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The arthropod leg is a form of jointed appendage of , 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 Neontology 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 Homeobox, 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 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 ( and ), and they are completely absent in insects.
The legs of and myriapoda 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, 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.
The tarsus of spiders has 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 , but with the addition of a pre-tarsus beyond the tarsus. The claws of the scorpion are not truly legs, but are , a different kind of appendage that is also found in spiders and is specialised for predation and mating.
In Horseshoe crab, there are no metatarsi or pretarsi, leaving six segments per leg.
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 Cercus, all of which are derived from paired legs on each segment of some common ancestor.
Some insects do however have extra walking legs on their abdominal segments; these extra legs are called . 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. (1977). 9780412613906, Springer. ISBN 9780412613906 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 Sawfly. 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 Diptera or grubs of Curculionidae. 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, 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 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 are called "crawlers" and they crawl around looking for a good place to feed, where they settle down and stay for life. Their later have no functional legs in most species. Among the Apterygota, the legs of immature specimens are in effect smaller versions of the adult legs.
Associated with the leg itself there are various around its base. Their functions are and have to do with how the leg attaches to the main exoskeleton of the insect. Such sclerites differ considerably between unrelated insects.
The distal segment of the typical insect leg is the pretarsus. In the Springtail, 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 Exoskeleton 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. It is present in many Hemiptera and almost all Heteroptera. Usually, the parempodia are bristly (setiform), but in a few species they are fleshy. (1995). 9780801420665, Cornell University Press. ISBN 9780801420665 Sometimes the parempodia are reduced in size so as to almost disappear. Above the unguitractor plate, the pretarsus expands forward into a median lobe, the arolium.
Webspinners (Embioptera) have an enlarged basal tarsomere on each of the front legs, containing the silk-producing glands.
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.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
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), (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.
| 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 |
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