Overview
A 'simple eye' refers to a type of
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
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 acuityVisual acuity is acuteness or clearness of vision, especially form vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
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 CCD (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 animals’ 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, 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 ).
Spider eyes
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.
Dorsal Ocelli
- For the Celtic god, see Ocelus
Ocelus is a Celtic god known from three inscriptions in Roman Britain. He is twice invoked on dedications at Caerwent: one stone is the base of a state of which only a pair of human feet and a pair of goose feet survive. The invocation is to Mars Lenus or Ocelus Vellaunus and the "numen" of the...
.
- For eye-like markings, see eyespot (mimicry)
An eyespot is an eye-like marking. They are found on butterflies, reptiles and birds. In members of the Felidae family , the white circular markings on the backs of the ears are termed ocelli, and they are functionally similar to eyespots in other animals.Eyespots may be a form of automimicry in...
.
- For lateral ocelli, see stemmata.
The term ocelli (singular ocellus) is derived from the latin oculus (eye), and literally means “little eye”. Two evolutionary distinct ocellus types exist : dorsal ocelli (or simply 'ocelli'), found in most insects, and lateral ocelli (or stemmata), which are found in the larvae of some insect orders. They are structurally and functionally very different. Simple eyes of other animals, e.g.
cnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
ns may also be referred to as 'ocelli', but again the structure and anatomy of these eyes is quite distinct from those of the dorsal ocelli of insects.
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 Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees, and ants. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and πτερόν : wing...
(bees, ants, wasps, sawflies), Diptera (flies),
OdonataOdonata is an order of insects, encompassing dragonflies and damselflies . The word dragonfly is also sometimes used to refer to all Odonata...
(dragonflies, damselflies) and
OrthopteraThe Orthoptera are an order of insects with paurometabolous or incomplete metamorphosis, including the grasshoppers, crickets and locusts.Many insects in this order produce sound by rubbing their wings against each other or their legs, the wings or legs containing rows of corrugated bumps...
(grasshoppers, locusts, mantises)). 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 (
corneaThe cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43...
) 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 humourThe vitreous humour or vitreous humor is the clear gel that fills the space between the lens and the retina of the eyeball of humans and other vertebrates...
). 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-numberIn optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
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
ultravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
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 .
Stemmata
Stemmata (singular stemma, also referred to as lateral ocelli) are the only eyes of the larvae of several orders of insects (
fleaFlea is the common name for insects of the order Siphonaptera which are wingless insects whose mouthparts are adapted for piercing skin and sucking blood....
s,
springtailSpringtails form the largest of the three lineages of modern hexapods that are no longer considered insects...
s,
silverfishLepisma saccharina is a small, wingless insect typically measuring from a half to one inch...
, and
StrepsipteraThe Strepsiptera are an order of insects with nine families making up about 600 species...
). They have a similar form to ommatidia, the constituent elements of compound eyes: thus they may not be considered as simple eyes. Behind each lens lies 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.
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