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.
All insects, from the largest to the most minute, contain internally a well-formed heart and a digestive, reproductive, respiratory, and nervous system ([Fig. 3]), while the spaces surrounding these organs are, for the most part, packed with a complex system of muscles. The heart is a delicate tube lying along the middle of the back or dorsal surface of the body, immediately under the skin, and extends almost from one end of the insect to the other; in an almost similar position, close to the lower or ventral surface of the body, the nervous system is situated, and consists of a chain of nerve centres, or ganglia, connected by a double nerve cord, the most anterior of these ganglia being in the head and forming the brain, the following three lying in the thorax, one to each segment, while the remainder are confined to the abdomen, one ganglion to each segment, as in the thorax. In many insects the number of nerve centres is reduced, owing to the fusion of two or more. The reproductive organs are located in the abdomen.
The digestive system consists of a tube ([Fig. 3]), with its appendages, opening at the mouth and at the posterior end of the body; this alimentary canal may be straight and simple, or convoluted and complex, according to the insect and the nature of its food. Respiration in insects is carried on by means of a system of air tubes ([Fig. 3]), which branch and re-branch to form an intricate system of delicate tubular airways, carrying the atmosphere to all tissues of the body; the main air tubes open at the surface by a series of breathing pores normally arranged along each side of the body, except on the head; these pores are best seen on a caterpillar or on the abdomen of adult insects.