Simple eye in invertebrates
Arthropods differ widely in the habitats in which they live, as well as their visual requirements for finding food or conspecifics, and avoiding predators.
Many have various forms of retinula (a retina-like cluster of photoreceptor cells), including the ommatidia of most insects and the central eyes of camel spiders.
In hunting or jumping spiders, for example, a forward-facing pair possesses the best resolution (and even some telescopic ability) to help spot prey from a distance.
Dorsal ocelli are light-sensitive organs found on the dorsal surface or frontal surface of the head of many insects, including Hymenoptera (bees, ants, wasps, sawflies), Diptera (flies), Odonata (dragonflies, damselflies), Orthoptera (grasshoppers, locusts) and Mantodea (mantises).
These ocelli coexist with compound eyes; thus, most insects possess two anatomically separate and functionally different visual pathways.
They tend to be larger and more strongly expressed in flying insects (particularly bees,[10] wasps, dragonflies and locusts) where they are typically found as a triplet.
These two factors have led to the conclusion that, with some exceptions in predatory insects, the ocelli are incapable of perceiving proper images 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 (amplification of photoreceptor signals), 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.
[11] 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.
Other theories of ocellar function have ranged from roles as light adaptors or global excitatory organs to polarization sensors and circadian entrainers.
Dragonfly ocelli are especially highly developed and specialised visual organs, which may support the exceptional acrobatic abilities of these animals.
Behind each lens of a typical functional stemma lies a single cluster of photoreceptor cells, termed a retinula.
One feature offering a clue to their ontogenetic 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.
