Arthropod eye

Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye. They are found in all arthropod groups, although they may have evolved more than once within this phylum.Some annelids and bivalves also have apposition eyes. They are also possessed by Limulus, the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point. Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion. Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye. They are found in all arthropod groups, although they may have evolved more than once within this phylum.Some annelids and bivalves also have apposition eyes. They are also possessed by Limulus, the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point. Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion. The arthropods ancestrally possessed compound eyes, but the type and origin of this eye varies between groups, and some taxa have secondarily developed simple eyes. The organ's development through the lineage can be estimated by comparing groups that branched early, such as the velvet worm and horseshoe crab to the advanced eye condition found in insects and other derived arthropods. Most arthropods have at least one of two types of eye: lateral compound eyes, and smaller median ocelli, which are simple eyes. When both are present, the two eye types are used in concert because each has its own advantage. Some insect larvae, e.g., caterpillars, have a different type of simple eye known as stemmata. These eyes usually provide only a rough image, but (as in sawfly larvae) they can possess resolving powers of 4 degrees of arc, be polarization sensitive and capable of increasing their absolute sensitivity at night by a factor of 1,000 or more. Flying insects can remain level with either type of eye surgically removed, but the two types combine to give better performance. Ocelli can detect lower light levels, and have a faster response time, while compound eyes are better at detecting edges and are capable of forming images. Most species of Arthropoda with compound eyes bear just two eyes that are located separately and symmetrically, one on each side of the head. This arrangement is called dichoptic. Examples include most insects, and most of the larger species of Crustacea, such as crabs. Many other organisms, such as vertebrates and Cephalopoda are similarly and analogously dichoptic, which is the common state in animals that are members of the Bilateria and have functionally elaborate eyes. However, there are variations on that scheme. In some groups of animals whose ancestors originally were dichoptic, the eyes of modern species may be crowded together in the median plane; examples include many of the Archaeognatha. In extreme cases such eyes may fuse, effectively into a single eye, as in some of the Copepoda, notably in the genus Cyclops. One term for such an arrangement of eyes is cycloptic. On the other hand, some modes of life demand enhanced visual acuity, which in compound eyes demands a larger number of ommatidia, which in turn demands larger compound eyes. The result is that the eyes occupy most of the available surface of the head, reducing the area of the frons and the vertex and crowding the ocelli, if any. Though technically such eyes still may be regarded dichoptic, the result in the extreme case is that borders of such eyes meet, effectively forming a cap over most of the head. Such an anatomy is called holoptic. Spectacular examples may be seen in the Anisoptera and various flies, such as some Acroceridae and Tabanidae. In contrast, the need for particular functions may not require extremely large eyes, but do require great resolution and good stereoscopic vision for precise attacks. Good examples may be seen in the Mantodea and Mantispidae, in which seeing prey from particular ommatidia in both compound eyes at the same time, indicates that it is in the right position to snatch in a close-range ambush. Their eyes accordingly are placed in a good position for all-round vision, plus particular concentration on the anterior median plane. The individual ommatidia are directed in all directions and accordingly, one may see a dark spot (the pseudopupil), showing which ommatidia are covering that field of view; from any position on the median plane, and nowhere else, the two dark spots are symmetrical and identical. Sometimes the needs for visual acuity in different functions conflict, and different parts of the eyes may be adapted to separate functions; for example, the Gyrinidae spend most of their adult lives on the surface of water, and have their two compound eyes split into four halves, two for underwater vision and two for vision in air. Again, particularly in some Diptera, ommatidia in different regions of the holoptic male eye may differ visibly in size; the upper ommatidia tend to be larger. In the case of some Ephemeroptera the effect is so exaggerated that the upper part of the eye is elevated like a risen cupcake, while its lower part that serves for routine vision looks like a separate organ. Compound eyes are often not completely symmetrical in terms of ommatidia count. For example, asymmetries have been indicated in honeybees and various flies. This asymmetry has been correlated with behavioural lateralization in ants (turning bias). The head patterning is controlled by orthodenticle, a homeobox gene which demarcates the segments from the top-middle of the head to the more lateral aspects. The ocelli are in an orthodenticle-rich area, and the gene is not expressed by the time one gets as lateral as the compound eyes.