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Apposition eyes are the most common form of eye, and are presumably the ancestral form ofcompound eye. They are found in allarthropod groups, although they may have evolved more than once within this phylum.[1]Someannelids andbivalves also have apposition eyes. They are also possessed byLimulus, the horseshoe crab, and there are suggestions that otherchelicerates developed theirsimple eyes by reduction from a compound starting point.[1] Somecaterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.[citation needed]
Thearthropods ancestrally possessedcompound 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 thevelvet worm andhorseshoe crab to the advanced eye condition found ininsects and otherderived arthropods.
Most arthropods have at least one of two types of eye: lateral compound eyes, and smaller median ocelli, which are simple eyes.[2] When both are present, the two eye types are used in concert because each has its own advantage.[3] Some insectlarvae, e.g.,caterpillars, have a different type of simple eye known asstemmata. These eyes usually provide only a rough image, but (as insawfly 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.[4] Flying insects can remain level with either type of eye surgically removed, but the two types combine to give better performance.[3] Ocelli can detect lower light levels,[a][5] and have a faster response time, while compound eyes are better at detecting edges and are capable of forming images.[3]
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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 calleddichoptic. 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 theBilateria 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 themedian plane; examples include many of theArchaeognatha. In extreme cases such eyes may fuse, effectively into a single eye, as in some of theCopepoda, notably in the genusCyclops. One term for such an arrangement of eyes iscycloptic.
On the other hand, some modes of life demand enhancedvisual acuity, which in compound eyes demands a larger number ofommatidia, 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 thefrons and thevertex 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 calledholoptic. Spectacular examples may be seen in theAnisoptera and various flies, such as someAcroceridae andTabanidae.
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 theMantodea andMantispidae, 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 theanterior median plane. The individual ommatidia are directed in all directions and accordingly, one may see a dark spot (thepseudopupil), 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, theGyrinidae 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 someEphemeroptera 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[6] and various flies.[7] This asymmetry has been correlated with behaviourallateralization in ants (turning bias).[8]
In the fruit flyDrosophila melanogaster (the best-studied arthropod species with respect to developmental biology), among the most important genes for patterning the eyes of insects are the Pax6 homologseyeless (ey) andtwin of eyeless (toy). Together, these genes drive the proliferation of cells early in eye development. Loss of either of these genes results in failure of eye formation. The activity ofey andtoy includes the activation of the retinal determination genessine oculis (so) andeyes absent (eya), which form a protein complex that regulates the transcription of downstream target genes.[9] Thereafter, the two visual systems ofD. melanogaster are patterned differently. Anterior head patterning is controlled byorthodenticle (otd), ahomeobox gene which demarcates the segments from the top-middle of the head to the more lateral aspects. The ocelli are in anotd-rich area and disruption ofotd results in loss of the ocelli, but does not affect the compound eyes.[10] Inversely, the transcription factordachshund (dac) is required for the patterning of compound eyes, but mutants lackingdac do not exhibit loss of the ocelli.[11] Differentopsins are used in the ocelli of compound eyes.[12]
The visual systems ofChelicerata (the sister group to the remaining Arthropoda) are less well understood. It has been shown that homologs of many eye patterning genes are variably expressed in the eyes of different spider species, but the functional significance of these changes in expression is not well understood, due to lack of functional data.[13][14] In addition, it has been shown in horseshoe crabs and spiders that Pax6 homologs are not expressed in the same way as their counterparts in insects, suggesting that Pax6 may not be required as a top-level eye patterning switch in chelicerates.[15][16] Most of the functional data on eye patterning in Chelicerata is drawn from the daddy-longlegsPhalangium opilio, which has been used to show thateyes absent plays a conserved role in patterning both the visual systems of this species (an example of conservation of gene function, with respect to insects) and thatdachshund affects the patterning of lateral eyes, but not median eyes (another example of conservation).[17]
Hexapods 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.[18] The eyes are strikingly different from themyriapods, 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,[19] and may be apomorphic with ocelli in other phyla,[20] such as the annelids.[21] Median ocelli are present inchelicerates andmandibulates; lateral ocelli are also present in chelicerates.[20]
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 theonychophorans: 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.[2]
