Arthropod leg

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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 are of Latin origin, and may be confused with terms for bones: coxa (meaning hip), trochanter (compare greater trochanter and lesser trochanter), femur, tibia, 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 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 the successive loss of function of a Hox-gene could result in parallel gains of leg segments.

Biramous and uniramous

File:CrustaceaAppendages.png
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 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.

Hexapoda

File:Insekt-bein.gif
Diagram of an insect leg

The legs of insects 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 level, for example beetles (Coleoptera), or to subfamily level among some dipteran flies (e.g. Cecidomyiidae). Tarsal claws, also called ungues (singular: unguis), may also be present [4][5].

The segmentation is similar in collembolans, 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 (grasshoppers and crickets), or pulvillus if it is located at the base of the claws, as in Diptera (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.


Chelicerata

File:Spiderlegdiagram.png
Diagram of a spider, with the leg segments colour-coded: coxa=grey, trochanter=red, femur=green, patella=blue, tibia=purple, metatarsus=brown, tarsus=cyan

Spiders' legs differ from those of insects by the addition of two segments, 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 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.

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

Crustacea

File:Crustaceanleg.jpg
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 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

Millipedes, centipedes and their relatives have seven-segmented legs, comprising coxa, trochanter, prefemur, femur, tibia, tarsus, and a tarsal claw.

References

  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 - Revue Canadienne de Zoologie. 70 (2): 236–255. 
  2. Fryer, G. (1996). "Reflections on arthropod evolution". Biol. J. Linn. Soc. B. 58 (1): 1–55. 
  3. Schram, F. R. & S. Koenemann (2001). "Developmental genetics and arthropod evolution: part I, on legs". Evolution & Development. 3 (5): 343–354. 
  4. Harbach, R. E. & K. L. Knight (1980). Taxonomist's Glossary of Mosquito Anatomy. Plexus, Marlton, NJ. 
  5. Richards, O. W. & R. G. Davies (1977). Imm's General Textbook of Entomology, 10th ed. (2 Volumes). Chapman and Hall, London. 
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