|
|
|
|
Ocellus
|
| |
|
| |
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 lenses (up to tens of thousands) which each focus light onto a small number of retinula cells.
rge variety of eye designs exist in arthropods, depending on the environment in which they live (for a review see ).
Discussion
Ask a question about 'Ocellus' Start a new discussion about 'Ocellus' Answer questions from other users |
Encyclopedia
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 lenses (up to tens of thousands) which each focus light onto a small number of retinula cells.
Types of simple eyes
A large variety of eye designs exist in arthropods, depending on the environment in which they live (for a review see ). Simple eyes in arthropods can be broken into three categories.
Traditional simple eyes
Traditional simple eyes 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
- For the Celtic god, see Ocelus.
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 orders. 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).
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.
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 detials 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 .
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).
Stemmata
Stemmata (singular stemma) 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.
External links
See also
|
| |
|
|
