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The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: coxa (meaning hip, pl.: coxae), trochanter, femur (pl.: femora), tibia (pl.: tibiae), tarsus (pl.: tarsi), ischium (pl.: ischia), metatarsus, carpus, dactylus (meaning finger), patella (pl.: patellae).

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 arthropods^[1] but modern arthropods have eight or fewer. It has been argued^[2]^[3] that the ancestral leg need not have been so complex, and that other events, such as successive loss of function of a Hox-gene, could result in parallel gains of leg segments.

In arthropods, each of the leg segments articulates with the next segment in a hinge joint and may only bend in one plane. This means that a greater number of segments is required to achieve the same kinds of movements that are possible in vertebrate animals, which have rotational ball-and-socket joints at the base of the fore and hind limbs.^[4]

Biramous and uniramous

Uniramous

Biramous

Generalized external morphology of uniramous and biramous 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 external branch (ramus) of the appendages of crustaceans is known as the exopod or exopodite, while the internal branch is known as the endopod or endopodite. Other structures aside from the latter two are termed exites (outer structures) and endites (inner structures). Exopodites can be easily distinguished from exites by the possession of internal musculature. The exopodites can sometimes be missing in some crustacean groups (amphipods and isopods), and they are completely absent in insects.^[5]

The legs of insects and 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, 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.

Chelicerata

Arachnid 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 tarsus of spiders have claws at the end as well as a hook that helps with web-spinning. Spider legs can also serve sensory functions, with hairs that serve as touch receptors, as well as an organ on the tarsus that serves as a humidity receptor, known as the tarsal organ.^[6]

The situation is identical in scorpions, but with the addition of a pre-tarsus beyond the tarsus. The claws of the scorpion are not truly legs, but are pedipalps, a different kind of appendage that is also found in spiders and is specialised for predation and mating.

In Limulus, there are no metatarsi or pretarsi, leaving six segments per leg.

Crustacea

The leg of a squat lobster, showing the segments; the ischium and merus are fused in many decapods

The legs of crustaceans 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 (chela) 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.

Myriapoda

Myriapods (millipedes, centipedes and their relatives) have seven-segmented walking legs, comprising coxa, trochanter, prefemur, femur, tibia, tarsus, and a tarsal claw. Myriapod legs show a variety of modifications in different groups. In all centipedes, the first pair of legs is modified into a pair of venomous fangs called forcipules. In most millipedes, one or two pairs of walking legs in adult males are modified into sperm-transferring structures called gonopods. In some millipedes, the first leg pair in males may be reduced to tiny hooks or stubs, while in others the first pair may be enlarged.

Insects

Insects and their relatives are hexapods, having six legs, connected to the thorax, each with five components. In order from the body they are the coxa, trochanter, femur, tibia, and tarsus. Each is a single segment, except the tarsus which can be from three to seven segments, each referred to as a tarsomere.

Except in species in which legs have been lost or become vestigial through evolutionary adaptation, adult insects have six legs, one pair attached to each of the three segments of the thorax. They have paired appendages on some other segments, in particular, mouthparts, antennae and cerci, all of which are derived from paired legs on each segment of some common ancestor.

Some larval insects do however have extra walking legs on their abdominal segments; these extra legs are called prolegs. They are found most frequently on the larvae of moths and sawflies. Prolegs do not have the same structure as modern adult insect legs, and there has been a great deal of debate as to whether they are homologous with them.^[7] Current evidence suggests that they are indeed homologous up to a very primitive stage in their embryological development,^[8] but that their emergence in modern insects was not homologous between the Lepidoptera and Symphyta.^[9] Such concepts are pervasive in current interpretations of phylogeny.^[10]

In general, the legs of larval insects, particularly in the Endopterygota, vary more than in the adults. As mentioned, some have prolegs as well as "true" thoracic legs. Some have no externally visible legs at all (though they have internal rudiments that emerge as adult legs at the final ecdysis). Examples include the maggots of flies or grubs of weevils. In contrast, the larvae of other Coleoptera, such as the Scarabaeidae and Dytiscidae have thoracic legs, but no prolegs. Some insects that exhibit hypermetamorphosis begin their metamorphosis as planidia, specialised, active, legged larvae, but they end their larval stage as legless maggots, for example the Acroceridae.

Among the Exopterygota, the legs of larvae tend to resemble those of the adults in general, except in adaptations to their respective modes of life. For example, the legs of most immature Ephemeroptera are adapted to scuttling beneath underwater stones and the like, whereas the adults have more gracile legs that are less of a burden during flight. Again, the young of the Coccoidea are called "crawlers" and they crawl around looking for a good place to feed, where they settle down and stay for life. Their later instars have no functional legs in most species. Among the Apterygota, the legs of immature specimens are in effect smaller versions of the adult legs.^{[citation needed]}

Fundamental morphology of insect legs

A representative insect leg, such as that of a housefly or cockroach, has the following parts, in sequence from most proximal to most distal:

coxa
trochanter
femur
tibia
tarsus
pretarsus.

Associated with the leg itself there are various sclerites around its base. Their functions are articular and have to do with how the leg attaches to the main exoskeleton of the insect. Such sclerites differ considerably between unrelated insects.^[7]

Coxa

The coxa is the proximal segment and functional base of the leg. It articulates with the pleuron and associated sclerites of its thoracic segment, and in some species it articulates with the edge of the sternite as well. The homologies of the various basal sclerites are open to debate. Some authorities suggest that they derive from an ancestral subcoxa. In many species, the coxa has two lobes where it articulates with the pleuron. The posterior lobe is the meron which is usually the larger part of the coxa. A meron is well developed in Periplaneta, the Isoptera, Neuroptera and Lepidoptera.

Trochanter

The trochanter articulates with the coxa but usually is attached rigidly to the femur. In some insects, its appearance may be confusing; for example it has two subsegments in the Odonata. In parasitic Hymenoptera, the base of the femur has the appearance of a second trochanter.

Femur

In most insects, the femur is the largest region of the leg; it is especially conspicuous in many insects with saltatorial legs because the typical leaping mechanism is to straighten the joint between the femur and the tibia, and the femur contains the necessary massive bipennate musculature.

Tibia

The tibia is the fourth section of the typical insect leg. As a rule, the tibia of an insect is slender in comparison to the femur, but it generally is at least as long and often longer. Near the distal end, there is generally a tibial spur, often two or more. In the Apocrita, the tibia of the foreleg bears a large apical spur that fits over a semicircular gap in the first segment of the tarsus. The gap is lined with comb-like bristles, and the insect cleans its antennae by drawing them through.

Tarsus

The ancestral tarsus was a single segment and in the extant Protura, Diplura and certain insect larvae the tarsus also is single-segmented. Most modern insects have tarsi divided into subsegments (tarsomeres), usually about five. The actual number varies with the taxon, which may be useful for diagnostic purposes. For example, the Pterogeniidae characteristically have 5-segmented fore- and mid-tarsi, but 4-segmented hind tarsi, whereas the Cerylonidae have four tarsomeres on each tarsus.

The distal segment of the typical insect leg is the pretarsus. In the Collembola, Protura and many insect larvae, the pretarsus is a single claw. On the pretarsus most insects have a pair of claws (ungues, singular unguis). Between the ungues, a median unguitractor plate supports the pretarsus. The plate is attached to the apodeme of the flexor muscle of the ungues. In the Neoptera, the parempodia are a symmetrical pair of structures arising from the outside (distal) surface of the unguitractor plate between the claws.^[11] It is present in many Hemiptera and almost all Heteroptera.^[11] Usually, the parempodia are bristly (setiform), but in a few species they are fleshy.^[12] Sometimes the parempodia are reduced in size so as to almost disappear.^[13] Above the unguitractor plate, the pretarsus expands forward into a median lobe, the arolium.

