Arthropod leg

Form of jointed appendage of arthropodsThe 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, : coxae), trochanter, femur (: femora), tibia (: tibiae), tarsus (: tarsi), ischium (: ischia), metatarsus, carpus, dactylus (meaning finger), patella (: 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 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 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.

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. 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.

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. 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.

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.

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 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. Current evidence suggests that they are indeed homologous up to a very primitive stage in their embryological development, but that their emergence in modern insects was not homologous between the Lepidoptera and Symphyta. Such concepts are pervasive in current interpretations of phylogeny. 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.

Fossorial legs are modified for digging and burrowing. For example: mole crickets have fossorial forelegs
Saltatorial legs are modified for jumping and leaping. For example: many orthopterans have saltatory hindlegs
Natatorial legs are modified for swimming. For example: whirligig beetles have natatorial mesothoracic and metathoracic legs
Cursorial legs are modified for running. For example: lots of cockroaches have very sensitive cursorial legs
Raptorial legs are modified for grasping, usually a hunting adaptation. For example: mantids use their raptorial forelegs to hunt