As Ocellus ( lat. "Little eyes" plurality Ocelli or Ocellen ) in the are Zoology the dot-shaped point eyes (light sense organs), and skin eyes referred various animal groups. In a narrower sense, the term ocellen is used today primarily for the individual eyes of the arthropods , which they wear on their heads in addition to their complex eyes .
Ocelles are found in almost all animal groups and are already found in the cnidarians (Cnidaria) and rib jellyfish (Ctenophora). Here, this probably originated several times independently of each other, but go for the various animal groups 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 back of the Hox gene is called.
Ocelles can have very different structures. The simplest type is the plate eye , in which part of the epithelium is sensitive to light. By sinking this piece of epithelium and the formation of auxiliary structures such as a lens and reflector layers, various types of eyes were created in the course of evolution via the pit eye and the cup eye, right up to the complex eyes and the lens eyes, which serve as complex light-sensing organs.
The ocelles normally have a single-layer retina , but there are also multi-layer retinas. The retinula cells form a closed rhabdom or a retinal retina. So that there are no optical problems with scattered light, Ocelles usually also have light-shielding cells that are colored by color pigments . Particularly efficient ocelles also have a lens or, as in arachnids , a glass body .
In arthropods, a distinction is made between medianocells , which are located in the middle of the head, and lateralocells on the sides of the head. The lateralocells are always remnants of dissolved or changed complex or compound eyes. Dissolved compound eyes can be found in arachnids or millipedes , for example, as well as in various larval forms of holometabolic insects (stemmata). Medianocells are common in all arthropods. In the woodlice spiders (Pantopoda), primitive crustaceans (Crustacea) and early hexapods (Hexapoda) such as the springtails (Collembola), these point eyes occur in large numbers, in the arachnids their number was reduced to two median or main eyes, in higher crustaceans to three Nauplius eyes and the insects reduced to three forehead ocelles.
In the case of flying insects, the forehead ocelles arranged in a triangle probably serve as a balance organ (horizon detector) and to control rapid flight movements. Experiments on dragonflies and grasshoppers provide information on this, the flight of which becomes unstable when the ocelles are covered. In addition, especially for hymenoptera (Hymenoptera), a light compass orientation was demonstrated, which is made possible by the forehead ocelles. Further theories see in the forehead ocelles sensory organs that support the function of the complex eyes, probably primarily as a "measuring device" for the light intensity , according to which the complex eyes can adapt. In flightless insects, the forehead ocelles are often absent, and nocturnal insects have more light-sensitive forehead ocelles. The forehead ocelles are absent in the larvae of holometabolic insects.
- G. Stange, J. Howard: An ocellar dorsal light response in a dragonfly. In: Journal of Experimental Biology. 83, 1979, pp. 351-355
- Gerhard Seifert: Entomological internship. Thieme Verlag, Stuttgart 1994, ISBN 3-13-455003-2 .