Webspinner, *Embia major*, front leg showing enlarged tarsomere, which contains the silk-spinning organs

Webspinners (Embioptera) have an enlarged basal tarsomere on each of the front legs, containing the silk-producing glands.^[14]

Under their pretarsi, members of the Diptera generally have paired lobes or pulvilli, meaning "little cushions". There is a single pulvillus below each unguis. The pulvilli often have an arolium between them or otherwise a median bristle or empodium, meaning the meeting place of the pulvilli. On the underside of the tarsal segments, there frequently are pulvillus-like organs or plantulae. The arolium, plantulae and pulvilli are adhesive organs enabling their possessors to climb smooth or steep surfaces. They all are outgrowths of the exoskeleton and their cavities contain blood. Their structures are covered with tubular tenent hairs, the apices of which are moistened by a glandular secretion. The organs are adapted to apply the hairs closely to a smooth surface so that adhesion occurs through surface molecular forces.^[7]^[15]

Insects control the ungues through muscle tension on a long tendon, the "retractor unguis" or "long tendon". In insect models of locomotion and motor control, such as Drosophila (Diptera), locusts (Acrididae), or stick insects (Phasmatodea), the long tendon courses through the tarsus and tibia before reaching the femur. Tension on the long tendon is controlled by two muscles, one in the femur and one in the tibia, which can operate differently depending on how the leg is bent. Tension on the long tendon controls the claw, but also bends the tarsus and likely affects its stiffness during walking. ^[16]

Variations in functional anatomy of insect legs

The typical thoracic leg of an adult insect is adapted for running (cursorial), rather than for digging, leaping, swimming, predation, or other similar activities. The legs of most cockroaches are good examples. However, there are many specialized adaptations, including:

The forelegs of mole crickets (Gryllotalpidae) and some scarab beetle (Scarabaeidae) are adapted to burrowing in earth (fossorial).
The raptorial forelegs of mantidflies (Mantispidae), mantises (Mantodea), and ambush bugs (Phymatinae) are adapted to seizing and holding prey in one way, while those of whirligig beetles Gyrinidae are long and adapted for grasping food or prey in quite a different way.
The forelegs of some butterflies, such as many Nymphalidae, are reduced so greatly that only two pairs of functional walking legs remain.
In most grasshoppers and crickets (Orthoptera), the hind legs are saltatorial; they have heavily bipinnately muscled femora and straight, long tibiae adapted to leaping and to some extent to defence by kicking. Flea beetles (Alticini) also have powerful hind femora that enable them to leap spectacularly.
Other beetles with spectacularly muscular hind femora may not be saltatorial at all, but very clumsy; for example, particular species of bean weevils (Bruchinae) use their swollen hind legs for forcing their way out of the hard-shelled seeds of plants such as Erythrina in which they grew to adulthood.
The legs of the Odonata, the dragonflies and damselflies, are adapted for seizing prey that the insects feed on while flying or while sitting still on a plant; they are nearly incapable of using them for walking.^[7]
The majority of aquatic insects use their legs only for swimming (natatorial), though many species of immature insects swim by other means such as by wriggling, undulating, or expelling water in jets.

Evolution and homology of arthropod legs

The embryonic body segments (somites) of different arthropods taxa have diverged from a simple body plan with many similar appendages which are serially homologous, into a variety of body plans with fewer segments equipped with specialised appendages.^[17] The homologies between these have been discovered by comparing genes in evolutionary developmental biology.^[18]

Somite (body segment)	Trilobite (Trilobitomorpha)	Spider (Chelicerata)	Centipede (Myriapoda)	Insect (Hexapoda)	Shrimp (Crustacea)
1	antennae	chelicerae (jaws and fangs)	antennae	antennae	1st antennae
2	1st legs	pedipalps	-	-	2nd antennae
3	2nd legs	1st legs	mandibles	mandibles	mandibles (jaws)
4	3rd legs	2nd legs	1st maxillae	1st maxillae	1st maxillae
5	4th legs	3rd legs	2nd maxillae	2nd maxillae	2nd maxillae
6	5th legs	4th legs	collum (no legs)	1st legs	1st legs
7	6th legs	-	1st legs	2nd legs	2nd legs
8	7th legs	-	2nd legs	3rd legs	3rd legs
9	8th legs	-	3rd legs	-	4th legs
10	9th legs	-	4th legs	-	5th legs

References

^ Kukalova-Peck, J. (1992). "The "Uniramia" do not exist - the ground plan of the Pterygota as revealed by Permian Diaphanopterodea from Russia (Insecta, Paleodictyopteroidea)". Canadian Journal of Zoology. 70 (2): 236–255. doi:10.1139/z92-037.
^ Fryer, G. (1996). "Reflections on arthropod evolution". Biol. J. Linn. Soc. 58 (1): 1–55. doi:10.1111/j.1095-8312.1996.tb01659.x.
^ Schram, F. R. & S. Koenemann (2001). "Developmental genetics and arthropod evolution: part I, on legs". Evolution & Development. 3 (5): 343–354. doi:10.1046/j.1525-142X.2001.01038.x. PMID 11710766. S2CID 25997101.
^ Pat Willmer; Graham Stone; Ian Johnston (12 March 2009). Environmental Physiology of Animals. John Wiley & Sons. p. 329. ISBN 978-1-4443-0922-5.
^ Geoff A. Boxshall & Damià Jaume (2009). "Exopodites, Epipodites and Gills in Crustaceans" (PDF). Arthropod Systematics & Phylogeny. 67 (2). Museum für Tierkunde Dresden: 229–254. doi:10.3897/asp.67.e31699. Archived (PDF) from the original on 2019-04-26. Retrieved 2012-01-14.
^ Pechmann, Matthias (November 2010). "Patterning mechanisms and morphological diversity of spider appendages and their importance for spider evolution". Arthropod Structure & Development. 39 (6): 453–467. doi:10.1016/j.asd.2010.07.007. PMID 20696272. Retrieved 20 August 2020.
^ ^a ^b ^c ^d Richards, O. W.; Davies, R.G. (1977). Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology. Berlin: Springer. ISBN 0-412-61390-5.
^ Panganiban, Grace; Nagy, Lisa; Carroll, Sean B. (1994). "The role of the Distal-less gene in the development and evolution of insect limbs". Current Biology. 4 (8): 671–675. doi:10.1016/S0960-9822(00)00151-2. PMID 7953552. S2CID 22980014.
^ Suzuki, Y; Palopoli, MF (Oct 2001). "Evolution of insect abdominal appendages: are prolegs homologous or convergent traits?". Dev Genes Evol. 211 (10): 486–92. doi:10.1007/s00427-001-0182-3. PMID 11702198. S2CID 1163446.
^ Galis, Frietson (1996). "The evolution of insects and vertebrates: homeobox genes and homology". Trends in Ecology & Evolution. 11 (10): 402–403. doi:10.1016/0169-5347(96)30038-4. PMID 21237897.
^ ^a ^b Friedemann, Katrin; Spangenberg, Rico; Yoshizawa, Kazunor; Beutel, Rolf G. (2013). "Evolution of attachment structures in the highly diverse Acercaria (Hexapoda)" (PDF). Cladistics. 30 (2): 170–201. doi:10.1111/cla.12030. PMID 34781597. S2CID 86195785. Archived from the original (PDF) on 25 January 2014. Retrieved 25 January 2014.
^ Schuh, Randall T. & Slater, James Alexander (1995). True Bugs of the World (Hemiptera:Heteroptera): Classification and Natural History. Ithaca, New York: Cornell University Press. p. 46. ISBN 978-0-8014-2066-5.
^ Goel, S. C. (1972). "Notes on the structure of the unguitractor plate in Heteroptera (Hemiptera)". Journal of Entomology Series A, General Entomology. 46 (2): 167–173. doi:10.1111/j.1365-3032.1972.tb00124.x.
^ Ross, Edward S. (1991). "Embioptera". In Naumann, I. D.; Carne, P. B.; et al. (eds.). The Insects of Australia. Volume 1 (2 ed.). Melbourne University Press. pp. 405–409.
^ Stanislav N Gorb. "Biological attachment devices: exploring nature's diversity for biomimetics Phil. Trans. R. Soc. A 2008; 366(1870): 1557-1574 doi:10.1098/rsta.2007.2172 1471-2962
^ RADNIKOW, G.; BÄSSLER, U. (1991-05-01). "Function of a Muscle Whose Apodeme Travels Through a Joint Moved by Other Muscles: Why the Retractor Unguis Muscle in Stick Insects is Tripartite and has no Antagonist". Journal of Experimental Biology. 157 (1): 87–99. doi:10.1242/jeb.157.1.87. ISSN 0022-0949.
^ Novartis Foundation; Hall, Brian (2008). Homology. John Wiley. p. 29. ISBN 978-0-470-51566-2.
^ Brusca, R.C.; Brusca, G.J. (1990). Invertebrates. Sinauer Associates. p. 669.

