally and functionally very different. Due to a strongly underfocused lens, the dorsal ocelli are usually considered to be incapable of perceiving any form at all (but see below for notable exceptions).
Stemmata have a similar form to ommatidia, the constituent elements of compound eyes. They contain a single cluster of photoreceptor cells, termed a retinula. Their lens is biconvex, and their body contains a vitreous or crystaline core. They may represent simplified compound eyes, reflected by their lateral position on the head. They are possessed by myriapods and some insect larvae.2
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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 (grasshoppers, 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. 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 below).
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.3
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 locusts4 and dragonflies5 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 layer67. In dragonflies it has been demonstrated that the receptive fields of both the photoreceptors8 and the second-order neurons9 can be quite restricted. Further research has demonstrated that these eyes not only resolve spatial detials of the world, they also perceive motion 10. 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 11.
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, Photoreceptors
See also
References
- ^ Warrant, E; Nilsson, Dan-Eric (2006). Invertebrate Vision.
- ^ a b Bitsch, C.; Bitsch, J. (2005), "Evolution of eye structure and arthropod phylogeny", Crustacea and Arthropod Relationships, http://books.google.ca/books?id=7ZHtG3aELesC&printsec=frontcover&dq=evolution+of+the+eye+review&lr=#PPA185,M1
- ^ Martin Wilson (1978). "The functional organisation of locust ocelli". Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology 124 (4): 297-316. doi:, http://www.springerlink.com/content/x0j046w843352w63/.
- ^ Taylor, Charles P. (08/01/1981). "Contribution of Compound Eyes and Ocelli to Steering of Locusts in Flight: I. Behavioural Analysis". Journal of Experimental Biology 93 (1): 1, http://jeb.biologists.org/cgi/content/abstract/93/1/1.
- ^ Stange, Gert; Howard, Jonathon (12/01/1979). "An Ocellar Dorsal Light Response in a Dragonfly". Journal of Experimental Biology 83 (1): 351, http://jeb.biologists.org/cgi/reprint/83/1/351.
- ^ Berry, Rp; Stange, G; Warrant, Ej (May 2007). "Form vision in the insect dorsal ocelli: an anatomical and optical analysis of the dragonfly median ocellus". Vision research 47 (10): 1394–409. doi:. PMID 17368709.
- ^ Warrant, Ej; Kelber, A; Wallén, R; Wcislo, Wt (Dec 2006). "Ocellar optics in nocturnal and diurnal bees and wasps". Arthropod structure & development 35 (4): 293–305. doi:. PMID 18089077.
- ^ van Kleef, J; James, A, C; Stange, G; (Oct 2005). "A Spatiotemporal White Noise Analysis of Photoreceptor Responses to UV and Green Light in the Dragonfly Median Ocellus". Journal of General Physiology 126 (5): 481-497. doi:.
- ^ Berry, R; van Kleef, J; Stange, G (May 2007). "The mapping of visual space by dragonfly lateral ocelli". Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology 193 (5): 495–513. doi:. PMID 17273849.
- ^ van Kleef, J; Berry, R; Stange, G; (March 2008). "Directional Selectivity in the Simple Eye of an Insect". The Journal of Neuroscience 28 (11): 2845-2855. doi:.
- ^ Stange, G; Berry, R; van Kleef, J; (September 2007). "Design concepts for a novel attitude sensor for Micro Air Vehicles, based on dragonfly ocellar vision". 3rd US-European Competition and Workshop on Micro Air Vehicle Systems (MAV07) & European Micro Air Vehicle Conference and Flight Competition (EMAV2007) 1: 17-21.
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