Arthropod eye
Encyclopedia
The arthropod
s ancestrally possessed compound eye
s, 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.
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.)
, a homeobox gene which demarkates the segments from the top-middle of the head to the more lateral aspects. The ocelli are in an orthrodenticle-rich area, and the gene is not expressed by the time one gets as lateral as the compound eyes.
The gene dachshund
is involved in the development of the compound eye.
Different opsins are used in the ocelli to the compound eyes.
are currently thought to fall within the Crustacean crown group; while molecular work paved the way for this association, their eye morphology and development is also markedly similar. The eyes are strikingly different from the myriapods
, which were traditionally considered to be a sister group to the hexapoda.
Both ocelli and compound eyes were probably present in the last common arthropod ancestor, and may be apomorphic with ocelli in other phyla, such as the annelids. Median ocelli are present in chelicerates
and mandibulates
; lateral ocelli are also present in chelicerates.
among the arthropods.
There were probably only a single pair of ocelli in the arthropod concestor; Cambrian lobopod fossils display a single pair, and while many arthropods today have three, four, or even six, the lack of common pathway suggests that a pair is the most probable ancestral state. The crustaceans and insects mainly have three ocelli, suggesting that such a formation was present in their concestor.
It is deemed probable that the compound eye arose as a result of the 'duplication' of individual ocelli. In turn, the disperseal of compound eyes seems to have created large networks of seemingly independent eyes in some arthropods, such as the larvae of certain insects. In some other insects and myriapods, lateral ocelli appear to have arisen by the reduction of lateral compound eyes.
s were of two forms, both of which grew by the addition of new ommatidia at the bottom of the eye, a row at a time. This growth form is today unique to the horseshoe crabs
. The holochroal eye
, consisting of many small lenses, appears to be the ancestral state. The more complex schizochroal eye was more derived.
s. Most other living chelicerates have lost their lateral compound eyes, evolving simple eyes in their place.
Limulus has two large compound eyes on the sides of its head. An additional simple eye is positioned at the rear of each of these structures. In addition to these obvious structures, it also has two smaller ocelli situated in the middle-front of its carapace, which may superficially be mistaken for nostrils. A further simple eye is located beneath these, on the underside of the carapace. A further pair of simple eyes are positioned just in front of the mouth. The simple eyes are probably important during the embryonic or larval stages of the organism, with the compound eyes and median ocelli becoming the dominant sight organisms during adulthood. These ocelli are less complex, and probably less derived, than those of the mandibulata
. Unlike the trilobites', the compound eyes of Limulus are triangular in shape; they also have a generative region at their base, but this elongates with time. Hence the one ommatidium at the apex of the triangle was the original "eye" of the larval organism, with subsequent rows added as the organism grew.
Crustacean can have stalked eyes on peduncles.
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s ancestrally possessed compound eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
s, 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
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
arthropods.
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.)
Eyes and functions
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. 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,They are about 5000 times more sensitive than apposition compound eyes. They can, for instance, respond to the position of the full moon and have a faster response time, while compound eyes are better at detecting edges and are capable of forming images.Genetic controls
The head patterning is controlled by orthodenticleOrthodenticle
Orthodenticle is a homeobox gene involved in head development. It produces a homeodomain protein which controls the development of cells in the developing head of embryos...
, a homeobox gene which demarkates the segments from the top-middle of the head to the more lateral aspects. The ocelli are in an orthrodenticle-rich area, and the gene is not expressed by the time one gets as lateral as the compound eyes.
The gene dachshund
Dachshund (gene)
dachshund is a gene involved in the development of the arthropod compound eye which also plays a role in leg development....
is involved in the development of the compound eye.
Different opsins are used in the ocelli to the compound eyes.
Evolution
HexapodsHexapoda
The subphylum Hexapoda constitutes the largest grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura . The Collembola are very abundant in terrestrial environments...
are currently thought to fall within the Crustacean crown group; while molecular work paved the way for this association, their eye morphology and development is also markedly similar. The eyes are strikingly different from the myriapods
Myriapoda
Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains 13,000 species, all of which are terrestrial...
, which were traditionally considered to be a sister group to the hexapoda.
Both ocelli and compound eyes were probably present in the last common arthropod ancestor, and may be apomorphic with ocelli in other phyla, such as the annelids. Median ocelli are present in chelicerates
Chelicerata
The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites...
and mandibulates
Mandible (arthropod)
thumb|250px|The mandibles of a [[Bull ant]]The mandible of an arthropod is either of a pair of mouthparts used for biting, cutting and holding food. Mandibles are often simply referred to as jaws. Mandibles are present in the extant subphyla Myriapoda , Crustacea and Hexapoda...
; lateral ocelli are also present in chelicerates.
