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 , but modern arthropods have eight or fewer. It has been argued  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.

Hexapoda

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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).

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.

Chelicerata

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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

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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 (branching), whereas all other arthropods have uniramous (unbranched) limbs.

Myriapoda

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

References

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Animal anatomy | Arthropods | Locomotion | Bein (insekt)