Coleoptera

Evolution and systematics

The remains of ancient beetles are abundant in the fossil record, entombed in amber or as impressions in sedimentary rock. Fossils of beetle-like insects are known from the Lower Permian rocks of eastern Europe, dating back about 250 million years ago (mya). These ancient insects were placed in the extinct family Tshekardocoleidae in the order Protocoleoptera.

Protocoleopterans resembled the modern insect order of Megaloptera (alderflies, dobsonflies, and fishflies) and most likely were an assemblage of precursors to several modern insect orders. Many Protocoleoptera are flattened, suggesting that they occupied tight spaces, such as those under loose bark. The tshekardocoleids had elytra with distinct ribbing and sculpturing, resembling extant beetles of the family Cupedidae. Tshekardocoleids differ from modern beetles in having elytra that are less regularly sculptured, loose fitting, and extending beyond the abdomen.

All protocoleopterans disappeared at the beginning of the Triassic, some 240 mya, replaced by the modern Coleoptera. By the middle of the Triassic, all four of today’s suborders of beetles (Archostemata, Myxophaga, Adephaga, and Polyphaga) were present. By the end of the Jurassic period, 210–145 mya, the evolutionary lineages of all of the modern Coleoptera were established.

Deposits of resin secreted by conifers began to appear during the Jurassic, suggesting that these ancient trees were at-

tempting to defend themselves from the attacks of woodboring insects similar to modern bark beetles (Curculionidae). At least 60 families of beetle are preserved in petrified sap deposits, or amber. The majority of these taxa are attributable to extant tribes and genera. Amber with fossil intrusions formed in tropical forests and other ancient habitats usually is poorly represented in the fossil record. Habitat types are inferred by comparing fossil beetles with the habits and distributions of extant species. Amber occurs throughout the world but is best known from deposits in the Baltic region, Dominican Republic, Mexico, China, Burma, Sicily, Canada, and Siberia. In the United States amber deposits are found in Alaska and New Jersey. Most fossil beetles from the Quaternary period, 1.6–0.5 mya, are identical to modern beetles. The remains of these beetles were not fossilized and instead were preserved among permanently frozen detritus, water-lain sediments, prehistoric dung middens, or asphalt seeps.

The relationship between beetles and plants is highly significant to their evolution. Already feeding on fungi, mosses, and algae for some 240 mya, beetles were poised to exploit the new range of vegetative structures offered by the appearance of angiosperms, or flowering plants, some 125 mya. The long and dynamic relationship between beetles and angiosperms has contributed to the amazing diversification of both groups. With approximately 350,000 species described thus far, beetles make up the largest known group of organisms.

Beetles are placed in the subclass Holometabola, also known as the Endopterygota. They are related closely to the

neuropteroid orders (Neuroptera, Megaloptera, and Rhaphidioptera) and the twisted-wing parasites (Strepsiptera). Adults are distinguished from other holometabolous orders by the presence of chewing mouthparts, with a sclerite underneath known as the gula. The mesothorax and metathorax fuse to form the pterothorax. The pterothorax is connected broadly to the abdomen and typically is covered by the elytra. The hind wings have reduced venation and are folded or rolled under the elytra. The typical three-part body plan for adult beetles therefore consists of a head, prothorax, and elytra covering the pterothorax and abdomen. The antennae usually consist of 11 or fewer antennomeres. The genitalia are retracted completely within the abdomen.

The four living suborders of Coleoptera (Archostemata, Myxophaga, Adephaga, and Polyphaga) are divided into 16 superfamilies and 166 families. The suborders are distinguished primarily by the structure of the adult prothorax, hind wing, abdomen, and reproductive organs. The suborder Archostemata contains four families of beetles that are associated with wood and resemble some of the earliest beetle fossils. The Myxophaga includes four families with fewer than 100 species. Myxophagans are small to minute and live in the watery film on stream rocks, among stream and river debris, or in interstitial habitats among sand grains. The Adephaga is the second-largest suborder of beetles, consisting of nine families and more than 40,000 known species. Most are predatory, but a few feed on algae or seeds. The abdomen has pygidial glands that produce acrid defensive chemicals. The

suborder Polyphaga contains the majority of beetles, with about 300,000 described species placed in 149 families. Among the commonly encountered polyphagans are the rove beetles (superfamily Staphylinoidea), scarabs and stag beetles (Scarabaeoidea), metallic wood-boring beetles (Buprestoidea), and click beetles and fireflies (Elateroidea), as well as fungus beetles, grain beetles, ladybird beetles, darkling beetles, blister beetles, longhorn beetles, leaf beetles, and weevils (all Cucujoidea). Aquatic families have evolved independently in the Hydrophiloidea and Byrrhoidea.

