At the lower surface of each thoracic segment is attached a pair of legs, the members of each pair being separated by the sternum of the segment to which they belong. The presence of three pairs of legs is a character by which insects can be distinguished from all other animals; indeed, on account of this feature, insects are sometimes called the hexapods, or six-legged animals. Each leg is covered by a continuation of the body cuticle, and is five-jointed; the first two joints at the attachment to the body are small; the next two are long, and form the greater part of the limb; while the fifth, or foot, consists of a varying number of small joints, the terminal one bearing a pair of claws. In the typical winged insects there are two pairs of wings: one pair attached to the middle thorax, and the other to the hind thorax; owing to the development of muscles controlling flight, the middle and hind thorax of winged insects are usually better developed than the fore thorax; this is especially noticeable in the thorax of two-winged flies (daddy-long-legs and blow-flies), where the hind wings are reduced to vestiges, the power of flight being thus confined to the middle thorax, which forms by far the greater portion of the whole thorax. Each wing, arising from the junction of the dorsal and lateral thoracic plates, is a bag-like extension of the cuticle, flattened leaf-like, so as to form a double flexible membrane. The wing membrane is supported by several ribs or veins, which may be very numerous (grasshopper) or few (aphid), while the fore edge, where it cuts the air in flight, is bordered by a stouter vein, ensuring rigidity. The fore and hind wings of some insects work independently, but in agreement of movement, while in others the fore and hind wings of each side are coupled along their adjoining margins, giving greater rigidity during flight. The abdomen of insects consists of a varying number of visible segments; each segment is covered by an upper and lower chitinous plate connected by membrane, there being no side plates as are found in the thorax. There are no organs of locomotion (except in a very few cases), the only appendages being those connected with reproduction; the latter are well developed in the female weta, where the egg-laying apparatus, or ovipositor, projects blade-like from the apex of the abdomen. In very many insects, however, the external reproductive organs are not readily seen without special study.