The mouth varies very considerably in different insects, but is often supplied with a pair of mandibles or biting jaws, and, below them, a pair of maxillæ or chewing jaws, both pairs being jointed to the head in such a manner as to be capable only of horizontal movements. Above and below these jaws are, respectively, the upper lip or labrum, and the lower lip or labium, the latter having appended to it a pair of jointed feelers called the labial palpi, and an additional pair of palpi are also frequently attached to the maxillæ, and therefore called the maxillary palpi. These organs of the mouth of an insect are modified in various ways according to the functions they are called upon to perform. Thus, in bees, the upper lip, as well as the mandibles, are adapted for chewing, while the maxillæ and the labium are grooved in such a manner that when brought together they form a tube through which fluids may be sucked into the mouth. Also, in the butterfly and the moth, the maxillæ are not constructed for chewing, but consist of two channelled rods which, when approximated, form a long tube or proboscis employed for suction; and in these insects the labial palps are large for the protection of the proboscis, which is retracted and closely coiled between them when not in use. Further, in the bugs, the labium is long and tubular, while the mandibles and maxillæ are often modified into sharp, stiff bristles that work within the tube, the whole thus forming a combined piercing and sucking arrangement. [Illustration: 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. [Illustration: FIG. 221.--TRACHEA OF AN INSECT, MAGNIFIED]