Simple eye in invertebrates
A simple eye or ocellus (sometimes called a pigment pit[1][2]) refers to a form of eye or an optical arrangement composed of a single lens without an elaborate retina as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-lensed "compound eye", and is not necessarily at all simple in the usual sense of the word.
The structure of an animal's eye is determined by the environment in which it lives, and the behavioural tasks it must fulfill to survive.
Use of the term simple eye is flexible, and must be interpreted in proper context; for example, the eyes of most large animals are have unambiguously compound eyes consisting of multiple lenses (up to tens of thousands), but achieve an effect similar to that of a camera eye, in that each ommatidium lens focuses light onto a number of neighbouring retinulae.
Ocelli or eye spots
Some
Many snails and slugs also have ocelli, either at the tips or bases of their tentacles.[6] Some other gastropods, such as the Strombidae, have much more sophisticated eyes. Giant clams have ocelli that allow light to penetrate their mantles.[7]
Simple eyes in arthropods
Spider eyes
Spiders do not have compound eyes, but instead have several pairs of simple eyes with each pair adapted for a specific task or tasks. The principal and secondary eyes in spiders are arranged in four, or occasionally fewer, pairs. Only the principal eyes have moveable retinas. 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 some telescopic ability) to help spot prey from a distance. Nocturnal spiders' eyes are very sensitive in low light levels and are large to capture more light, equivalent to f/0.58 in the rufous net-casting spider.[8]
Dorsal ocelli
The term "ocellus" (plural ocelli) is derived from the
Dorsal ocelli are light-sensitive organs found on the dorsal surface or frontal surface of the head of many insects, including
The number, forms, and functions of the dorsal ocelli vary markedly throughout insect orders. They tend to be larger and more strongly expressed in flying insects (particularly bees,[10] wasps, dragonflies and locusts) where they are typically found as a triplet. Two ocelli are directed to either side of the head, while a central (median) ocellus is directed forwards. In some terrestrial insects (e.g. some ants and cockroaches), the median ocellus is absent. The sideways-facing ocelli can be called "lateral ocelli", referring to their direction and position in the triplet, however this is not to be confused with the stemmata of some insect larvae, which are also known as lateral ocelli.
A dorsal ocellus consists of a lens element (
Two somewhat unusual features of ocelli are particularly notable and generally common between insect orders.
- The refractive powerof the lens is not typically sufficient to form an image on the photoreceptor layer, essentially it is out of focus.
- Dorsal ocelli ubiquitously have massive convergence ratios from first-order (photoreceptor) to second-order neurons.[clarification needed]
These two factors have led to the conclusion that, with some exceptions in predatry insects, the ocelli are incapable of perceiving proper images 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 (amplification of photoreceptor signals), 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.[11]
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
Recent studies have shown the ocelli of some insects (most notably the dragonfly, but also some wasps) are capable of "form vision" similar to camera eyes, as the ocellar lens forms an image within, or close to, the photoreceptor layer.[14][15] In dragonflies it has been demonstrated that the receptive fields of both the photoreceptors[16] and the second-order neurons[17] can be quite restricted. Further research has demonstrated these eyes not only resolve spatial details of the world, but also perceive motion.[18] Second-order neurons in the dragonfly median ocellus respond more strongly to upwards-moving bars and gratings than to downwards-moving bars and gratings, but 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
Stemmata
Stemmata (singular stemma) are a class of simple eyes. Many kinds of
Behind each lens of a typical functional stemma lies a single cluster of photoreceptor cells, termed a retinula. The lens is
Although stemmata are simple eyes, some kinds (such as those of the larvae of Lepidoptera and especially those of Tenthredinidae, a family of sawflies) are only "simple" in that they represent immature or embryonic forms of the compound eyes of the adult. They can possess a considerable degree of acuity and sensitivity, and can detect polarized light.[21] They may be optimized for light sensitivity, as opposed to detailed image formation.[22] In the pupal stage, such stemmata develop into fully fledged compound eyes. One feature offering a clue to their ontogenetic role is their lateral position on the head; ocelli, that in other ways resemble stemmata, tend to be borne in sites median to the compound eyes, or nearly so. Among some researchers, this distinction has led to the use of the term "lateral ocelli" for stemmata.[9]
Genetic controls
A number of genetic pathways are responsible for the occurrence and positioning of the ocelli. The gene orthodenticle is allelic to ocelliless, a mutation that stops ocelli from being produced.[23] In Drosophila, the rhodopsin Rh2 is only expressed in simple eyes.[24]
While (in Drosophila at least) the genes eyeless and dachshund are both expressed in the compound eye but not the simple eye, no reported 'developmental' genes are uniquely expressed in the simple eye.[25]
Epidermal growth factor receptor (Egfr) promotes the expression of orthodenticle and possibly eyes absent (Eya) and as such is essential for simple eye formation.[25]
See also
References
- ^ "Catalog - Mendeley". www.mendeley.com. Archived from the original on 24 March 2012. Retrieved 4 May 2018.
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- ^ "Eye (invertebrate)". McGraw-Hill Encyclopedia of Science & Technology. Vol. 6. 2007. p. 790.
- ^ Martin, Vicki J. (2002). "Photoreceptors of cnidarians" (PDF). University of California, Irvine. Archived from the original (PDF) on 2013-10-05.
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- ^ Blest, AD; Land (1997). "The Physiological optics of Dinopis Subrufus L.Koch: a fisheye lens in a spider". Proceedings of the Royal Society (196): 198–222.
- ^ ISBN 978-0-8493-3498-6.
- ^ Brindley, Hal (2023-03-07). "How many eyes does a bee have?". Our Wild Yard. Retrieved 2023-08-30.
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- ^ Gert Stange, R. Berry & J. van Kleef (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). Vol. 1. pp. 17–21.
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Further reading
- Warrant, Eric; Nilsson, Dan-Eric (2006). Invertebrate Vision. Cambridge University Press. ISBN 978-0-521-83088-1.
External links
- Meyer, John R. (5 March 2006). "Photoreceptors". Department of Entomology. cals.ncsu.edu. Ent 425 (General Entomology) course tutorial. North Carolina State University. Archived from the original on 2011-07-16.