Everything about Uniramous totally explained
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.
Biramous and uniramous
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 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.
Hexapoda
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 (
for example Cecidomyiidae). Tarsal claws, also called
ungues (singular:
unguis), may also be present .
The segmentation is similar in
collembolans, in which each leg has a coxa, trochanter, femur, tibia, and a foot-complex.
In addition there's 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's located between the claws, as in the case of
Orthoptera (
grasshoppers and
crickets), or
pulvillus if it's 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.
Insects' legs are attached to the three thoracic segments, the prothorax, mesothorax and metathorax. This terminology is also sometimes applied to the leg segments, so that the mesofemur is the femur of the second pair of legs, and the protarsus is the tarsus of the first pair of legs.
Millipedes,
centipedes and their relatives have seven-segmented legs, comprising coxa, trochanter, prefemur, femur, tibia, tarsus, and a tarsal claw.
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