The phylogenetic relationships among the beetle suborders are poorly known. Only recently have the morphological features of these groups been subjected to phylogentic analysis, and DNA sequencing information is only now being gathered. The two most widely discussed hypotheses regard the Polyphaga either as the sister group of Myxophaga or the sister group of all remaining beetles. Evidence for the Polyphaga plus Myxophaga hypothesis includes their shared reduction in the number of segments in the larval leg. The Adephaga have been proposed as sister group to Polyphaga plus Myxophaga based on the following shared characters: completely sclerotized elytra, reduced venation in the hind wings, and folded (as opposed to rolled) hind wings. Evidence for the Polyphaga as the sister group to all remaining beetles is grounded primarily on wing structure and the loss of the neck, or cervical sclerites, in the other three suborders. Recent phylogenetic analyses supports the Polyphaga plus Myxophaga hypothesis.

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Physical characteristics

Beetles are very diverse in form and are elongate or spherical, cylindrical or flattened, slender or robust. The integument generally is tough and rigid, although in some families, such as the fireflies (Lampyridae), soldier beetles (Cantharidae), and net-winged beetles (Lycidae), it typically is soft and pliable. They range widely in length; some featherwing beetles (Anobiidae) measure less than 0.02 in (0.55 mm), while one of the world’s largest species, Titanus gigantea (Cerambycidae), reaches 6.7 in (17 cm).

The beetle head is a hardened capsule bearing chewing mouthparts and is attached to the prothorax by a flexible, membranous neck. The clypeus is absent in weevils (Curculionidae) and other beetles with a rostrum, or elongated mouthparts. The mandibles usually are conspicuous and curved and sometimes toothed along the interior margin; they may be monstrously developed in some male stag beetles (Lucanidae) and longhorns (Cerambycidae). The mandibles are modified variously to cut, grind, and strain foodstuffs, but those of wrinkled bark beetles (Rhysodidae) are incapable of biting. The maxilla and labium may possess delicate fingerlike structures, or palpi, that manipulate food. The mouthparts may be directed downward or may be hypognathous, as in leaf beetles (Chrysomelidae) and weevils. The mouthparts of some net-winged beetles and weevils and their relatives are mounted at the very tip of an extended rostrum, an adaptation often associated with floweror seed-feeding habits. In the predatory whirligigs and ground beetles, as well as in some longhorns, the mouthparts are prognathous, or directed forward.

The antennae are equipped with supersensitive receptors that detect food, locate egg-laying sites, identify vibrations, and assess temperature and humidity. Some ground beetles and rove beetles (Staphylinidae) possess specialized structures on their tibia and tarsi that are used to clean the antennae regularly to maintain their sensitivity. The antennae generally are shorter than the body but are much longer in some longhorns and brentid weevils (Brentidae). Males may have elaborate antennal structures that increase the surface area for special chemical receptors, a modification typical among species that use attractant odors, or pheromones, to locate mates.

Antennal segments lack internal musculature and are referred to as “antennomeres.” The basic number of antennomeres for beetles is 11, but reductions to as few as seven are common. Each antenna consists of a basal scape that articulates with a smaller pedicel. The antennomeres are described variously as filiform (threadlike), moniliform (beadlike), serrate (saw-toothed), pectinate (comblike), flabellate (featherlike), clavate or capitate (distinctly clubbed), lamellate (terminal antennomeres flattened or platelike), or geniculate (elbowed). Antennal shapes sometimes are characteristic of families.

