What Is a Beetle?

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Photo by Anthony Davies, © Her Majesty the Queen in Right of Canada
The study of a superdiverse group of organisms such as beetles requires the close examination of a wide range of anatomical structures in order to recognize and classify species and larger groups correctly.

Humans may think they rule the world, but it’s beetles that really dominate it. In The Book of Beetles, editor Patrice Bouchard goes in-depth about these creatures that make up a quarter of the world’s animals. With 400,000 known species, these insects are about as abundant as they are interesting. This excerpt, which discusses defining characteristics of beetles, is from the section “What is a Beetle?”

The English word “beetle” comes from the Middle English bityl or betyll and the Old English bitula, all of which mean “little biter.” Other commonly used names, such as “weevil” and “chafer” derived from Old English and Old High German, also relate to biting. Coleoptera, first coined by Aristotle in the fourth century bce and later adopted as an order of insects by Carl Linnaeus in 1758, is derived from the Greek words koleos, meaning “sheath,” and pteron, or “winged,” and was inspired by the tough elytra of beetles.

Distinguishing Features of Beetles

Among other adaptations, beetles are distinguished from other insects by their chewing mouthparts, the conversion of their forewings into hardened elytra, their hind wings that fold lengthwise and across beneath the elytra, and their holometabolous development. Holometabolous insects pass through four very distinct life stages: egg, larva, pupa, and adult. The larvae and adults frequently differ in habits and habitat, functioning in the environment as if they were two separate species.

Beetles, like other insects, crustaceans, arachnids, millipedes, centipedes, and their kin with segmented exoskeletons and jointed appendages (antennae, mouthparts, legs), are classified in the phylum Arthropoda. Light and durable, the beetle exoskeleton is incredibly tough and rigid or characteristically soft and pliable, and provides protection and support. It serves as a platform for important tactile and chemosensory structures externally, while providing an internal framework that supports muscles and organs. The exoskeletal surface is smooth and shiny, or dulled by waxy secretions or microscopic networks of cracks (alutaceous) resembling that of human skin. The surface is variously festooned with spines, hairlike setae, or flattened setae called scales, and sculpted with tiny bumps (tubercles), pit-like punctures, ridges, grooves (striae), or rows of punctures.

Colors of Beetles

The colors of beetles are derived either from chemical pigments obtained from their food or structural properties of the outer layers of the exoskeleton. Most beetles are black as a result of melanin deposition during sclerotization, the chemical hardening process of the exoskeleton that occurs after emergence from the pupa, or eclosion. Microscopic surface sculpturing also influences beetle colors, as do patterns of setae, scales, or waxy secretions. Black desert darkling beetles (Tenebrionidae) are sometimes partially or completely covered with a white, yellow, or bluish-gray waxy bloom that reflects light and helps to keep the beetle cool.

The brilliant iridescent and metallic colors of beetles are created by multiple reflective layers in the exoskelton and scales, or a layer of highly complex photonic crystals that reflect light at different wavelengths to create specific metallic colors and shimmering iridescence. These structures are determined genetically, but their final form in individual beetles is determined by conditions experienced during growth and development.

The Structure of Beetles

As with other insects, the beetle body is divided into three distinct regions: head, thorax, and abdomen.

The Head

The hardened head capsule is attached to the thorax by a flexible and membranous neck, and is clearly visible from above or partially withdrawn inside the thorax. It bears chewing mouthparts that typically consist of an upper lip (labrum), two sets of jaws (mandibles, maxillae), and a lower lip (labium). The usually conspicuous mandibles are modified to cut, grind, or strain various foodstuffs. The maxilla and labium may possess delicate fingerlike structures, or palps, that help manipulate food. Beetle mouthparts are directed forward, or prognathous (e.g., whirligig beetles [Gyrinidae], ground beetles [Carabidae]), or downward (hypognathous), as in leaf beetles (Chrysomelidae) and weevils. The mouthparts of some beetles, especially weevils and their relatives (Curculionoidea), are borne at the very tip of a snout-like rostrum, an adaptation often associated with flower- or seedfeeding habits.

