Arthropod leg
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/* Chelicerata */
The '''arthropod leg''' is a form of jointed [[appendage]] of [[arthropod]]s, usually used for [[walking]]. Many of the terms used for arthropod leg segments are of [[Latin]] origin, and may be confused with terms for bones: ''coxa'' (meaning [[hip (anatomy)|hip]]), ''trochanter'' (compare [[greater trochanter]] and [[lesser trochanter]]), ''[[femur]]'', ''[[tibia]]'', ''[[tarsus (skeleton)|tarsus]]'', ''[[ischium]]'', ''[[metatarsus]]'', ''[[carpus]]'', ''dactylus'' (meaning [[finger]]), ''[[patella]]''.
Homologies of leg segments between groups are difficult to prove and are the source of much argument. Some authors posit up to eleven segments per leg for the [[most recent common ancestor]] of extant [[arthropod]]s <ref>{{cite journal | quotes=no | title=The "Uniramia" do not exist - the ground plan of the Pterygota as revealed by Permian Diaphanopterodea from Russia (Insecta, Paleodictyopteroidea) |author = Kukalova-Peck, J. |journal=Canadian Journal of Zoology - Revue Canadienne de Zoologie |volume=70 |issue=2 |pages=236–255 |year=1992}}</ref>, but modern arthropods have eight or fewer. It has been argued <ref>{{cite journal | quotes=no |journal=Biol. J. Linn. Soc. B |author=Fryer, G. |year=1996 |title=Reflections on arthropod evolution |volume=58 |issue=1 |pages=1–55}}</ref><ref>{{cite journal | quotes=no |author = [[Frederick Schram|Schram, F. R.]] & S. Koenemann |title=Developmental genetics and arthropod evolution: part I, on legs |journal=Evolution & Development |volume=3 |issue=5 |pages=343–354 |year=2001 |doi=10.1046/j.1525-142X.2001.01038.x}}</ref> that the ancestral leg need not have been so complex, and that other events, such as the successive loss of function of a [[Homeobox|''Hox''-gene]] could result in [[parallelism|parallel]] gains of leg segments.
==Biramous and uniramous==<!-- This section is linked from [[Arthropod]] -->
[[Image:CrustaceaAppendages.png|thumb|Crustacean appendages]]
The appendages of arthropods may be either '''biramous''' or '''uniramous'''. A uniramous limb comprises a single series of segments attached end-to-end. A biramous limb, however, branches into two, and each branch consists of a series of segments attached end-to-end.
The legs of insects and [[myriapoda|myriapods]] are uniramous. In crustaceans, the first antennae are uniramous, but the second antennae are biramous, as are the legs in most species.
For a time, possession of uniramous limbs was believed to be a shared, [[cladistics|derived character]], so uniramous arthropods were grouped into a taxon called [[Uniramia]]. It is now believed that several groups of arthropods evolved uniramous limbs independently from ancestors with biramous limbs, so this taxon is no longer used.
==Hexapoda==
[[Image:Insekt-bein.gif|thumb|right|180px|Diagram of an insect leg]]
The legs of [[insect]]s are always found on the [[thorax]]. They have five segments per leg (coxa, trochanter, femur, tibia, tarsus), with the tarsus being divided into several sub-sections called '''tarsomeres'''. In some groups, the number of tarsomeres is important for identification to [[family (biology)|family]] level, for example [[beetle]]s (Coleoptera), or to [[subfamily]] level among some dipteran [[fly|flies]] (''e.g.'' [[Cecidomyiidae]]). Tarsal claws, also called ''ungues'' (singular: ''unguis''), may also be present <ref>{{cite book |author=Harbach, R. E. & K. L. Knight |year=1980 |title=Taxonomist's Glossary of Mosquito Anatomy |publisher=Plexus, Marlton, NJ}}</ref><ref>{{cite book |author=Richards, O. W. & R. G. Davies |year=1977 |title=Imm's General Textbook of Entomology, 10th ed. (2 Volumes) |publisher=[[Chapman and Hall]], London}}</ref>.
The segmentation is similar in [[collembola]]ns, in which each leg has a coxa, trochanter, femur, tibia, and a foot-complex.
In addition there is a sixth segment called pretarsus, which consists of claws and various other structures on the end of the tarsus. A pad or lobe-like structure is called an '''arolium''' when it is located between the claws, as in the case of [[Orthoptera]] ([[grasshopper]]s and [[crickets]]), or '''pulvillus''' if it is located at the base of the claws, as in [[Diptera]] ([[Fly|flies]]). These structures usually serve to increase adherence on various surfaces (as for the flies) and/or to cushion a fall, such as the jump of a grasshopper.
Insects' legs are attached to the three thoracic segments, the prothorax, mesothorax and metathorax. This terminology is also sometimes applied to the leg segments, so that the mesofemur is the femur of the second pair of legs, and the protarsus is the tarsus of the first pair of legs.
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==Chelicerata==
[[Image:Spiderlegdiagram.png|thumb|220px|Diagram of a spider, with the leg segments colour-coded: coxa=grey, trochanter=red, femur=green, patella=blue, tibia=purple, metatarsus=brown, tarsus=cyan]]
[[Spider|Spiders']] legs differ from those of insects by the addition of two segments on either side of the tibia, the patella between the femur and the tibia, and the metatarsus (sometimes called basitarsus) between the tibia and the tarsus (sometimes called telotarsus), making a total of seven segments.
The situation is identical in [[scorpion]]s, but with the addition of a pre-tarsus beyond the tarsus. The claws of the scorpion are not truly legs, but are [[pedipalp]]s, a different kind of [[appendage]] that is also found in spiders and is specialised for predation.
In ''[[Horseshoe crab|Limulus]]'', there are no metatarsi or pretarsi, leaving six segments per leg.
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==Crustacea==
[[Image:Crustaceanleg.jpg|thumb|260px|The leg of a [[squat lobster]], showing the segments; the ischium and merus are fused in many [[Decapoda|decapods]].]]
The legs of [[crustacean]]s are divided primitively into seven segments, which do not follow the naming system used in the other groups. They are: coxa, basis, ischium, merus, carpus, propodus, and dactylus. In some groups, some of the limb segments may be fused together. The [[claw]] of a lobster or crab is formed by the articulation of the dactylus against an outgrowth of the propodus. Crustacean limbs also differ in being biramous, whereas all other extant arthropods have uniramous limbs.
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==[[Myriapoda]]==
[[Millipede]]s, [[centipede]]s and their relatives have seven-segmented legs, comprising coxa, trochanter, prefemur, femur, tibia, tarsus, and a tarsal claw.
==References==
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[[Category:Arthropod anatomy]]
[[Category:Locomotion]]
[[Category:Spider anatomy]]
[[fr:patte des arthropodes]]
[[ja:関節肢]]
[[de:Insektenbein]]
[[no:Bein (insekt)]]