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'.
Photoreceptors
In
insects, ocelli are involved (along with
compound eyes) in the horizon-detecting response, decreasing the latency between horizon changes and the
insect's response.
Dorsal Ocelli
Dorsal ocelli are light-sensitive organs on the
dorsal surface or on the top of the head. There are generally three, forming a triangle.
They consist of a few dozen
rod cells covered with a
corneal lens. In some species, such as
locusts, dorsal ocelli may contain several hundreds of retinula cells. Insects with dorsal ocelli also have
compound eyes.
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.
Lateral ocelli are the only eyes of the larvae of several orders of insects (
fleas,
springtails,
silverfish, and
Strepsiptera).
External links
★ John R. Meyer,
Photoreceptors
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