[1] Kukalova-Peck, J. (1992). "The "Uniramia" do not exist - the ground plan of the Pterygota as revealed by Permian Diaphanopterodea from Russia (Insecta, Paleodictyopteroidea)". Canadian Journal of Zoology. 70 (2): 236–255. doi:10.1139/z92-037.

[2] Fryer, G. (1996). "Reflections on arthropod evolution". Biol. J. Linn. Soc. 58 (1): 1–55. doi:10.1111/j.1095-8312.1996.tb01659.x.

[3] Schram, F. R. & S. Koenemann (2001). "Developmental genetics and arthropod evolution: part I, on legs". Evolution & Development. 3 (5): 343–354. doi:10.1046/j.1525-142X.2001.01038.x. PMID 11710766. S2CID 25997101.

[WillmerStone2009-4] Pat Willmer; Graham Stone; Ian Johnston (12 March 2009). Environmental Physiology of Animals. John Wiley & Sons. p. 329. ISBN 978-1-4443-0922-5.

[boxhall-5] Geoff A. Boxshall & Damià Jaume (2009). "Exopodites, Epipodites and Gills in Crustaceans" (PDF). Arthropod Systematics & Phylogeny. 67 (2). Museum für Tierkunde Dresden: 229–254. doi:10.3897/asp.67.e31699. Archived (PDF) from the original on 2019-04-26. Retrieved 2012-01-14.

[6] Pechmann, Matthias (November 2010). "Patterning mechanisms and morphological diversity of spider appendages and their importance for spider evolution". Arthropod Structure & Development. 39 (6): 453–467. doi:10.1016/j.asd.2010.07.007. PMID 20696272. Retrieved 20 August 2020.

[isbn0-412-61390-5-7] Richards, O. W.; Davies, R.G. (1977). Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology. Berlin: Springer. ISBN 0-412-61390-5.

[8] Panganiban, Grace; Nagy, Lisa; Carroll, Sean B. (1994). "The role of the Distal-less gene in the development and evolution of insect limbs". Current Biology. 4 (8): 671–675. doi:10.1016/S0960-9822(00)00151-2. PMID 7953552. S2CID 22980014.

[9] Suzuki, Y; Palopoli, MF (Oct 2001). "Evolution of insect abdominal appendages: are prolegs homologous or convergent traits?". Dev Genes Evol. 211 (10): 486–92. doi:10.1007/s00427-001-0182-3. PMID 11702198. S2CID 1163446.

[10] Galis, Frietson (1996). "The evolution of insects and vertebrates: homeobox genes and homology". Trends in Ecology & Evolution. 11 (10): 402–403. doi:10.1016/0169-5347(96)30038-4. PMID 21237897.

[Friedemann-2013-11] Friedemann, Katrin; Spangenberg, Rico; Yoshizawa, Kazunor; Beutel, Rolf G. (2013). "Evolution of attachment structures in the highly diverse Acercaria (Hexapoda)" (PDF). Cladistics. 30 (2): 170–201. doi:10.1111/cla.12030. PMID 34781597. S2CID 86195785. Archived from the original (PDF) on 25 January 2014. Retrieved 25 January 2014.

[12] Schuh, Randall T. & Slater, James Alexander (1995). True Bugs of the World (Hemiptera:Heteroptera): Classification and Natural History. Ithaca, New York: Cornell University Press. p. 46. ISBN 978-0-8014-2066-5.

[13] Goel, S. C. (1972). "Notes on the structure of the unguitractor plate in Heteroptera (Hemiptera)". Journal of Entomology Series A, General Entomology. 46 (2): 167–173. doi:10.1111/j.1365-3032.1972.tb00124.x.

[14] Ross, Edward S. (1991). "Embioptera". In Naumann, I. D.; Carne, P. B.; et al. (eds.). The Insects of Australia. Volume 1 (2 ed.). Melbourne University Press. pp. 405–409.

[15] Stanislav N Gorb. "Biological attachment devices: exploring nature's diversity for biomimetics Phil. Trans. R. Soc. A 2008; 366(1870): 1557-1574 doi:10.1098/rsta.2007.2172 1471-2962

[16] RADNIKOW, G.; BÄSSLER, U. (1991-05-01). "Function of a Muscle Whose Apodeme Travels Through a Joint Moved by Other Muscles: Why the Retractor Unguis Muscle in Stick Insects is Tripartite and has no Antagonist". Journal of Experimental Biology. 157 (1): 87–99. doi:10.1242/jeb.157.1.87. ISSN 0022-0949.

[17] Novartis Foundation; Hall, Brian (2008). Homology. John Wiley. p. 29. ISBN 978-0-470-51566-2.

[Brusca-18] Brusca, R.C.; Brusca, G.J. (1990). Invertebrates. Sinauer Associates. p. 669.

