Arthropod leg

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Thearthropod legis a form of jointedappendageofarthropods,usually used forwalking.Many of the terms used for arthropod leg segments (calledpodomeres) are ofLatinorigin, and may be confused with terms for bones:coxa(meaninghip,pl.:coxae),trochanter,femur(pl.:femora),tibia(pl.:tibiae),tarsus(pl.:tarsi),ischium(pl.:ischia),metatarsus,carpus,dactylus(meaningfinger),patella(pl.:patellae).

Homologiesof 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 themost recent common ancestorofextantarthropods[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 aHox-gene,could result inparallelgains of leg segments.

In arthropods, each of the leg segments articulates with the next segment in ahinge jointand 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 rotationalball-and-socket jointsat the base of the fore and hind limbs.[4]

Biramous and uniramous

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Uniramous
Biramous
Generalized external morphology of uniramous and biramous appendages

The appendages of arthropods may be eitherbiramousoruniramous.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 ofcrustaceansis known as theexopodorexopodite,while the internal branch is known as theendopodorendopodite.Other structures aside from the latter two are termedexites(outer structures) andendites(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 (amphipodsandisopods), and they are completely absent in insects.[5]

The legs ofinsectsandmyriapodsare 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 calledUniramia.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

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Diagram of a spider leg and pedipalp – the pedipalp has one fewer segment

Arachnidlegs 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 thetarsal organ.[6]

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

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

Crustacea

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The leg of asquat lobster,showing the segments; the ischium and merus are fused in manydecapods

The legs ofcrustaceansare 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

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Seven-segmented legs ofScutigera coleoptrata

Myriapods(millipedes,centipedesand 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 calledgonopods.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

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Zabaliusaridusshowing full leg anatomy, including plantulae under each tarsomere

Insects and their relatives are hexapods, having six legs, connected to thethorax,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 atarsomere.

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,antennaeandcerci,all of which are derived from paired legs on each segment of somecommon ancestor.

Somelarvalinsects do however have extra walking legs on their abdominal segments; these extra legs are calledprolegs.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 theLepidopteraandSymphyta.[9]Such concepts are pervasive in current interpretations of phylogeny.[10]

In general, the legs of larval insects, particularly in theEndopterygota,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 finalecdysis). Examples include the maggots offliesor grubs ofweevils.In contrast, the larvae of otherColeoptera,such as theScarabaeidaeandDytiscidaehave thoracic legs, but no prolegs. Some insects that exhibithypermetamorphosisbegin their metamorphosis asplanidia,specialised, active, legged larvae, but they end their larval stage as legless maggots, for example theAcroceridae.

Among theExopterygota,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 immatureEphemeropteraare 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 theCoccoideaare called "crawlers" and they crawl around looking for a good place to feed, where they settle down and stay for life. Their laterinstarshave no functional legs in most species. Among theApterygota,the legs of immature specimens are in effect smaller versions of the adult legs.[citation needed]

Fundamental morphology of insect legs

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Diagram of a typical insect leg

A representative insect leg, such as that of ahouseflyorcockroach,has the following parts, in sequence from mostproximalto mostdistal:

  • coxa
  • trochanter
  • femur
  • tibia
  • tarsus
  • pretarsus.

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

Coxa

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The coxa is the proximal segment and functional base of the leg. It articulates with thepleuronand 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 themeronwhich is usually the larger part of the coxa. A meron is well developed in Periplaneta, the Isoptera, Neuroptera and Lepidoptera.

Trochanter

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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

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Acanthacrisruficornis,legs saltatorial, femora withbipennate muscleattachments, 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 withsaltatoriallegs because the typical leaping mechanism is to straighten the joint between the femur and the tibia, and the femur contains the necessary massivebipennatemusculature.

Tibia

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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 theApocrita,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

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Robber fly (Asilidae), showing tarsomeres and pretarsi with ungues, pulvilli and empodia

The ancestral tarsus was a single segment and in the extantProtura,Dipluraand 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 thetaxon,which may be useful for diagnostic purposes. For example, thePterogeniidaecharacteristically have 5-segmented fore- and mid-tarsi, but 4-segmented hind tarsi, whereas theCerylonidaehave four tarsomeres on each tarsus.

The distal segment of the typical insect leg is the pretarsus. In theCollembola,Protura and many insect larvae, the pretarsus is a single claw. On the pretarsus most insects have a pair of claws (ungues,singularunguis). Between the ungues, a median unguitractor plate supports the pretarsus. The plate is attached to theapodemeof the flexor muscle of the ungues. In theNeoptera,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 allHeteroptera.[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, thearolium.

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 thesilk-producing glands.[14]

Under their pretarsi, members of theDipteragenerally 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 orempodium,meaning the meeting place of the pulvilli. On the underside of the tarsal segments, there frequently are pulvillus-like organs orplantulae.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 asDrosophila(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

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Bruchine with powerful femora used for escape from hard-shelled seed

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 mostcockroachesare good examples. However, there are many specialized adaptations, including:

  • The forelegs ofmole crickets(Gryllotalpidae) and some scarab beetle (Scarabaeidae) are adapted to burrowing in earth (fossorial).
  • Theraptorialforelegs of mantidflies (Mantispidae),mantises(Mantodea), and ambush bugs (Phymatinae) are adapted to seizing and holding prey in one way, while those ofwhirligig beetlesGyrinidae are long and adapted for grasping food or prey in quite a different way.
  • The forelegs of some butterflies, such as manyNymphalidae,are reduced so greatly that only two pairs of functional walking legs remain.
  • In most grasshoppers and crickets (Orthoptera), the hind legs aresaltatorial;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 ofbean weevils(Bruchinae) use their swollen hind legs for forcing their way out of the hard-shelled seeds of plants such asErythrinain which they grew to adulthood.
  • The legs of theOdonata,thedragonfliesanddamselflies,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.
Expression ofHox genesin thebody segmentsof different groups ofarthropod,as traced byevolutionary developmental biology.TheHox genes7, 8, and 9 correspond in these groups but are shifted (byheterochrony) by up to three segments. Segments with maxillopeds have Hox gene 7. Fossiltrilobitesprobably had three body regions, each with a unique combination of Hox genes.

Evolution and homology of arthropod legs

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The embryonic body segments (somites) of differentarthropodstaxa 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 comparinggenesinevolutionary 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 1stmaxillae 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

See also

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References

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  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.PMID11710766.S2CID25997101.
  4. ^Pat Willmer; Graham Stone; Ian Johnston (12 March 2009).Environmental Physiology of Animals.John Wiley & Sons. p. 329.ISBN978-1-4443-0922-5.
  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.Retrieved2012-01-14.
  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.PMID20696272.Retrieved20 August2020.
  7. ^abcdRichards, O. W.; Davies, R.G. (1977).Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology.Berlin: Springer.ISBN0-412-61390-5.
  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.PMID7953552.S2CID22980014.
  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.PMID11702198.S2CID1163446.
  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.PMID21237897.
  11. ^abFriedemann, 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.PMID34781597.S2CID86195785.Archived fromthe original(PDF)on 25 January 2014.Retrieved25 January2014.
  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.ISBN978-0-8014-2066-5.
  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. A2008; 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.ISSN0022-0949.
  17. ^Novartis Foundation; Hall, Brian (2008).Homology.John Wiley. p. 29.ISBN978-0-470-51566-2.
  18. ^Brusca, R.C.; Brusca, G.J. (1990).Invertebrates.Sinauer Associates. p.669.