_Head._—Almost all insects have in the adult state a compound eye, _i.e._ an eye made up of a large number (up to ten thousand) of smaller eyes. In many insects one finds in addition to this a few simple eyes on the top of the head. The feelers are very unlike in different insects; they serve as organs of touch, and perhaps also help some other sense. The mouth-parts consist of three pairs of jaws, of which the first (the mandibles) and the second pair (maxillæ) are freely movable from side to side, while the third consists of two jaws immovably fused together (lower lip, or labium). A downward projection of the skin of the head (the upper lip, or labrum) overhangs the three pairs of jaws (Fig. 58). In insects which feed on solid matters, tearing or chewing them, the jaws are short and sharp-edged (biting mouth-parts); in those which take up fluid food (blood, the juices of plants, etc.) they are elongated, and adapted for licking, sucking, or piercing. [Illustration: FIG. 58.—Head and Mouth-parts of a Ground Beetle, enlarged four times. A, from above; B, from below. _a_, labrum; _b_, mandible; _f_, maxilla with palpi (_g_); _c_, labium with palp (_e_). ] _Thorax._—The names of the parts of the legs are for the most part those used in the case of mammalian limbs; though the similarity between the limbs of Mammals and Insects is quite superficial. The following parts are distinguished in the leg of an insect: (1) the generally spherical hip (_coxa_) (Fig. 59, _a_); (2) the very short _trochanter_ (_b_); (3) the elongated thigh (_femur_) (_c_); (4) the shank (_tibia_) provided with movable spines at its tip (_d_); (5) the three- to five-jointed foot (_tarsus_) (_e_), whose last joint ends in claws, and often also in flap-like outgrowths. The _Wings_ are expansions of the skin, which consist of two layers. There are tracheæ between the upper and lower lamellæ. At first (in the pupa) the wings are folded up; but the forcing of air into the tracheæ quickly causes them to unfold. A firm substance is now deposited in the larger tracheæ, which are thus converted into veins or ribs giving support to the wings. In beetles the fore wings are quite hard and horny, and serve for protecting the delicate hind wings and soft back rather than for flight; they are therefore named “wing-covers” (_elytra_). In many insects the fore wings (wasps) or the hind wings (beetles, grasshoppers) are folded when at rest. [Illustration: FIG. 59.—Leg of a Ground Beetle (× 2). ] [Illustration: FIG. 60.—Silkworm Moth (_Bombyx mori_). Female moth, caterpillar, pupa and cocoon. ]