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-{{Short description|Form of jointed appendage of arthropods}}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 (called '''podomeres''') are of [[Latin]] origin, and may be confused with terms for bones: ''coxa'' (meaning [[hip (anatomy)|hip]], plural ''coxae''), ''trochanter'', ''femur'' (plural ''femora''), ''tibia'' (plural ''tibiae''), ''tarsus'' (plural ''tarsi''), ''ischium'' (plural ''ischia''), ''metatarsus'', ''carpus'', ''dactylus'' (meaning [[finger]]), ''patella'' (plural ''patellae'').
+{{Short description|Form of jointed appendage of arthropods}}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 (called '''podomeres''') are of [[Latin]] origin, and may be confused with terms for bones: ''coxa'' (meaning [[hip (anatomy)|hip]], {{plural form}}: ''coxae''), ''trochanter'', ''femur'' ({{plural form}}: ''femora''), ''tibia'' ({{plural form}}: ''tibiae''), ''tarsus'' ({{plural form}}: ''tarsi''), ''ischium'' ({{plural form}}: ''ischia''), ''metatarsus'', ''carpus'', ''dactylus'' (meaning [[finger]]), ''patella'' ({{plural form}}: ''patellae'').
-[[Homology (biology)|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 | 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 |volume=70 |issue=2 |pages=236–255 |year=1992 | doi=10.1139/z92-037}}</ref> but modern arthropods have eight or fewer. It has been argued<ref>{{cite journal |journal=Biol. J. Linn. Soc. |author=Fryer, G. |year=1996 |title=Reflections on arthropod evolution |volume=58 |issue=1 |pages=1&ndash;55 | doi=10.1111/j.1095-8312.1996.tb01659.x|doi-access=free}}</ref><ref>{{cite journal |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 |pmid=11710766}}</ref> that the ancestral leg need not have been so complex, and that other events, such as successive loss of function of a [[Homeobox|''Hox''-gene]], could result in [[Parallel evolution|parallel]] gains of leg segments.
+[[Homology (biology)|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 [[Neontology|extant]] arthropods<ref>{{cite journal | 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 |volume=70 |issue=2 |pages=236–255 |year=1992 | doi=10.1139/z92-037}}</ref> but modern arthropods have eight or fewer. It has been argued<ref>{{cite journal |journal=Biol. J. Linn. Soc. |author=Fryer, G. |year=1996 |title=Reflections on arthropod evolution |volume=58 |issue=1 |pages=1&ndash;55 | doi=10.1111/j.1095-8312.1996.tb01659.x|doi-access=free}}</ref><ref>{{cite journal |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 |pmid=11710766|s2cid=25997101 }}</ref> that the ancestral leg need not have been so complex, and that other events, such as successive loss of function of a [[Homeobox|''Hox''-gene]], could result in [[Parallel evolution|parallel]] gains of leg segments.
 In arthropods, each of the leg segments articulates with the next segment in a [[hinge joint]] and may only bend in one plane. This means that a greater number of segments is required to achieve the same kinds of movements that are possible in vertebrate animals, which have rotational [[ball-and-socket joint]]s at the base of the fore and hind limbs.<ref name="WillmerStone2009">{{cite book|author1=Pat Willmer|author2=Graham Stone|author3=Ian Johnston|title=Environmental Physiology of Animals|url=https://books.google.com/books?id=r9gvbjRFyRgC&pg=PA329|date=12 March 2009|publisher=John Wiley & Sons|isbn=978-1-4443-0922-5|page=329}}</ref>
 ==Biramous and uniramous==<!-- This section is linked from [[Arthropod]] -->
+{{Multiple image
+| image1            = 20200213 arthropod uniramous arthropodized appendage morphology en.png
-[[File:Agnostus leg.jpg|thumb|Diagram of biramous leg of a [[trilobite]]; ''[[Agnostus]]'' spp.]]
+| image2            = 20211214 arthropod biramous arthropodized appendage morphology en.png
+| footer            = Generalized external morphology of uniramous and biramous appendages
+| total_width       = 400
+| caption1          = Uniramous
+| caption2          = Biramous
+| caption_align     = center
+}}
 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 external branch (ramus) of the appendages of [[crustaceans]] is known as the '''exopod''' or '''exopodite''', while the internal branch is known as the '''endopod''' or '''endopodite'''. Other structures aside from the latter two are termed '''exites''' (outer structures) and '''endites''' (inner structures). Exopodites can be easily distinguished from exites by the possession of internal musculature. The exopodites can sometimes be missing in some crustacean groups ([[amphipod]]s and [[isopod]]s), and they are completely absent in insects.<ref name="boxhall">{{cite journal|author1=Geoff A. Boxshall|author2=Damià Jaume|name-list-style=amp|year=2009|title=Exopodites, Epipodites and Gills in Crustaceans|url=https://research.nhm.org/pdfs/31363/31363.pdf|url-status=live|journal=Arthropod Systematics & Phylogeny|publisher=Museum für Tierkunde Dresden|volume=67|issue=2|pages=229–254|archive-url=https://web.archive.org/web/20190426152013/http://www.arthropod-systematics.de/ASP_67_2/ASP_67_2_Boxshall_229-254.pdf|archive-date=2019-04-26|access-date=2012-01-14}}</ref>
+The external branch (ramus) of the appendages of [[crustaceans]] is known as the '''exopod''' or '''exopodite''', while the internal branch is known as the '''endopod''' or '''endopodite'''. Other structures aside from the latter two are termed '''exites''' (outer structures) and '''endites''' (inner structures). Exopodites can be easily distinguished from exites by the possession of internal musculature. The exopodites can sometimes be missing in some crustacean groups ([[amphipod]]s and [[isopod]]s), and they are completely absent in insects.<ref name="boxhall">{{cite journal|author1=Geoff A. Boxshall|author2=Damià Jaume|name-list-style=amp|year=2009|title=Exopodites, Epipodites and Gills in Crustaceans|url=https://research.nhm.org/pdfs/31363/31363.pdf|url-status=live|journal=Arthropod Systematics & Phylogeny|publisher=Museum für Tierkunde Dresden|volume=67|issue=2|pages=229–254|doi=10.3897/asp.67.e31699 |archive-url=https://web.archive.org/web/20190426152013/http://www.arthropod-systematics.de/ASP_67_2/ASP_67_2_Boxshall_229-254.pdf|archive-date=2019-04-26|access-date=2012-01-14 |doi-access=free }}</ref>
 The legs of [[insect]]s 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.
-{{Clear}}
-[[File:CrustaceaAppendages.png|thumb|left|Crustacean appendages]]
-[[File:Micrograph of house-fly leg.jpeg|thumb|Micrograph of [[housefly]] leg]]
-{{Clear}}
 ==Chelicerata==
 {{See also|Spider anatomy|Glossary of spider terms}}
+[[File:Spider external anatomy appendages en.png|thumb|upright=1.2|Diagram of a spider leg and pedipalp – the pedipalp has one fewer segment]]
 [[Arachnid]] 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 tarsus of spiders have claws at the end as well as a hook that helps with web-spinning. Spider legs can also serve sensory functions, with hairs that serve as touch receptors, as well as an organ on the tarsus that serves as a humidity receptor, known as the [[Glossary of spider terms#tarsal organ|tarsal organ]].<ref>{{cite journal |last1=Pechmann |first1=Matthias |title=Patterning mechanisms and morphological diversity of spider appendages and their importance for spider evolution |journal=Arthropod Structure & Development |date=November 2010 |volume=39 |issue=6 |pages=453–467 |url=https://www.sciencedirect.com/science/article/pii/S1467803910000551 |access-date=20 August 2020}}</ref>
