Beetles are typical insects in that the head, thorax, and abdomen are very well defined, and can be readily distinguished from each other; the insect is more or less protected with a stout horny integument. But the joints are flexible, so that though the parts fit close and the body appears ensheathed in regular armour plate, most of the species are very active. They are all furnished with cutting, biting, or chewing jaws, and are therefore called mandibulate insects; and with very few exceptions have well developed eyes and antennae, the latter produced into all kinds of curious shapes in some groups, but usually slender, filiform and many jointed. The thorax consists of one solid segment, the three portions, so apparent in some insects, being soldered together to form one uniform mass when viewed from above. The large abdomen is said to contain ten distinct segments on dissection, but when viewed from the under-surface generally only five can be seen. Instead of the thin flying, or membranous fore-wings of other insects, the first pair in the beetles are transformed into two horny plates completely covering the dorsal surface of the abdomen and called the elytra. When at rest they fit close together over the back, but can be readily opened out in flight. Though of little use in flying, they probably assist a large heavy beetle in balancing or steering through the air, and always cover the two large pointed membranous hind flying wings, which when not in use are folded up beneath them. In some beetles the elytra are not divided, but form a solid shield; and the hind wings are wanting, or if they exist are simple pads. The various families have the head, mouth parts, and legs admirably adapted to their different habits and diet. Their larvae are also as variable in form as the perfect insects; many are active, slender grubs with three pairs of legs, and large powerful jaws, as in the carnivorous species; elongate cylindrical jointed creatures with scaly heads, or short and wrinkled grubs like the wood borers; others quite slug-like feed upon the surface of the foliage; and a few are clothed with fine hairs. [Illustration: =Fig. 54.=--Diagram of a Water-Beetle, showing the Dorsal surface. 1, Labrum; 2, clypeus; 3, head; 4, prothorax; 5, maxillary palpus; 6, antennae; 7, eyes; 8, elytron; 9, wings; 10, scutellum; 11, abdominal segments; 12, scutellum of the metathorax; 13, claws of the feet or the fore leg; 14, tarsus; 15, tibia; 16, femur; 17, middle leg; 18, spines or spurs on tibia; 19, tarsus; 20, hind leg. (Redrawn from Westwood [Griffiths’ Animal Kingdom].)]