Head of a wasp with three ocelli (centre), and the dorsal part of compound eyes (left)

An ocellus (plural: ocelli) is a type of photoreceptor organ in animals. A wasp is any Insect of the order Hymenoptera and suborder Apocrita that is neither a Bee nor Ant. In Anatomy, the dorsum is the upper side of animals that typically run fly or swim in a horizontal position and the back side of animals (like humans that walk upright Photoreceptor can refer to In anatomy/cell biology Photoreceptor cell: a photosensitive cell most commonly referring to a specialized type of neuron Also called "simple eyes", ocelli are miniature eyes capable of sensing light but not distinguishing its direction. Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain See also stemmata, which are structurally similar. For information regarding Stemma in textual analysis see Stemmatics For "stoma" or "stomata" gas exchange pores on plant leaves see Ocelli are found in many invertebrates. An invertebrate is an Animal lacking a Vertebral column. The group includes 98% of all animal Species — all animals except those in the Chordate Insects in particular have two types of ocelli, dorsal ocelli and lateral ocelli. Insects ( Class Insecta) are a major group of Arthropods and the most diverse group of Animals on the Earth with over a million described

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 (grasshopers, locusts). Hymenoptera is one of the larger orders of Insects comprising the sawflies, Wasps Bees and Ants The name refers to Odonata is an order of Insects encompassing dragonflies (Anisoptera and damselflies (Zygoptera The Orthoptera (from the Greek ορθός orthos = "straight" and πτεрόν pteron = "wing" are an order of Insects 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). Lepidoptera is an order of Insect that includes Moths and butterflies.

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.

A dorsal ocellus consists of a lens element (cornea) and a layer of photoreceptors (rod cells). The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber. Rod cells, or rods, are Photoreceptor cells in the Retina of the Eye that can function in less intense Light than can the other type 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 [1].

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 [2] and dragonflies [3]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 (doi:10. 1016/j. visres. 2007. 01. 019; doi:10. 1016/j. asd. 2006. 08. 012). In dragonflies it has been demonstrated that the receptive fields of both the photoreceptors (doi:10. 1085/jgp. 20050931)and the second-order neurons (doi:10. 1007/s00359-006-0204-8) can be quite restricted. Dragonfly ocelli appear to be especially highly developed and specialised visual organs, which may underlie their exceptional acrobatic abilities.

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. Rod cells, or rods, are Photoreceptor cells in the Retina of the Eye that can function in less intense Light than can the other type Cone cells, or cones, are Photoreceptor cells in the Retina of the Eye which function best in relatively bright Light.

Lateral ocelli are the only eyes of the larvae of several orders of insects (fleas, springtails, silverfish, and Strepsiptera). Flea is the Common name for any of the small wingless Insects of the order Siphonaptera (some authorities use the name Aphaniptera Springtails ( Collembola) form the largest of the three lineages of modern hexapods that are no longer considered Insects (the other two are the Lepisma saccharina (commonly called the fishmoth, urban silverfish or just the silverfish) is a small wingless Insect typically The Strepsiptera (known in older literature as twisted-winged parasites) are an order of Insects with nine families making up about 600 species

External links

• John R. Meyer, Photoreceptors