It is almost certain that the distal of these two segments really belongs to the thigh, but the ordinary nomenclature will be used in the present article, as this character is of great importance in discriminating families, and the two segments in question are referred to the trochanter by most systematic writers.
The trochanter is simple in some genera and divided in others.
In most insects the leg is built up of nine segments: (1) a broad triangular, sub-globular, conical or cylindrical haunch (coxa); (2) a small trochanter; (3) an elongate stout thigh (femur); (4) a more slender shin (tibia); and (5-9) a foot consisting of five tarsal segments.
The vertebrae of the neck unite by nearly flat surfaces, the humerus has lost the foramen, or perforation, at the lower end, and the third trochanter to the femur may also be wanting.
In many families the trochanter appears to be represented by two small segments, there being thus an extra joint in the leg.
Hartig's character drawn from the trochanter - whether of two segments or undivided - the groups being termed respectively Ditrocha and Monotrocha.
The femur has a small third trochanter, the radius and ulna and tibia and fibula are respectively separate, at least in the young, and the fibula articulates with the astragalus.
In the hinder limb the femur is marked, as in other perissodactyles, by the presence of a " third trochanter," a flattened process, curving forwards and arising from the outer side of the bone, about one-third of the distance from the upper end.
It is remarkable that it should persist in the spectacled bear of the Andes, although it has disappeared in all other living members of the group. The third trochanter of the femur, on the other hand, can scarcely be regarded as primitive, seeing that it is absent in several of the lower groups of mammals.