Origin
No fossil organisms have been identified as similar to the last common ancestor of arthropods; hence the eyes possessed by the first arthropod remains a matter of conjecture. The largest clue into their appearance comes from the onychophorans: a stem group lineage that diverged soon before the first true arthropods. The eyes of these creatures are attached to the brain using nerves which enter into the centre of the brain, and there is only one area of the brain devoted to vision. This is similar to the wiring of the median ocelli (small simple eyes) possessed by many arthropods; the eyes also follow a similar pathway through the early development of organisms. This suggests that onychophoran eyes are derived from simple ocelli, and the absence of other eye structures implies that the ancestral arthropod lacked compound eyes, and only used median ocelli to sense light and dark. However, a conflicting view notes that compound eyes appeared in many early arthropods, including the trilobites and eurypterids, suggesting that the compound eye may have developed after the onychopohran and arthropod lineages split, but before the radiation of arthropods. This view is supported if a stem-arthropod position is supported for compound-eye bearing Cambrian organisms such as the Anomalocaridids. An alternative, however, is that compound eyes evolved multiple timesConvergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
among the arthropods.
There were probably only a single pair of ocelli in the arthropod concestor; Cambrian lobopod fossils display a single pair, and while many arthropods today have three, four, or even six, the lack of common pathway suggests that a pair is the most probable ancestral state. The crustaceans and insects mainly have three ocelli, suggesting that such a formation was present in their concestor.
It is deemed probable that the compound eye arose as a result of the 'duplication' of individual ocelli. In turn, the disperseal of compound eyes seems to have created large networks of seemingly independent eyes in some arthropods, such as the larvae of certain insects. In some other insects and myriapods, lateral ocelli appear to have arisen by the reduction of lateral compound eyes.
Trilobite eyes
The eyes of trilobiteTrilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s were of two forms, both of which grew by the addition of new ommatidia at the bottom of the eye, a row at a time. This growth form is today unique to the horseshoe crabs
Xiphosura
Xiphosura is an order of marine chelicerates which includes a large number of extinct lineages and only four recent species in the family Limulidae, which include the horseshoe crabs...
. The holochroal eye
Holochroal eye
Holochroal eyes are compound eyes with a many tiny lenses . They are the oldest and most widespread type of trilobite eye, and found in all orders of trilobite from the Cambrian to the Permian periods. Lenses show hexagonal close packing, and a single corneal membrane covered all lenses...
, consisting of many small lenses, appears to be the ancestral state. The more complex schizochroal eye was more derived.
Limulus
Limulus, the horseshoe crab, has traditionally been used in investigations into the eye, because it has relatively large ommatidia with large nerve fibres (making them easy to experiment on). It also falls in the stem group of the chelicerates; its eyes are believed to represent the ancestral condition because they have changed so little over evolutionary time. Indeed the horseshoe crabs are often considered to be living fossilLiving fossil
Living fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...
s. Most other living chelicerates have lost their lateral compound eyes, evolving simple eyes in their place.
Limulus has two large compound eyes on the sides of its head. An additional simple eye is positioned at the rear of each of these structures. In addition to these obvious structures, it also has two smaller ocelli situated in the middle-front of its carapace, which may superficially be mistaken for nostrils. A further simple eye is located beneath these, on the underside of the carapace. A further pair of simple eyes are positioned just in front of the mouth. The simple eyes are probably important during the embryonic or larval stages of the organism, with the compound eyes and median ocelli becoming the dominant sight organisms during adulthood. These ocelli are less complex, and probably less derived, than those of the mandibulata
Mandibulata
The Mandibulata or mandibulates is a clade of arthropods that comprises the extant subphyla Myriapoda , Crustacea and Hexapoda . Mandibulata is currently believed to be the sister group of the clade Arachnomorpha, which comprises the rest of arthropods...
. Unlike the trilobites', the compound eyes of Limulus are triangular in shape; they also have a generative region at their base, but this elongates with time. Hence the one ommatidium at the apex of the triangle was the original "eye" of the larval organism, with subsequent rows added as the organism grew.
Insects and crustaceans
These two groups are probably monophyletic; their eyes certainly develop in a very similar fashion. Their larvae only possess a pit-eye ocellus, termed Bolwig's organ. The compound eyes of adults develop in a separate region of the head. New ommatidia are added in semicircular rows at the rear of the eye; during the first phase of growth, this leads to individual ommatidia being square, but later in development they become hexagonal. The hexagonal pattern will only become visible when the carapace of the stage with square eyes is molted.Crustacean can have stalked eyes on peduncles.
Myriapods
Most myriapods bear stemmata - that is, single lensed eyes which evolved by the reduction of a compound eye. However, the genus Scutigera has secondarily re-evolved a compound eye composed of repeated stemmata. These appear to grow in rows which are inserted between existing rows of ocelli.See also
- Mollusc eyeMollusc eyeThe molluscs have the widest variety of eye morphologies of any phylum, and a large degree of variation in their function. Cephalopods' eyes are as complex as those of vertebrates; scallops have up to 100 simple eyes; and some bivalves have compound eyes....
- Parietal eyeParietal eyeA parietal eye, also known as a parietal organ or third-eye or pineal eye, is a part of the epithalamus present in some animal species...
- Simple eye in invertebrates
- Vision in fish