Simple eye in invertebrates
As ocellus (Latin for " little eyes ", plural ocelli or ocelli ) the point light sensory organs ( eye point ) and skin eyes of various animal groups are referred to in zoology. In a narrower sense, the term ocelli is today mainly used for the individual eyes of arthropods that carry these in addition to their compound eyes on the head. In botany, one uses the term for single, striking enlarged cells in the lamina of the liverworts.
Occurrence
Ocelli are found in almost all animal groups and are already used in the cnidarians ( Cnidaria ) and comb jellies ( Ctenophora ) ago. Here, these are probably multiple arose independently, but go with the different groups of animals along with the formation of other types of eyes as the lens eye of vertebrates, or the compound eye of insects always a homologous group of genes of a type returned, which is called Hox gene.
Construction
Ocelli can be constructed very differently. The simplest type is the eye plate, in which a portion of the epithelium is sensitive to light. By depression of this Epithelstücks and the training of auxiliary structures such as a lens and reflector layers built up over the pit eye and the cup eye in the course of evolution different types of eyes to the compound eyes and the lens eyes that serve as complex light organs of sense.
The ocelli are normally for a single-layer retina ( retina), but there are also multi-layered retina. The Retinulazellen thereby form a closed rhabdome or a reticular retina. So that there is no optical problems with stray light and light shielding generally have Ocellen cells are colored by pigments. Particularly powerful ocelli also have a lens or, as in the arachnids, also a glass body.
In arthropods, a distinction Medianocellen, which lie in the center of the head, and on the sides of the head Lateralocellen. The Lateralocellen are always traces of dissolved or modified complex or compound eyes. Resolution compound eyes are found for example in arachnids or centipedes and at various larval forms holometabolic insects ( stemmata ). Medianocellen are common to all arthropods. In the sea spiders ( Pantopoda ), original crustaceans (Crustacea ) and early Sechsfüßern ( Hexapoda ) as the springtails ( Collembola ) come these simple eyes in variety before, at the arachnids their number was two median or main eyes at higher crabs on three Naupliusaugen and reduces the insects on three Stirnocellen.
When flying insects which are arranged in a triangle Stirnocellen probably serve as organs of equilibrium (horizon detector ) and to control flight movements faster. Information on this offer experiments on dragonflies and grasshoppers, whose flight is unstable when covered ocelli. In addition, a light - compass orientation was detected mainly for hymenopterans (Hymenoptera ), which is made possible by the Stirnocellen. See more theories in the Stirnocellen sense organs that support the function of the compound eyes, probably mainly as a " meter " for the light intensity according to which the compound eyes can adapt. In flightless insects Stirnocellen are often lacking, and nocturnal insects have light sensitive Stirnocellen. The larvae holometabolic insects Stirnocellen missing.