The shape of the compound eyes may be entire (rounded or oval), emarginate (kidney-shaped), or partially or completely divided. In whirligigs the eyes are completely divided, with the lenses of the exposed upper portion best suited for seeing in the air and those of the submerged portion of the eyes gathering images underwater. Compound eyes with few lenses are typical among flightless species, whereas those

Order: Coleoptera

A Colorado beetle (Leptinotarsa decemilineata) under the leaf of a potato plant. (Photo by J. C. Carton. Bruce Coleman, Inc. Reproduced by permission.)

species living in total darkeness, such as cave and litter dwellers, may lack them altogether. Ocelli, or simple eyes, rarely are encountered in the Coleoptera but are present in most hide beetles (Dermestidae), a few rove beetles, some leiodids (Leiodidae), and derodontids (Derodontidae).

Horns are the most conspicuous and extraordinary modification of the beetle head and prothorax, jutting out from the upper surface of the prothorax or sweeping symmetrically back over the body. In the male Hercules beetle, Dynastes hercules (Scarabaeidae), the head and thoracic horns work in concert, like a vice. Horns generally are found in male beetles only and also may arise from the mandibles, legs, and elytra. Body size, nutrition, environmental factors, and heredity influence horn size and development.

The mesothorax supports a pair of elytra, whereas the more delicate and intricately folded flight wings are attached to the metathorax. Desert-dwelling darkling beetles and weevils frequently have elytra that are fused and lack hind wings. The legs are modified variously for burrowing, swimming, crawling, or running. The male harlequin beetle, Acrocinus longimanus (Cerambycidae); the long-armed chafer, Euchirus longimanus (Scarabaeidae); and several large weevils possess elongated front legs probably used for mating or defense. Several genera of darkling beetles living in the Kalahari Desert use their long, spindly legs as stilts to distance themselves along hot, blistering sands. The legs of aquatic beetles are flattened like oars, fringed with setae, and used like paddles to propel them through the water. The segments of the feet, or tarsi, lack musculature and are called tarsomeres. The tarsi, if present, are tipped with one or two claws. The tarsi of some leaf beetles are equipped with brushy, setose pads and oil

glands that adhere to plant surfaces. Certain male predaceous diving beetles have front tarsi similar to suction cups, to grasp the elytra of the female while copulating underwater.

The soft abdominal tergites usually are covered by the elytra. But in rove, clown (Histeridae), and many sap beetles (Nitidulidae), the elytra are short, leaving the tergites exposed. The tip of the abdomen is modified to facilitate egg laying and insemination. An elongate egg-laying tube, or ovipositor, is characteristic of many beetle families that develop in wood. Short, stout ovipositors are present in species adapted to lay their eggs directly on the surfaces of plants and rocks. The tip of the abdomen may be extremely flexible for use as a rudder (whirligigs) or to direct noxious defensive chemicals at attackers (ground beetles).

Beetle larvae usually are grublike or wormlike in appearance, but certain predatory forms may be flattened. They have distinct head capsules with chewing mouthparts modified for crushing, grinding, or tearing. Predatory species may have suctorial mouthparts for taking up the liquified tissues of their prey. The antennae vary in appearance, usually with two to four segments. The simple eyes vary in number from one to six on each side, but they may be absent altogether.

The three thoracic segments are very similar to one another, but the prothorax may have a more heavily sclerotized

dorsal plate. When present, the legs may have six (Adephaga), five (Polyphaga), or fewer segments. The abdomen usually is divided into 10 segments, but there may be nine or even eight segments; it typically is completely membranous in nature. The segments may bear ambulatory warts, or ampullae. Ampullae help the larvae gain purchase while moving through the substrate. Fleshy leglike structures, or prolegs, may appear on the first two abdominal segments, while the last segment sometimes has a fleshy lobe that forms an anal foot or pad. The ninth abdominal segment may possess a pair of fixed or articulated appendages, known as urogomphi.

The pupae of beetles are either adecticous, with legs tightly pressed against (appressed to) the body, or exarate, with the legs and wings held free from the body. The pupal abdomen may have functional muscles, allowing for some movement. “Gin-traps,” or sclerotized teeth that form on the opposite surfaces of the abdominal segments of some beetles, snap together to protect the vulnerable intersegmental grooves that often are targets for small predators and parasitic mites.

