Beetles

As insects, beetles share common traits with all other arthropods. The legs are jointed, and there is an external skeleton called the exoskeleton, an inert compound made mostly of a carbohydrate called chitin (polyacetylglucosamine). Those sections of the exoskeleton that do not need to be flexible to allow for movement are further strengthened by sclerotin, a hard, proteinaceous substance similar in composition to human fingernails. The exoskeleton serves in both protection and in muscle attachment. A superficial layer of wax secreted on the outside of the exoskeleton prevents water loss through evaporation.

Beetles share with all insects the body form that differentiates them from other arthropods. The body of insects is divided into three main sections: head, thorax and abdomen. In Coleoptera, however, two of the three segments of the thorax (mesothorax and metathorax) are attached to the abdomen, while the third one (prothorax) is isolated between the head and trunk and is covered by a dorsal plate called the pronotum. The insect thorax usually has three pairs of legs and two pairs of wings. This body section also contains the powerful muscles that operate both the wings and legs. The abdomen has nine or ten segments, some not externally visible, each bearing a pair of spiracles, or respiratory openings, which direct air through the exoskeleton into the body.

Beetles can fly from hostile environments, escape enemies, and seek mates over wide areas. The first pair of wings of beetles, which arise from the mesothorax, is modified as the elytra-forming the protective cover for the hind wings and abdomen. This is a particular advantage for these insects, because they spend so much time on the ground rummaging through decaying plant matter, wood, and soil. The hind wings are membranous and usually fold beneath the elytra—when not in use. When the beetle flies, the elytra are held open at an angle, providing additional stability and lift as the back wings beat.

Beetles have three pairs of legs that are usually well-developed, with a strong femur and tibia, and five or fewer tarsal (end) segments tipped with a paired claw. The front pair of legs arises from cavities under the pronotum, with a spiracle positioned just to the rear of the base of each of the front legs. The mesothorax bears the second pair of legs, while the third pair of legs arises from the metathorax.

The legs of beetles may be modified for running, swimming, jumping, digging, or clasping, depending on the species. For example, the hind legs of some species of water beetles are long, flattened, and covered with long, matted hairs that serve as paddles for swimming. The water strider has slender legs, which, together with a lightweight body covered with tiny hairs that buoy it up, permit it to skitter over the surface of the water.

The head bears a pair of compound eyes, a pair of antennae (usually with 11 segments), and the mouthparts. The eyes consist of many tiny individual units (facets), which together resemble a honeycomb. Under each facet is a group of six or seven retinal cells surrounding a rodlike light-receptive zone (rhabdom). Each of these tiny, individual "eyes" has its own nerve, which together with the nerves of the other eyes, form the optic nerve.

The beetle eye, like that of other insects, does not move, and its lenses cannot focus. Instead, each individual eye contributes a tiny bit of the image; these combine to form a crude mosaic of the scene rather than a clear, continuous picture. In addition, insects can't close their eyes, and can see well only to a distance of a few feet (about 90 cm). The whirligig beetle, which is found on the surface of bodies of water, has eyes divided into an upper part, with which the insect observes the surface environment, and a lower part, for underwater viewing.

The antennae (feelers) are sense organs that gather information about the touch, sound, taste, smell, temperature and humidity of the beetle's environment. The maxillae hold a pair of lobed sense organs, called palps, which may detect smells. The beetle's mouth is a simple hole that lacks jaws, but is surrounded by specialized structures for grasping and grinding. Behind the upper "lip," or labrum, a pair of jawlike appendages (called mandibles) serves as pincers. Behind the mandibles are a pair of bladelike appendages (called maxillae), followed by a second pair of maxillae that are fused in the midline to form the lower lip, or labium.

While most beetles have mouth parts designed for chewing solid food, many of the beetles of the superfamily Curculionoidea have a distinct snout that can bore into wood and suck sap. The snout has mouthparts at its end and is used for penetration and feeding, and for boring holes for egg-laying. These beetles are mostly plant feeders and are economically important pests of crops. For example, the 30,000 species of weevils in the family Curculionidae include many insect pests, such as the cotton boll weevil, the apple blossom weevil, and the rice weevil. The Curculionidae are also called true weevils, or snout weevils.

The chewed food is passed into the mouth (which secretes the digestive enzyme amylase), then into the muscular pharynx, and then to the esophagus. From there food enters the midgut, where digestive enzymes break it down further. Attached to the end of the midgut are the malpighian tubules, the insect's kidney-like organs of excretion that empty into the hindgut (located just past the midgut). The hindgut is followed by the rectum, which ends in the anus. Digested food enters the hemocoele, or body cavity, and is transported to the organs by means of the circulatory fluid, or hemolymph.

Beetles have an open circulatory system, that is, they lack an extensive system of arteries and veins and their hemolymph bathes their tissues directly. A tube-like "heart" in the abdomen pumps the hemolymph forward through a dorsal tube ("aorta") in the thorax to the head. Tiny pumps send the hemolymph to the wings, antennae and legs, after which the fluid flows back passively to the heart in the abdomen. The hemolymph transports nutrients throughout the body, and carries waste products from the organs to the malpighian tubules. Free cells called hemocytes travel in the hemolymph and serve to devour foreign microorganisms. Unlike the blood of the vertebrates, the hemolymph is not involved in oxygen transport; that function is performed by the spiracles.