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The term "ocellus" (plural ocelli) is derived from the Latin oculus (eye), and literally means "little eye". In insects, two distinct ocellus types exist: [9] dorsal (top-most) ocelli, and lateral ocelli (often referred to as ocelli and stemmata, respectively), most insects have dorsal ocelli while stemmata are found in the larvae of some ...
[11] [21] The ocelli are not homologous to the simple eyes of caterpillars which are differently named as stemmata. [5] The ocelli of Lepidoptera are reduced externally in some families; where present, they are unfocussed, unlike stemmata of larvae which are fully focussed. The utility of ocelli is not understood at present. [5]
Ocelli (the plural of ocellus) are a type of simple eye, or eyespot. They are photoreceptive, but very simple; differing from complex eyes with compound lenses, ocelli cannot morph the sensation of light into a complete image, and are utilized to sense movement and the absence and presence of light.
Some reptiles, such as the sand lizard of Europe, have eyespots; in the sand lizard's case, there is a row of spots along the back, and a row on each side. [12]Many species of cat, including Geoffroy's cats, jungle cats, pampas cats, and servals, have white markings, whether spots or bars, on the backs of their ears; it is possible that these signal "follow me" to the young of the species.
Arthropods use combinations of compound eyes and pigment-pit ocelli for vision. In most species, the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey.
Chalcidoid wasps are small wasps (most within the range 0.5–5 mm). However the group does include the smallest known insect (Dicopomorpha echmepterygis males have a body length of 0.14–0.24 mm); the largest chalcidoids include Leucospis gigas with a body length of up to 21 mm [2] and Doddifoenus wallacei with a body length of up to 19.6 mm. [3]
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The ocelli, when present, are located in the top of the head, arranged at the corners of a triangle in an area called stemmaticum or ocellar triangle. For the purpose of systematics, the presence, the arrangement, and the conformation of the cephalic bristles is important and they have a specific terminology.