+The tarsus of spiders have claws at the end as well as a hook that helps with web-spinning. Spider legs can also serve sensory functions, with hairs that serve as touch receptors, as well as an organ on the tarsus that serves as a humidity receptor, known as the [[Glossary of spider terms#tarsal organ|tarsal organ]].<ref>{{cite journal |last1=Pechmann |first1=Matthias |title=Patterning mechanisms and morphological diversity of spider appendages and their importance for spider evolution |journal=Arthropod Structure & Development |date=November 2010 |volume=39 |issue=6 |pages=453–467 |doi=10.1016/j.asd.2010.07.007 |pmid=20696272 |url=https://www.sciencedirect.com/science/article/pii/S1467803910000551 |access-date=20 August 2020}}</ref>
 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 and mating.
 In ''[[Horseshoe crab|Limulus]]'', there are no metatarsi or pretarsi, leaving six segments per leg.
-{{Clear}}
-[[File:Spider external anatomy appendages en.png|thumb|upright=1.2|left|Diagram of a spider leg and pedipalp – the pedipalp has one fewer segment]]
-{{Clear}}
 ==Crustacea==
+[[Image:Crustaceanleg.jpg|thumb|260px|left|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 ([[chela (organ)|chela]]) 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.
-{{Clear}}
-[[Image:Crustaceanleg.jpg|thumb|260px|left|The leg of a [[squat lobster]], showing the segments; the ischium and merus are fused in many [[Decapoda|decapods]].]]
-{{Clear}}
 ==Myriapoda==
-[[File:Scutigera coleoptrata MHNT.jpg|thumb|right|Seven-segmented legs of ''[[Scutigera coleoptrata]]''.]]
+[[File:Scutigera coleoptrata 205306445.jpg|thumb|right|Seven-segmented legs of ''[[Scutigera coleoptrata]]'']]
 [[Myriapod]]s ([[millipede]]s, [[centipede]]s and their relatives) have seven-segmented walking legs, comprising coxa, trochanter, prefemur, femur, tibia, tarsus, and a tarsal claw. Myriapod legs show a variety of modifications in different groups. In all centipedes, the first pair of legs is modified into a pair of venomous fangs called forcipules. In most millipedes, one or two pairs of walking legs in adult males are modified into sperm-transferring structures called [[gonopod (millipedes)|gonopods]]. In some millipedes, the first leg pair in males may be reduced to tiny hooks or stubs, while in others the first pair may be enlarged.
-{{Clear}}
 == Insects {{anchor|tarsus}} ==
+[[File:Tettigoniidae Zabalius aridus True Leaf Katydid 2012 06 04 6713.JPG|thumb|left|''[[Zabalius]] aridus'' showing full leg anatomy, including plantulae under each tarsomere]]
 {{See also|Insect morphology}}
 {{anchor|tarsomere}}
@@ Line 54: / Line 53: @@
 Insects and their relatives are hexapods, having six legs, connected to the [[Thorax (insect anatomy)|thorax]], each with five components. In order from the body they are the coxa, trochanter, femur, tibia, and tarsus. Each is a single segment, except the tarsus which can be from three to seven segments, each referred to as a ''tarsomere''.
-Except in species in which legs have been lost or become vestigial through evolutionary adaptation, adult insects have six legs, one pair attached to each of the three segments of the thorax. They have paired appendages on some other segments, in particular, [[Insect mouthparts|mouthparts]], [[Antenna (biology)|antennae]] and [[Cercus|cerci]], all of which are derived from paired legs on each segment of some [[Onychophora#Evolutionary history|common ancestor]].
+Except in species in which legs have been lost or become vestigial through evolutionary adaptation, adult insects have six legs, one pair attached to each of the three segments of the thorax. They have paired appendages on some other segments, in particular, [[Insect mouthparts|mouthparts]], [[Antenna (biology)|antennae]] and [[Cercus|cerci]], all of which are derived from paired legs on each segment of some [[Onychophora#Evolution|common ancestor]].
-Some [[larva]]l insects do however have extra walking legs on their abdominal segments; these extra legs are called [[proleg]]s. They are found most frequently on the larvae of moths and sawflies. Prolegs do not have the same structure as modern adult insect legs, and there has been a great deal of debate as to whether they are homologous with them.<ref name="isbn0-412-61390-5">{{cite book |author1=Richards, O. W. |author2=Davies, R.G. |title=Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology |publisher=Springer |location=Berlin |year=1977 |isbn=0-412-61390-5 }}</ref> Current evidence suggests that they are indeed homologous up to a very primitive stage in their embryological development,<ref>{{cite journal | last1=Panganiban | first1=Grace | last2=Nagy | first2=Lisa | last3=Carroll | first3=Sean B. | author3-link=Sean B. Carroll | title=The role of the Distal-less gene in the development and evolution of insect limbs | journal=Current Biology | volume=4 | issue=8 | pages=671–675 | doi=10.1016/S0960-9822(00)00151-2}}</ref> but that their emergence in modern insects was not homologous between the [[Lepidoptera]] and [[Sawfly|Symphyta]].<ref>{{cite journal | last1=Suzuki | first1=Y | last2=Palopoli | first2=MF | date=Oct 2001 | title=Evolution of insect abdominal appendages: are prolegs homologous or convergent traits? | journal=Dev Genes Evol. | volume=211 | issue=10| pages=486–92 | doi=10.1007/s00427-001-0182-3 | pmid=11702198}}</ref> Such concepts are pervasive in current interpretations of phylogeny.<ref>{{cite journal | last1=Galis | first1=Frietson | year=1996 | title=The evolution of insects and vertebrates: homeobox genes and homology | journal=Trends in Ecology & Evolution | volume=11 | issue=10| pages=402–403 | doi=10.1016/0169-5347(96)30038-4}}</ref>
+Some [[larva]]l insects do however have extra walking legs on their abdominal segments; these extra legs are called [[proleg]]s. They are found most frequently on the larvae of moths and sawflies. Prolegs do not have the same structure as modern adult insect legs, and there has been a great deal of debate as to whether they are homologous with them.<ref name="isbn0-412-61390-5">{{cite book |author1=Richards, O. W. |author2=Davies, R.G. |title=Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology |publisher=Springer |location=Berlin |year=1977 |isbn=0-412-61390-5 }}</ref> Current evidence suggests that they are indeed homologous up to a very primitive stage in their embryological development,<ref>{{cite journal | last1=Panganiban | first1=Grace | last2=Nagy | first2=Lisa | last3=Carroll | first3=Sean B. | author3-link=Sean B. Carroll | title=The role of the Distal-less gene in the development and evolution of insect limbs | journal=Current Biology | year=1994 | volume=4 | issue=8 | pages=671–675 | doi=10.1016/S0960-9822(00)00151-2| pmid=7953552 | s2cid=22980014 }}</ref> but that their emergence in modern insects was not homologous between the [[Lepidoptera]] and [[Sawfly|Symphyta]].<ref>{{cite journal | last1=Suzuki | first1=Y | last2=Palopoli | first2=MF | date=Oct 2001 | title=Evolution of insect abdominal appendages: are prolegs homologous or convergent traits? | journal=Dev Genes Evol. | volume=211 | issue=10| pages=486–92 | doi=10.1007/s00427-001-0182-3 | pmid=11702198| s2cid=1163446 }}</ref> Such concepts are pervasive in current interpretations of phylogeny.<ref>{{cite journal | last1=Galis | first1=Frietson | year=1996 | title=The evolution of insects and vertebrates: homeobox genes and homology | journal=Trends in Ecology & Evolution | volume=11 | issue=10| pages=402–403 | doi=10.1016/0169-5347(96)30038-4| pmid=21237897 }}</ref>
-In general the legs of larval insects, particularly in the [[Endopterygota]], vary more than in the adults. As mentioned, some have prolegs as well as "true" thoracic legs. Some have no externally visible legs at all (though they have internal rudiments that emerge as adult legs at the final [[ecdysis]]). Examples include the maggots of [[Diptera|flies]] or grubs of [[Curculionidae|weevils]]. In contrast, the larvae of other [[Coleoptera]], such as the [[Scarabaeidae]] and [[Dytiscidae]] have thoracic legs, but no prolegs. Some insects that exhibit [[hypermetamorphosis]] begin their metamorphosis as [[Planidium|planidia]], specialised, active, legged larvae, but they end their larval stage as legless maggots, for example the [[Acroceridae]].
