 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Arthropod leg" – news ·newspapers ·books ·scholar ·JSTOR(March 2025) (Learn how and when to remove this message) |
Thearthropod leg is a form of jointedappendage ofarthropods, usually used forwalking. Many of the terms used for arthropod leg segments (calledpodomeres) are ofLatin origin, 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).
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 themost recent common ancestor ofextant 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 successive loss of function of aHox-gene, could result inparallel gains of leg segments.
In arthropods, each of the leg segments articulates with the next segment in ahinge 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 rotationalball-and-socket joints at the base of the fore and hind limbs.[4]
The appendages of arthropods may be eitherbiramous oruniramous. 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 ofcrustaceans is known as theexopod orexopodite, while the internal branch is known as theendopod orendopodite. 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 (amphipods andisopods), and they are completely absent in insects.[5]
The legs ofinsects andmyriapods 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 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.[citation needed]
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 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 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 ofappendage that is also found in spiders and is specialised for predation and mating.
InLimulus, there are no metatarsi or pretarsi, leaving six segments per leg.
The legs ofcrustaceans 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.[citation needed]
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 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 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,antennae andcerci, all of which are derived from paired legs on each segment of somecommon ancestor.
Somelarval insects 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 theLepidoptera andSymphyta.[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 offlies or grubs ofweevils. In contrast, the larvae of otherColeoptera, such as theScarabaeidae andDytiscidae have thoracic legs, but no prolegs. Some insects that exhibithypermetamorphosis begin 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 immatureEphemeroptera 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 theCoccoidea are called "crawlers" and they crawl around looking for a good place to feed, where they settle down and stay for life. Their laterinstars have no functional legs in most species.Among theApterygota, the legs of immature specimens are in effect smaller versions of the adult legs.[citation needed]
A representative insect leg, such as that of ahousefly orcockroach, has the following parts, in sequence from mostproximal to mostdistal:
Associated with the leg itself there are varioussclerites around its base. Their functions arearticular and have to do with how the leg attaches to the main exoskeleton of the insect. Such sclerites differ considerably between unrelated insects.[7]
The coxa is the proximal segment and functional base of the leg. It articulates with thepleuron and 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 themeron which is usually the larger part of the coxa. A meron is well developed in Periplaneta, the Isoptera, Neuroptera and Lepidoptera.[citation needed]
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.[citation needed] In parasitic Hymenoptera, the base of the femur has the appearance of a second trochanter.[citation needed]
In most insects, the femur is the largest region of the leg; it is especially conspicuous in many insects withsaltatorial legs because the typical leaping mechanism is to straighten the joint between the femur and the tibia, and the femur contains the necessary massivebipennate musculature.[citation needed]
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.[citation needed]
The ancestral tarsus was a single segment and in the extantProtura,Diplura and 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, thePterogeniidae characteristically have 5-segmented fore- and mid-tarsi, but 4-segmented hind tarsi, whereas theCerylonidae have 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 theapodeme of 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.
Webspinners (Embioptera) have an enlarged basal tarsomere on each of the front legs, containing thesilk-producing glands.[14]
Under their pretarsi, members of theDiptera 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 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]
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 mostcockroaches are good examples. However, there are many specialized adaptations, including:
The embryonic body segments (somites) of differentarthropods taxa 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 comparinggenes inevolutionary developmental biology.[18]
| Somite (body segment) | Trilobite (Trilobitomorpha)  | Spider (Chelicerata)  | Centipede (Myriapoda) .png) | 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 |
