aridus'' showing full leg anatomy, including plantulae under each tarsomere 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.
Fundamental morphology of insect legs A representative insect leg, such as that of a
housefly or
cockroach, has the following parts, in sequence from most
proximal to most
distal: coxa, trochanter, femur, tibia, tarsus, and pretarsus. Associated with the leg itself there are various
sclerites around its base. Their functions are
articular and have to do with how the leg attaches to the main exoskeleton of the insect. Such sclerites differ considerably between unrelated insects. It is present in many Hemiptera and almost all
Heteroptera. 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. ,
Embia major, front leg showing enlarged tarsomere, which contains the silk-spinning organs 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. 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),
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.
Variations in functional anatomy of insect legs 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 most
cockroaches are good examples. However, there are many specialized adaptations, including: • The forelegs of
mole crickets (Gryllotalpidae) and some scarab beetle (
Scarabaeidae) are adapted to burrowing in earth (
fossorial). • The
raptorial forelegs of mantidflies (
Mantispidae),
mantises (Mantodea), damsel bugs (
Nabidae) and ambush bugs (
Phymatinae) are adapted to seizing and holding prey in one way, while those of
whirligig beetles Gyrinidae are long and adapted for grasping food or prey in quite a different way. • The forelegs of some butterflies, such as many
Nymphalidae, are reduced so greatly that only two pairs of functional walking legs remain. • In most grasshoppers and crickets (
Orthoptera), the hind legs are
saltatorial; they have heavily bipinnately muscled femora and straight, long tibiae adapted to leaping and to some extent to defence by kicking.
Flea beetles (Alticini) also have powerful hind femora that enable them to leap spectacularly. • Other beetles with spectacularly muscular hind femora may not be saltatorial at all, but very clumsy; for example, particular species of
bean weevils (Bruchinae) use their swollen hind legs for forcing their way out of the hard-shelled seeds of plants such as
Erythrina in which they grew to adulthood. • The legs of the
Odonata, the
dragonflies and
damselflies, are adapted for seizing prey that the insects feed on while flying or while sitting still on a plant; they are nearly incapable of using them for walking. • The majority of aquatic insects use their legs only for swimming (natatorial), though many species of immature insects swim by other means such as by wriggling, undulating, or expelling water in jets. s in the
body segments of different groups of
arthropod, as traced by
evolutionary developmental biology. The
Hox genes 7, 8, and 9 correspond in these groups but are shifted (by
heterochrony) by up to three segments. Segments with maxillopeds have Hox gene 7. Fossil
trilobites probably had three body regions, each with a unique combination of Hox genes. ==Evolution and homology of arthropod legs==