+In general, the legs of larval insects, particularly in the [[Endopterygota]], vary more than in the adults. As mentioned, some have prolegs as well as "true" thoracic legs. Some have no externally visible legs at all (though they have internal rudiments that emerge as adult legs at the final [[ecdysis]]). Examples include the maggots of [[Diptera|flies]] or grubs of [[Curculionidae|weevils]]. In contrast, the larvae of other [[Coleoptera]], such as the [[Scarabaeidae]] and [[Dytiscidae]] have thoracic legs, but no prolegs. Some insects that exhibit [[hypermetamorphosis]] begin their metamorphosis as [[Planidium|planidia]], specialised, active, legged larvae, but they end their larval stage as legless maggots, for example the [[Acroceridae]].
-Among the [[Exopterygota]] the legs of larvae tend to resemble those of the adults in general, except in adaptations to their respective modes of life. For example, the legs of most immature [[Mayfly|Ephemeroptera]] are adapted to scuttling beneath underwater stones and the like, whereas the adults have more gracile legs that are less of a burden during flight. Again, the young of the [[Scale insect|Coccoidea]] are called "crawlers" and they crawl around looking for a good place to feed, where they settle down and stay for life. Their later [[instar]]s have no functional legs in most species.
+Among the [[Exopterygota]], the legs of larvae tend to resemble those of the adults in general, except in adaptations to their respective modes of life. For example, the legs of most immature [[Mayfly|Ephemeroptera]] are adapted to scuttling beneath underwater stones and the like, whereas the adults have more gracile legs that are less of a burden during flight. Again, the young of the [[Scale insect|Coccoidea]] are called "crawlers" and they crawl around looking for a good place to feed, where they settle down and stay for life. Their later [[instar]]s have no functional legs in most species.
-Among the [[Apterygota]] the legs of immature specimens are in effect smaller versions of the adult legs.{{citation needed|date=December 2017}}
+Among the [[Apterygota]], the legs of immature specimens are in effect smaller versions of the adult legs.{{citation needed|date=December 2017}}
 ===Fundamental morphology of insect legs===
@@ Line 78: / Line 77: @@
 ====Coxa====
-[[File:Tettigoniidae Zabalius aridus True Leaf Katydid 2012 06 04 6713.JPG|thumb|left|''[[Zabalius]] aridus'' showing full leg anatomy, including plantulae under each tarsomere]]
 The coxa is the proximal segment and functional base of the leg. It articulates with the [[Pleuron (insect anatomy)|pleuron]] and associated sclerites of its thoracic segment, and in some species it articulates with the edge of the sternite as well. The homologies of the various basal sclerites are open to debate. Some authorities suggest that they derive from an ancestral subcoxa.
-In many species the coxa has two lobes where it articulates with the pleuron. The posterior lobe is the [[Insect morphology|meron]] which is usually the larger part of the coxa. A meron is well developed in Periplaneta, the Isoptera, Neuroptera and Lepidoptera.
+In many species, the coxa has two lobes where it articulates with the pleuron. The posterior lobe is the [[Insect morphology|meron]] which is usually the larger part of the coxa. A meron is well developed in Periplaneta, the Isoptera, Neuroptera and Lepidoptera.
 ====Trochanter====
-The trochanter articulates with the coxa but usually is attached rigidly to the femur. In some insects its appearance may be confusing; for example it has two subsegments in the Odonata. In parasitic Hymenoptera the base of the femur has the appearance of a second trochanter.
+The trochanter articulates with the coxa but usually is attached rigidly to the femur. In some insects, its appearance may be confusing; for example it has two subsegments in the Odonata. In parasitic Hymenoptera, the base of the femur has the appearance of a second trochanter.
 ====Femur====
 [[File:Acanthacris ruficornis Acrididae IMG 2127s.jpg|thumb|''[[Acanthacris]] ruficornis'', legs saltatorial, femora with [[bipennate muscle]] attachments, spines on tibiae painfully effective in a defensive kick]]
-In most insects the femur is the largest region of the leg; it is especially conspicuous in many insects with [[Glossary of entomology terms|saltatorial]] legs because the typical leaping mechanism is to straighten the joint between the femur and the tibia, and the femur contains the necessary massive [[Bipennate muscle|bipennate]] musculature.
+In most insects, the femur is the largest region of the leg; it is especially conspicuous in many insects with [[Glossary of entomology terms|saltatorial]] legs because the typical leaping mechanism is to straighten the joint between the femur and the tibia, and the femur contains the necessary massive [[Bipennate muscle|bipennate]] musculature.
 ====Tibia====
-The tibia is the fourth section of the typical insect leg. As a rule the tibia of an insect is slender in comparison to the femur, but it generally is at least as long and often longer. Near the distal end there is generally a tibial spur, often two or more. In the [[Apocrita]] the tibia of the foreleg bears a large apical spur that fits over a semicircular gap in the first segment of the tarsus. The gap is lined with comb-like bristles, and the insect cleans its antennae by drawing them through.
+The tibia is the fourth section of the typical insect leg. As a rule, the tibia of an insect is slender in comparison to the femur, but it generally is at least as long and often longer. Near the distal end, there is generally a tibial spur, often two or more. In the [[Apocrita]], the tibia of the foreleg bears a large apical spur that fits over a semicircular gap in the first segment of the tarsus. The gap is lined with comb-like bristles, and the insect cleans its antennae by drawing them through.
 ====Tarsus====
 [[File:Pegesimallus sp robberfly.jpg|thumb|left|Robber fly ([[Asilidae]]), showing tarsomeres and pretarsi with ungues, pulvilli and empodia]]
+{{see also|Tarsal formula}}
 The ancestral tarsus was a single segment and in the extant [[Protura]], [[Diplura]] and certain insect larvae the tarsus also is single-segmented. Most modern insects have tarsi divided into subsegments (tarsomeres), usually about five. The actual number varies with the [[taxon]], which may be useful for diagnostic purposes. For example, the [[Pterogeniidae]] characteristically have 5-segmented fore- and mid-tarsi, but 4-segmented hind tarsi, whereas the [[Cerylonidae]] have four tarsomeres on each tarsus.