Although usually shorter than the body, the antennae are much longer in many longhorns (Cerambycidae) and brentid weevils (Brentidae). Equipped with incredibly sensitive receptors, these appendages help beetles detect food, locate egg-laying sites, identify vibrations, and assess temperature and humidity. Those of males are sometimes quite elaborate and are packed with chemical receptors for detecting pheromones, sexually attractant odors released by females. Each antennal segment is referred to as an antennomere. The basic number of antennomeres for beetles is 11, but reductions to as few as seven are common, while 12 or more occur in some species. Antennal modifications are described as filiform (threadlike), moniliform (bead-like), serrate (saw-toothed), pectinate (comb-like), flabellate (feather-like), clavate (gradually clubbed at the tip), capitate (abruptly clubbed at tip), lamellate (terminal antennomeres flattened or plate-like), or geniculate (elbowed).

The compound eyes are entire (rounded or oval in shape), emarginate (kidney-shaped), or are completely divided, as in the whirligigs (Gyrinidae). They are often reduced in flightless species, or absent altogether in cave and litter species that dwell in total darkness. Ocelli, simple eyes limited to detecting light and dark, are found only in a few rove beetles (Staphylinidae), some leiodids (Leiodidae) and derodontids (Derodontidae), and in most hide beetles (Dermestidae).

Male heads are sometimes adorned with extraordinary antler- or tusklike horns that may vary greatly in size depending on overall body size, larval nutrition, and other environmental and genetic factors. Horns enhance the reproductive capabilities of males and are used in battles against rival males to butt, block, pry, or lift them out of the way.

The Thorax

The beetle thorax consists of three segments, each bearing a pair of legs. The clearly visible midsection of a beetle is the prothorax, the upper or dorsal surface of which is covered by a plate, the pronotum. The pronotum may bear horns that work in concert with those of the head, or is scooped out to facilitate burrowing in the soil or rotten wood. The mesothorax and metathorax bear the elytra and membranous flight wings, respectively. They are broadly connected to one another and are hidden under the elytra. The ventral sclerotized plates for these two thoracic segments are termed the meso- and metaventrite. Dorsally, the elytra usually meet along a straight line down the back called the elytral suture. A small triangular plate of the mesothorax called the scutellum is often visible between the bases of the elytra.

The legs are modified for burrowing, swimming, crawling, running, or jumping, and usually consists of six distinct segments. The coxa is generally short and stout, and firmly anchors the leg into the coxal cavity of the thorax, yet allows for the horizontal to and fro movement of the legs. The trochanter is usually small and freely movable in relation to the coxa, but fixed to the femur. The femur is the largest and most powerful leg segment and is greatly enlarged in species that jump (e.g., Scirtidae). The tibia is usually long and slender, but may be modified into a rake-like structure on the forelegs of burrowing species, or fringed with long setae to enhance their use as oars by aquatic beetles. The tarsus is typically divided into multiple articles called tarsomeres, and ends by a claw-bearing segment referred to as the pretarsus. Each tarsus consists of up to five tarsomeres (including the pretarsus), the exact number of which may be of diagnostic value and is expressed as a three-digit tarsal formula (e.g., 5-5-5 or 2-2-2), which indicates the number of tarsomeres on the fore, mid, and hind legs, respectively. The penultimate tarsomere is often difficult to see without examination under high magnification, a fact denoted by the statement “appears 4-4-4, but actually 5-5-5.” 

The Abdomen

Beetles typically have five visible abdominal segments, although there can be up to eight. Each segment is comprised of four plates, or sclerites: a dorsal tergum or tergite, the ventral sternum, and two lateral pleura. The dorsal terga are thin and flexible in beetles with abdomens completely covered by the elytra, but are thicker and more rigid in rove beetles (Staphylinidae), clown beetles (Histeridae), and others with short elytra. The penultimate and ultimate terga are called the propygidium and pygidium, respectively. The lateral pleura are usually small, more or less hidden from view, and have a single breathing pore, or spiracle. The ventrally visible abdominal sterna are called ventrites. Ventrites are numbered beginning at the base of the abdomen and are separated by deep to shallow transversal divisions called sutures, or by narrow membranes. The internal copulatory organs of males are often of great value in species delimitation and identification.

More about Beetles:

The Evolution and Diversity of Beetles
Beetles in Art, Myth and Society
Beetle Conservation Efforts and Rare Species

This excerpt has been reprinted with permission from The Book of Beetles, edited by Patrice Bouchard and published by The University of Chicago Press, 2014.