A conflicting view notes, however, that compound eyes appeared in many early arthropods, including the trilobites and eurypterids. That suggests that the compound eye may have developed after the onychophoran and arthropod lineages split, but before the radiation of arthropods.[20] This view is supported if a stem-arthropod position is supported for compound-eye bearing Cambrian organisms such as theRadiodontids. Yet another alternative is that compound eyesindependently evolved, multiple times within the arthropods.[21]
There were probably only a single pair of ocelli in the arthropod concestor, since Cambrian lobopod fossils display a single pair. And while many arthropods today have three, four, or even six, the lack of a 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 intheir concestor.[2]
It is deemed probable that the compound eye arose as a result of the 'duplication' of individual ocelli.[20] In turn, the dispersal of compound eyes seems to have created large networks of seemingly independent eyes in some arthropods, such as the larvae of certain insects.[20] In some other insects and myriapods, lateral ocelli appear to have arisen by the reduction of lateral compound eyes.[20]
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The eyes oftrilobites came in three forms, calledholochroal,schizochroal, andabathochroal eyes. The eye morphology of trilobites is useful for inferring their mode of life, and can function as indicators of the palaeo-environment conditions.[22]
Theholochroal eye was the most common and most primitive. It consisted of many small lenses – between 100 and 15,000 – covered by a single corneal membrane. This was the most ancient kind of eye. This eye morphology was found in the Cambrian trilobites (the earliest) and survived until thePermian extinction.[22]
The more complexschizochroal eye was found only in one sub-order of trilobite, thePhacopina (Ordovician-Silurian). There is no exact counterpart to theschizochroal eye in modern animals, but a somewhat similar eye structure is found in adult male insects in the orderStrepsiptera.Schizochroal eyes developed as an improvement onholochroal; they were more powerful, with overlapping visual fields, and were particularly useful for nocturnal vision and possibly for colour anddepth perception. Schizochroal eyes have up to 700 large lenses (large compared toholochroal lenses). Each lens has a cornea, and each has an individualsclera that separates it from the surrounding lenses. The multiple lenses for the eye were each constructed from a singlecalcite crystal. Earlyschizochroal eye designs appear haphazard and irregular – possibly constrained by the geometrical complications of packing identical sized lenses on a curved surface. Laterschizochroal eyes had size graduated lens.[22]
Theabathochroal eye is the third eye morphology of trilobites, but it has found only within theEodiscina. This form of eye consisted of up to 70 much smaller lenses. The cornea separated each lens, and the sclera on each lens terminated on top of each cornea.[22]
Thehorseshoe 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 near the base of thechelicerates; its eyes are believed to represent the ancestral condition because they have changed so little over evolutionary time. Most other living chelicerates have lost their lateral compound eyes, evolving simple eyes in their place that vary in number.[24] Up to five pairs of lateral eyes occur in scorpions, whereas three pairs of lateral eyes are typical forTetrapulmonata (e.g.,spiders;Amblypygi).[25]
Horseshoe crabs have two large compound eyes on the sides of its head. An additional simple eye is positioned at the rear of each of these structures.[24] 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.[24] A further simple eye is located beneath these, on the underside of the carapace; this eye is initially paired during embryonic stages and fuses later in development.[24][15] A further pair of simple eyes are positioned just in front of the mouth.[24] 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 organs during adulthood.[24] These ocelli are less complex, and probably less derived, than those of theMandibulata.[20] Unlike the compound eyes of trilobites, those of horseshoe crabs 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.[18]
It is generally thought thatinsects are a clade within the Crustacea, and that the Crustacea aremonophyletic. This is consistent with the observation that their eyes develop in a very similar fashion. While most crustacean and some insect larvae possess only simple median eyes, such as theBolwig organs ofDrosophila and thenaupliar eye of most crustaceans, several groups have larvae with simple or compound lateral eyes. The compound eyes of adults develop in a region of the head separate from the region in which the larval median eye develops.[18] 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 become visible only when the carapace of the stage with square eyes is molted.[18]
Although stalked eyes onpeduncles occur in some species of crustaceans and some insects, only some of the Crustacea, such as crabs, bear their eyes on articulated peduncles that permit the eyes to be folded out of the way of trouble.
Most myriapods bear stemmata – single lensed eyes which are thought to have evolved by the reduction of a compound eye.[20] However, members of the chilopod genusScutigera have a compound eye, which is composed of facets[26] and not, as earlier interpretations had it, of clustered stemmata.[21] that were thought to grow in rows, inserted between existing rows of ocelli.[18]
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