-The distal segment of the typical insect leg is the pretarsus. In the [[Springtail|Collembola]], Protura and many insect larvae, the pretarsus is a single claw. On the pretarsus most insects have a pair of claws ([[Glossary of entomology terms#unguis|''ungues'', singular ''unguis'']]). Between the ungues a median unguitractor plate supports the pretarsus. The plate is attached to the [[Exoskeleton|apodeme]] of the flexor muscle of the ungues. In the [[Neoptera]] the parempodia are a symmetrical pair of structures arising from the outside (distal) surface of the unguitractor plate between the claws.<ref name="Friedemann-2013">{{Cite journal |author1=Friedemann, Katrin |author2=Spangenberg, Rico |author3=Yoshizawa, Kazunor |author4=Beutel, Rolf G. |year=2013 |title=Evolution of attachment structures in the highly diverse Acercaria (Hexapoda) |journal=Cladistics |doi=10.1111/cla.12030 |url=http://insect3.agr.hokudai.ac.jp/psoco-web/pdf/2013clad.pdf |archive-url=https://web.archive.org/web/20140125061403/http://insect3.agr.hokudai.ac.jp/psoco-web/pdf/2013clad.pdf|archive-date=25 January 2014 |url-status=dead |volume=30 |pages=170–201 |access-date=25 January 2014}}</ref> It is present in many Hemiptera and almost all [[Heteroptera]].<ref name="Friedemann-2013" /> Usually the parempodia are bristly (setiform), but in a few species they are fleshy.<ref>{{Cite book|author1=Schuh, Randall T. |author2=Slater, James Alexander |name-list-style=amp |year=1995 |title=True Bugs of the World (Hemiptera:Heteroptera): Classification and Natural History |location=Ithaca, New York |publisher=Cornell University Press |page=[https://books.google.com/books?id=wmSc-2KDmZ4C&pg=PA46 46] |isbn=978-0-8014-2066-5}}</ref> Sometimes the parempodia are reduced in size so as to almost disappear.<ref>{{Cite journal |author=Goel, S. C. |year=1972 |title=Notes on the structure of the unguitractor plate in Heteroptera (Hemiptera)|journal=Journal of Entomology, Series A |volume=46 |issue=2 |pages=167–173 |doi=10.1111/j.1365-3032.1972.tb00124.x}}</ref> Above the unguitractor plate the pretarsus expands forward into a median lobe, the '''arolium'''.
+The distal segment of the typical insect leg is the pretarsus. In the [[Springtail|Collembola]], Protura and many insect larvae, the pretarsus is a single claw. On the pretarsus most insects have a pair of claws ([[Glossary of entomology terms#unguis|''ungues'', singular ''unguis'']]). Between the ungues, a median unguitractor plate supports the pretarsus. The plate is attached to the [[Exoskeleton|apodeme]] of the flexor muscle of the ungues. In the [[Neoptera]], the parempodia are a symmetrical pair of structures arising from the outside (distal) surface of the unguitractor plate between the claws.<ref name="Friedemann-2013">{{Cite journal |author1=Friedemann, Katrin |author2=Spangenberg, Rico |author3=Yoshizawa, Kazunor |author4=Beutel, Rolf G. |year=2013 |title=Evolution of attachment structures in the highly diverse Acercaria (Hexapoda) |journal=Cladistics |doi=10.1111/cla.12030 |url=http://insect3.agr.hokudai.ac.jp/psoco-web/pdf/2013clad.pdf |archive-url=https://web.archive.org/web/20140125061403/http://insect3.agr.hokudai.ac.jp/psoco-web/pdf/2013clad.pdf|archive-date=25 January 2014 |url-status=dead |volume=30 |issue=2 |pages=170–201 |pmid=34781597 |s2cid=86195785 |access-date=25 January 2014}}</ref> It is present in many Hemiptera and almost all [[Heteroptera]].<ref name="Friedemann-2013" /> Usually, the parempodia are bristly (setiform), but in a few species they are fleshy.<ref>{{Cite book|author1=Schuh, Randall T. |author2=Slater, James Alexander |name-list-style=amp |year=1995 |title=True Bugs of the World (Hemiptera:Heteroptera): Classification and Natural History |location=Ithaca, New York |publisher=Cornell University Press |page=[https://books.google.com/books?id=wmSc-2KDmZ4C&pg=PA46 46] |isbn=978-0-8014-2066-5}}</ref> Sometimes the parempodia are reduced in size so as to almost disappear.<ref>{{Cite journal |author=Goel, S. C. |year=1972 |title=Notes on the structure of the unguitractor plate in Heteroptera (Hemiptera)|journal=Journal of Entomology Series A, General Entomology |volume=46 |issue=2 |pages=167–173 |doi=10.1111/j.1365-3032.1972.tb00124.x}}</ref> Above the unguitractor plate, the pretarsus expands forward into a median lobe, the '''arolium'''.
 [[File:Embia major female front leg showing enlarged tarsomere.jpg|thumb|[[Webspinner]], ''[[Embia major]]'', front leg showing enlarged tarsomere, which contains the silk-spinning organs]]
@@ Line 107: / Line 106: @@
 Webspinners ([[Embioptera]]) have an enlarged basal tarsomere on each of the front legs, containing the [[silk]]-producing glands.<ref>{{cite book |chapter=Embioptera |pages=405–409 |title=The Insects of Australia. Volume 1 |edition=2 |year=1991 |publisher=Melbourne University Press |editor1=Naumann, I. D.| editor2=Carne, P. B. |editor3=Lawrence, J. F. |editor4=Nielsen, E. S. |editor5=Spradbery, J. P. |editor6=Taylor, R. W. |editor7=Whitten, M. J. |editor8=Littlejohn, M. J. |display-editors=2 |author=Ross, Edward S.}}</ref>
-Under their pretarsi, members of the [[Diptera]] generally have paired lobes or pulvilli, meaning "little cushions". There is a single pulvillus below each unguis. The pulvilli often have an arolium between them or otherwise a median bristle or '''empodium''', meaning the meeting place of the pulvilli. On the underside of the tarsal segments there frequently are pulvillus-like organs or '''plantulae'''. The arolium, plantulae and pulvilli are adhesive organs enabling their possessors to climb smooth or steep surfaces. They all are outgrowths of the exoskeleton and their cavities contain blood. Their structures are covered with tubular tenent hairs, the apices of which are moistened by a glandular secretion. The organs are adapted to apply the hairs closely to a smooth surface so that adhesion occurs through surface molecular forces.<ref name="isbn0-412-61390-5"/><ref>Stanislav N Gorb. "Biological attachment devices: exploring nature's diversity for biomimetics Phil. ''Trans. R. Soc. A'' 2008; 366(1870): 1557-1574 doi:10.1098/rsta.2007.2172 1471-2962</ref>
+Under their pretarsi, members of the [[Diptera]] generally have paired lobes or pulvilli, meaning "little cushions". There is a single pulvillus below each unguis. The pulvilli often have an arolium between them or otherwise a median bristle or '''empodium''', meaning the meeting place of the pulvilli. On the underside of the tarsal segments, there frequently are pulvillus-like organs or '''plantulae'''. The arolium, plantulae and pulvilli are adhesive organs enabling their possessors to climb smooth or steep surfaces. They all are outgrowths of the exoskeleton and their cavities contain blood. Their structures are covered with tubular tenent hairs, the apices of which are moistened by a glandular secretion. The organs are adapted to apply the hairs closely to a smooth surface so that adhesion occurs through surface molecular forces.<ref name="isbn0-412-61390-5"/><ref>Stanislav N Gorb. "Biological attachment devices: exploring nature's diversity for biomimetics Phil. ''Trans. R. Soc. A'' 2008; 366(1870): 1557-1574 doi:10.1098/rsta.2007.2172 1471-2962</ref>
-Insects control the ungues through muscle tension on a long tendon, the "retractor unguis" or "long tendon". In insect models of locomotion and motor control, such as ''[[Drosophila melanogaster|Drosophila]]'' ([[Diptera]]), [[Locust|locusts]] ([[Acrididae]]), or stick insects ([[Phasmatodea]]), the long tendon courses through the tarsus and tibia before reaching the femur. Tension on the long tendon is controlled by two muscles, one in the femur and one in the tibia, which can operate differently depending on how the leg is bent. Tension on the long tendon controls the claw, but also bends the tarsus and likely affects its stiffness during walking. <ref>{{Cite journal|last=RADNIKOW|first=G.|last2=BÄSSLER|first2=U.|date=1991-05-01|title=Function of a Muscle Whose Apodeme Travels Through a Joint Moved by Other Muscles: Why the Retractor Unguis Muscle in Stick Insects is Tripartite and has no Antagonist|url=https://doi.org/10.1242/jeb.157.1.87|journal=Journal of Experimental Biology|volume=157|issue=1|pages=87–99|doi=10.1242/jeb.157.1.87|issn=0022-0949}}</ref>
