Mouth masticatory, furnished with an upper lip or labrwn, two mandibles, two maxillae, with maxillary palpi (generally four-jointed), and a movable lower lip or labium, with two jointed labial palpi. The four wings are usually present, and the anterior pair are not adapted for flight, but are hardened by chitine, so as to form protective cases (elytra) for the posterior wings (fig. 194). The inner margins of the elytra are generally straight, and when in contact they form a longitudinal suture. The posterior wings are membranous, and when not in use are folded transversely beneath the elytra. (Amongst deviations from this state of parts may be mentioned the occasional absence or rudimentary condition of the hinder wings, the soldering together of the elytra, the soft and yielding condition of the elytra, or the absence of both elytra and wings.) The eyes are always compound, generally circular, oval, or reniform, but sometimes completely divided. The antennae are extremely variable in form, generally of eleven joints, sometimes of fewer, rarely of twelve or more. The thorax is composed of a pro- meso- and meta-thorax, but when the elytra are closed, only the prothorax and a little plate ("scutellum") belonging to the mesothorax are visible.
Fig. 194. - Coleoptera. Common Cockchafer (Melolontha vulgaris).
Fig. 195. - a Rose-chafer (Cetonia aurata) and larva.
The tarsus is generally composed of five joints, sometimes fewer, never more, and its last joint is usually furnished with two hooked claws.
The larvae of Coleoptera are generally composed of thirteen segments, including the head. The body is generally soft and fleshy, the head horny, and the mouth adapted for mastication, the food being sometimes of an animal and sometimes of a vegetable nature. The antennae are small, usually of three or four joints, with ocelli at their base. They have three pairs of legs attached to the thorax, and rarely anal pro-legs or fleshy tubercles; or they may be devoid of feet (as in the weevils). The pupa is sometimes enclosed in a cocoon, and is always quiescent; and the parts of the perfect insect are always distinctly recognisable in the pupa.
The order Coleoptera includes all those insects commonly known as "Beetles," and comprises an enormous number of genera and species. They are remarkable, as a general rule, for their hard polished integument, their glittering, often metallic colours, and their voracious habits.
The order Coleoptera was divided by Latreille into four sections, in accordance with the number of the joints in the tarsi; and though the resulting arrangement is not strictly natural, this classification is generally followed. The four sections founded by Latreille are :