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A simple eye or ocellus (sometimes called a pigment pit) is a form of eye or an optical arrangement which has a single lens without the sort of elaborate retina that occurs in most . These eyes are called "simple" to distinguish them from "compound eye", which have multiple lenses. They are not necessarily simple in the sense of being uncomplicated or basic.
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. differ widely in the habitats in which they live, as well as their visual requirements for finding food or conspecifics, and avoiding predators. Consequently, arthropod eye to overcome visual problems or limitations.
Use of the term simple eye is flexible, and must be interpreted in proper context; for example, the eyes of most large animals are camera eyes and are sometimes considered "simple" because a single lens collects and focuses an entire image onto the retina (analogous to a camera). By other criteria, the presence of a complex retina distinguishes the vertebrate camera eye from the simple stemma or Ommatidium which make up compound eyes. Additionally, not all invertebrate ocelli and ommatidium have simple photoreceptors. Many have various forms of retinula (a retina-like cluster of photoreceptor cells), including the ommatidia of most insects and the central eyes of Solifugae. Salticidae and some other predatory with seemingly simple eyes also emulate retinal vision in various ways. Many 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.
Many Snail and Slug also have ocelli, either at the tips or bases of their tentacles. Some other gastropods, such as the Strombidae, have much more sophisticated eyes. Giant clam have ocelli that allow light to penetrate their mantles.
Dorsal ocelli are light-sensitive organs found on the dorsal surface or frontal surface of the head of many insects, including Hymenoptera (, , , sawfly), Diptera (flies), Odonata (dragonfly, damselfly), Orthoptera (, ), Mantodea (mantises), and various groups within Heteroptera (true bugs). These ocelli coexist with compound eyes; thus, most insects possess two anatomically separate and functionally different visual pathways.
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, 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.
In many members of the suborder Heteroptera (true bugs), such as stink bugs Pentatomidae, assassin bugs Reduviidae, and seed bugs Lygaeidae, only two dorsal ocelli are present. These are typically positioned symmetrically on the vertex, between or slightly behind the compound eyes. This two-ocelli arrangement is a distinguishing trait in several Heteropteran families and differs from the more common three-ocelli configuration found in other insect orders.
A dorsal ocellus consists of a lens element (cornea) and a layer of photoreceptors (rod cells). The ocellar lens may be strongly curved or flat. The photoreceptor layer may also be separated from the lens by a clear vitreous humour. The number of photoreceptors also varies widely, but may number in the hundreds or thousands for well-developed ocelli. In bees, locusts, and dragonflies, the lens is strongly curved; while in cockroaches it is flat. Locusts possess vitreous humour while blowflies and dragonflies do not.
Two somewhat unusual features of ocelli are particularly notable and generally common between insect orders.
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. Locusts and dragonflies in tethered flight have been observed to try and "correct" their flight posture based on changes in light. Other theories of ocellar function have ranged from roles as light adaptors or global excitatory organs to polarization sensors and circadian entrainers.
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. 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 these eyes not only resolve spatial details of the world, but 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, 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 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 to overcome these challenges.
Behind each lens of a typical functional stemma lies a single cluster of photoreceptor cells, termed a retinula. The lens is biconvex, and the body of the stemma has a vitreous or crystalline core.
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. They may be optimized for light sensitivity, as opposed to detailed image formation. In the stage, such stemmata develop into fully fledged compound eyes. One feature offering a clue to their ontogeny 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.
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.
Epidermal growth factor receptor ( Egfr) promotes the expression of orthodenticle and possibly eyes absent ( Eya) and as such is essential for simple eye formation.
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