+Insects control the ungues through muscle tension on a long tendon, the "retractor unguis" or "long tendon". In insect models of locomotion and motor control, such as ''[[Drosophila melanogaster|Drosophila]]'' ([[Diptera]]), [[Locust|locusts]] ([[Acrididae]]), or stick insects ([[Phasmatodea]]), the long tendon courses through the tarsus and tibia before reaching the femur. Tension on the long tendon is controlled by two muscles, one in the femur and one in the tibia, which can operate differently depending on how the leg is bent. Tension on the long tendon controls the claw, but also bends the tarsus and likely affects its stiffness during walking. <ref>{{Cite journal|last1=RADNIKOW|first1=G.|last2=BÄSSLER|first2=U.|date=1991-05-01|title=Function of a Muscle Whose Apodeme Travels Through a Joint Moved by Other Muscles: Why the Retractor Unguis Muscle in Stick Insects is Tripartite and has no Antagonist|url=https://doi.org/10.1242/jeb.157.1.87|journal=Journal of Experimental Biology|volume=157|issue=1|pages=87–99|doi=10.1242/jeb.157.1.87|issn=0022-0949}}</ref>
 ===Variations in functional anatomy of insect legs===
 [[File:Starr 020615-0037 Erythrina sandwicensis.jpg|thumb|left|Bruchine with powerful femora used for escape from hard-shelled seed]]
-The typical thoracic leg of an adult insect is adapted for running, rather than for digging, leaping, swimming, predation, or other similar activities. The legs of most [[cockroach]]es are good examples. However, there are many specialized adaptations, including:
+The typical thoracic leg of an adult insect is adapted for running ([[cursorial]]), rather than for digging, leaping, swimming, predation, or other similar activities. The legs of most [[cockroach]]es are good examples. However, there are many specialized adaptations, including:
-*The forelegs of the [[Mole cricket|Gryllotalpidae]] and some [[Scarabaeidae]] are adapted to burrowing in earth.
+*The forelegs of [[Mole cricket|mole crickets]] (Gryllotalpidae) and some scarab beetle ([[Scarabaeidae]]) are adapted to burrowing in earth ([[fossorial]]).
-*The forelegs of the [[Mantispidae]], [[Mantodea]], and [[Phymatinae]] are adapted to seizing and holding prey in one way, while those of the [[Gyrinidae]] are long and adapted for grasping food or prey in quite a different way.
+*The [[raptorial]] forelegs of mantidflies ([[Mantispidae]]), [[Mantis|mantises]] (Mantodea), and ambush bugs ([[Phymatinae]]) are adapted to seizing and holding prey in one way, while those of [[Whirligig beetle|whirligig beetles]] Gyrinidae are long and adapted for grasping food or prey in quite a different way.
 *The forelegs of some butterflies, such as many [[Nymphalidae]], are reduced so greatly that only two pairs of functional walking legs remain.
-*In most [[Orthoptera]] the hind legs are [[saltatorial]]; they have heavily bipinnately muscled femora and straight, long tibiae adapted to leaping and to some extent to defence by kicking. [[Flea beetle]]s also have powerful hind femora that enable them to leap spectacularly.
+*In most grasshoppers and crickets ([[Orthoptera]]), the hind legs are [[saltatorial]]; they have heavily bipinnately muscled femora and straight, long tibiae adapted to leaping and to some extent to defence by kicking. [[Flea beetle]]s (Alticini) also have powerful hind femora that enable them to leap spectacularly.
-*Other beetles with spectacularly muscular hind femora may not be saltatorial at all, but very clumsy; for example, particular species of [[Bruchinae]] use their swollen hind legs for forcing their way out of the hard-shelled seeds of plants such as ''[[Erythrina]]'' in which they grew to adulthood.
+*Other beetles with spectacularly muscular hind femora may not be saltatorial at all, but very clumsy; for example, particular species of [[Bean weevil|bean weevils]] (Bruchinae) use their swollen hind legs for forcing their way out of the hard-shelled seeds of plants such as ''[[Erythrina]]'' in which they grew to adulthood.
 *The legs of the [[Odonata]], the [[dragonflies]] and [[damselflies]], are adapted for seizing prey that the insects feed on while flying or while sitting still on a plant; they are nearly incapable of using them for walking.<ref name="isbn0-412-61390-5"/>
-*The majority of aquatic insects use their legs only for swimming, though many species of immature insects swim by other means such as by wriggling, undulating, or expelling water in jets.
+*The majority of aquatic insects use their legs only for swimming (natatorial), though many species of immature insects swim by other means such as by wriggling, undulating, or expelling water in jets.
-==Evolution and homology of arthropod legs==
 [[File:Arthropod segment Hox gene expression.svg|thumb|upright=1.5|Expression of [[Hox gene]]s in the [[Segmentation (biology)|body segments]] of different groups of [[arthropod]], as traced by [[evolutionary developmental biology]]. The [[Hox gene]]s 7, 8, and 9 correspond in these groups but are shifted (by [[heterochrony]]) by up to three segments. Segments with maxillopeds have Hox gene 7. Fossil [[trilobite]]s probably had three body regions, each with a unique combination of Hox genes.]]
+==Evolution and homology of arthropod legs==
 The embryonic body segments ([[somite]]s) of different [[arthropod]]s taxa have diverged from a simple body plan with many similar appendages which are serially homologous, into a variety of body plans with fewer segments equipped with specialised appendages.<ref>{{cite book |author1=Novartis Foundation |author2=Hall, Brian |title=Homology |url=https://books.google.com/books?id=vptaNfbkd8sC&pg=PA29 |date=2008 |publisher=John Wiley |isbn=978-0-470-51566-2 |page=29}}</ref> The homologies between these have been discovered by comparing [[gene]]s in [[evolutionary developmental biology]].<ref name=Brusca>{{cite book |author1=Brusca, R.C. |author2=Brusca, G.J. |year=1990 |title=Invertebrates |url=https://archive.org/details/invertebrates0000brus |url-access=registration |publisher=Sinauer Associates |page=[https://archive.org/details/invertebrates0000brus/page/669 669]}}</ref>
@@ Line 132: / Line 132: @@
 |-
 ! [[Somite]]<br />(body<br />segment)
-! [[Trilobite]]<br />([[Trilobitomorpha]])<br />[[File:Acadoparadoxides sp 4343.JPG|50px]]
+! [[Trilobite]]<br />([[Trilobitomorpha]])<br />[[File:202003 Trilobite.svg|50px]]
-! [[Spider]]<br />([[Chelicerata]])<br />[[File:Araneus quadratus MHNT.jpg|50px]]
+! [[Spider]]<br />([[Chelicerata]])<br />[[File:202201 Common house spider.svg|50px]]
-! [[Centipede]]<br />([[Myriapoda]])<br />[[File:Scolopendridae - Scolopendra cingulata.jpg|50px]]
+! [[Centipede]]<br />([[Myriapoda]])<br />[[File:Scolopendra subspinipes japonica (no background).png|80px]]
-! [[Insect]]<br />([[Hexapoda]])<br />[[File:Cerf-volant MHNT Dos.jpg|50px]]
+! [[Insect]]<br />([[Hexapoda]])<br />[[File:202101 Chrysodeixis eriosoma.svg|50px]]
-! [[Shrimp]]<br />([[Crustacea]])<br />[[File:GarneleCrystalRed20.jpg|50px]]
+! [[Shrimp]]<br />([[Crustacea]])<br />[[File:202112 Japanese tiger prawn.svg|50px]]
 |-
 | 1
@@ Line 210: / Line 210: @@
 |}
+==See also==
-===Leg modification classifications===
+* [[Limb (anatomy)|Limb]]
-* [[Fossorial]] legs are modified for [[digging]] and [[burrow]]ing. For example: [[mole cricket]]s have fossorial forelegs
+* [[Tentacle]]
-* [[Saltatorial]] legs are modified for [[jumping]] and leaping. For example: many [[orthoptera]]ns have saltatory hindlegs
+* [[Tube foot]]
-* Natatorial legs are modified for [[aquatic locomotion|swimming]]. For example: [[whirligig beetle]]s have natatorial mesothoracic and metathoracic legs
-* [[Cursorial]] legs are modified for [[Terrestrial locomotion#Legged locomotion|running]]. For example: lots of [[cockroach]]es have very sensitive cursorial legs
-* [[Raptorial]] legs are modified for [[grasp]]ing, usually a hunting adaptation. For example: [[Mantidae|Mantids]] use their raptorial forelegs to hunt
 ==References==
-{{reflist|28em}}
+{{reflist|22em}}
 {{Portal|Arthropods}}