An ocellus (plural: ocelli) is a type of photoreceptor organ in animals. Also called "simple eyes", ocelli are miniature eyes capable of sensing light but not distinguishing its direction. See also stemmata, which are structurally similar. Ocelli are found in many invertebrates. Insects in particular have two types of ocelli, dorsal ocelli and lateral 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, 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.
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 (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.