Fig. 220.—The Leg of an Insect
The leg of an insect is built up much in the same manner as that of the typical crustacean. It consists of a basal hip joint or coxa, a ring segment or trochanter, a thigh (femur), a shin (tibia), and the tarsus or foot of several joints which terminates in a claw or claws, and is often provided with sucking-pads. The wings, when present, are attached to the second and third segments of the thorax, if two pairs, but if, as in the case of the house fly, the insect has only one pair of wings, these are always appended to the second segment.
Insects are developed from eggs, but in their young state they are segmented larvæ, with strong jaws, antennæ, simple eyes, and usually three pairs of legs attached to the first three segments next to the head.
As regards internal structure, we need only mention here that the body is traversed by numerous branching tubes (tracheæ) that open at the exterior and constitute the respiratory apparatus; that the insect is provided with a contractile, tubular heart by means of which the blood is propelled through a system of blood-vessels; that the nervous system consists of a chain of ganglia, connected by a nerve cord, sending nerve filaments to all parts of the body; and that the digestive tube is often a complicated structure, especially in the case of those insects that feed on herbivorous matter.
Fig. 221.—Trachea of an Insect, magnified
The above outline will be sufficient to show that insects are not very unlike the crustaceans in their general characteristics; and, indeed, when we examine certain forms, noting the distinct segmentation of the body, the hardened exo-skeleton of chitinous material, and the unhardened skin between the segments to admit of freedom of movement, we see a striking resemblance in external appearance to some of the typical crustaceans.