Beetles and Weevils: Coleoptera

Beetles are the largest order in the animal kingdom, with approximately 350,000 species known worldwide. Beetles come in a wide variety of body shapes and sizes. They range in length from 0.02 to 6.7 inches (0.55 to 170 millimeters) and are long or round, cylinder-shaped or flat, slender or heavy-bodied. Their bodies are usually very hard and rigid, but some groups, such as fireflies, soldier beetles, and net-winged beetles, typically have bodies that are soft and flexible. Although many species of beetles are plain black or brown, others have amazing metallic or shimmering colors and patterns. These colors are created by chemicals inside their exoskeletons, or hard outer coverings, or by the physical structure of the surface of the exoskeleton itself. Additional structures on the surface of the exoskeleton also influence color, such as surface texture, waxy coatings, scales, and other hairlike coverings. The colors and coatings of beetles help them to recognize one another and to protect themselves from extreme temperatures, water loss, and predators (PREH-duh-ters) that hunt them for food.

The head has chewing mouthparts that are directed forward or downward. Their jaws are used for cutting or grinding plant and animal tissues or for straining small particles from liquids. However, wrinkled beetles are unable to use their jaws for chewing. Instead, they use their other finger-like mouthparts to handle their food. The mouthparts of some net-winged beetles and weevils are place on the tip of a snout, which makes it easier for them to feed on flowers and seeds.

Beetle antennae (an-TEH-nee), or sense organs, are equipped with very sensitive receptors that help them find food, water, mates, and egg-laying sites. Antennae also help them to determine temperature or detect approaching enemies by sensing changes in air currents. Some ground beetles and rove beetles have special structures on their front legs that are used regularly to clean the antennae to maintain their sensitivity. The antennae are usually shorter than the body but are often much longer in some longhorns and brentid weevils. Males may have elaborate antennal structures that increase the surface area for special chemical receptors that are sensitive to pheromones (FEH-re-moans) released by females of the same species. Pheromones are very special airborne chemicals that help potential mates find one another, sometimes over large distances. The antennae are threadlike, beadlike, elbowed or bent, sawtoothed, comblike, feathery, swollen at the tips or clubshaped, or have some segments flat or platelike.

The compound eyes, or eyes with multiple lenses, are usually well developed and may be partially or completely divided into two parts. For example, the eyes of whirligigs are completely divided. They live on the surface of water. The upper portions of the eyes are used for seeing in the air, while the portions in the water are specialized for seeing under water. Simple eyes, or eyes with only a single lens, are rarely found in beetles but are present in most hide beetles and some rove beetles, as well as a few other groups.

Some beetles, usually males, have horns on their heads, jaws, or the first section of their thorax, or midsection. In the male Hercules beetle, the head and thoracic horns work together to form a powerful pinching device. Other beetles have horns on their legs or wing covers. The size of horns varies within the same species and is influenced by adult body size and conditions that influence larval development. These include diet, temperatures, and moisture. Heredity (hih-REH-dih-ti), the physical traits passed from parent to offspring through genes, also plays an important role in horn development.

Most beetles have two pairs of wings. The hard or leathery forewings are called elytra (el-EE-tra). The elytra cover the last two segments of the thorax, the second pair of wings, and some or all of the abdomen. The elytra not only protect beetles from predators and parasites (PAE-rih-saits), or animals that live on another organism from which they obtain food; the elytra also keep beetles from getting scuffed up as they burrow through the soil and rotten wood. The air space between the elytra and the abdomen in desert species helps to insulate their body from sudden changes in temperature. This same space also gives some aquatic beetles a place to store air so they can breathe under water. The second pair of wings are also called flight wings. They do most of the work in flying beetles, while the elytra are used to help them keep their balance. Desert-dwelling darkling beetles and weevils are often unable to fly because their elytra are permanently joined together and they lack fully developed flight wings. However, this arrangement helps them to conserve moisture.

The legs of beetles are used for burrowing, swimming, crawling, or running. Males of the harlequin beetle, the long-armed chafer, and several large weevils have very long front legs that are probably used for mating defense. Desert darkling beetles living in the Kalahari Desert have long, thin legs that lift them as high as possible above the hot sands. Aquatic beetles have flat and fringed legs and use them like paddles as they swim through water. Their feet are tipped with one or two claws and are sometimes equipped with sticky or brushy pads that help them walk on plants and other surfaces. Some male diving beetles have front feet that work like suction cups to help them hold on to a female as they mate underwater.

The abdomen is usually hidden underneath the elytra, but in rove, clown, and many sap beetles, the elytra are short, leaving some or most of the abdomen exposed. The tip of the abdomen sometimes has small structures that are for egg-laying and mating. The tip of the abdomen is very flexible in some beetles. For example, whirligig beetles use their abdomen to help steer themselves on the surface of the water. In ground beetles the flexible abdominal tip is used as a defensive weapon to aim harmful chemicals at their attackers.

Beetle larvae (LAR-vee), or the young of an animal that must go through certain changes in form before becoming adults, usually do not look like the adults. They are grublike with short legs or wormlike with legs reduced in size or entirely absent. Some predatory species are flat with long legs. The distinct head has chewing mouthparts that are adapted for crushing, grinding, or tearing food. Predatory species may have sucking mouthparts for drinking liquified tissues of their prey. The antennae are small and sometimes difficult to see. They have zero to six simple eyes on either side of the head. The thorax has three distinct sections. The first section may be covered with a broad, dark plate. The legs have six or fewer segments, if they are present at all. The soft abdomen is usually divided into ten segments, but some species have only eight or nine. It is sometimes covered with wartlike or other fleshy structures that help give them traction while moving through soil or rotten wood. The next to last segment may have a pair of projections that are sometimes pincherlike.

The features of adult beetles are clearly visible in the pupae, or insects at the life stage between larvae and adults. Some pupae have their legs and developing wings tightly pressed against the body and cannot move them. In other species these structures are not firmly attached to the body and are flexible and capable of limited movement. In some species the abdomen is capable of some movement. The seams between the movable abdominal segments may have small pinchers on both sides called "gin-traps." The pupa snaps these pinchers together to protect itself from small predators and parasitic mites.