The colors of adult beetles derive from the chemical (pigment) or physical (structural) properties of the outer layer of the cuticle. The vast majority of beetles are black owing to melanin deposition during the tanning, or hardening process, of the cuticle. Other pigments contribute to beetle colors,

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making ladybird beetles (Coccinellidae) reddish or yellowish or tortoise beetles (Chrysomelidae) green. Setae, scales, or microscopic sculpturing also influences color and pattern. Densely packed, recumbent setae may give beetles an overall velvety look, while fine, parallel rows of pits, or punctures, and ridges may produce a shimmering sheen. Black desert darkling beetles (Tenebrionidae) sometimes are covered with a waxy bloom secreted by an underlying layer of the cuticle (epidermis), which creates a distinct white or yellow pattern or an overall bluish-gray hue. These waxy coatings and paler colors reflect light and reduce the chances that the beetle will overheat in the hot sun.

Structural properties of the cuticle create the shiny, metallic colors of beetles. The iridescent qualities are the result of light interference from thin films in the cuticle. The colors vary with the incidence of light and among species. Because green pigments are especially rare in the Coleoptera, structural green is a useful alternative for achieving cryptic coloration among diurnal leaf beetles. Structural greens and blues are the result of concentrated reflected light, whereas the silvery and golden reflections of some neotropical jewel scarabs of the genus Chrysina are produced by broader, more scattered wavelengths.

Distribution

Beetles live in nearly every habitat on the earth, save for the oceans, polar ice caps, and some of the tallest mountain peaks. The vast majority of beetles are endemic to their habitat, species specifically adapted to soils, climates, and foods that restrict their distributions to a particular continent, mountain range, or valley. The distributions of plant-feeding and parasitic beetles are limited by (but seldom coincide perfectly with) those of their food plant or host. A few species are nearly cosmopolitan in distribution through human activity, either introduced purposely or accidentally transported from their native lands. Depending upon predators, parasites, and pathogens found in their new home, exotic species may become established or simply may vanish.

Habitat

Small and compact, beetles are well equipped for seeking shelter, searching for food, and reproducing in nearly every terrestrial and freshwater habitat, from coastal sand dunes to wind-swept rocky fields 10,000 ft (3,050 m) above sea level. They are equally well adapted to humid tropical forests, cold mountain streams, and parched deserts, where they forage high in the canopy, hunt in roiling water, or scavenge in deep, dark caverns. Others have managed to colonize nearby continental or distant oceanic islands.

Beetles have evolved numerous morphological and behavioral adaptations that enable them to survive and reproduce in widely different habitats. Desert darkling beetles and weevils have thickened exoskeletons with fused elytra and waxy coatings to prevent desiccation. By closing their spiracles, beetles can minimize water vapor loss through respiration. Other desert species may rely on a thick coating of insulating setae or may remain buried where temperatures are significantly

Order: Coleoptera

lower during the heat of the day. Some species, such as aquatic beetles, are flat and streamlined, enabling them to dive quickly into loose sand.

The elytra protect beetles from abrasion, desiccation, parasites, and predators. In addition, the space below, or the subelytral cavity, is an important adaptation used by both desert and aquatic species. In desert species this air space insulates the body from sudden changes in temperature, and predaceous diving beetles use it to store oxygen beneath the water. Some aquatic beetles are plastron breathers, trapping a layer of air elsewhere on their body. Water scavenger beetles (Hydrophilidae) capture a layer of air on the undersurface of the abdomen, whereas long-toed water beetles (Dryopidae) and riffle beetles (Elmidae) envelop their entire bodies with the aid of a thick layer of water-repellant setae.

Beetles capable of flight increase their chances of avoiding predation, locating mates, and finding food and also boost their ability to colonize new habitats. Weevils, ground beetles, and rove beetles living at high elevations in exposed and windy conditions often lose their wings. Flight is of little use in this type of harsh, restricted habitat.

Many cave-dwelling beetles have extremely thin and permeable cuticles that readily absorb moisture from the humid atmosphere. These species quickly die from desiccation if they are placed in outside air. In the absence of light, cave beetles must rely on organic materials transported into the cave by bats in the form of guano and carcasses. Water also carries small quantities of organic material into the deepest recesses of caves. Dry caves that lack water-carrying, nutrient-rich loads generally are devoid of beetles.

The beetle faunas of continental islands strongly resemble those of nearby continental fauna and harbor relatively few endemic species. Remote, volcanic oceanic islands tend to support the greater number of endemic beetles. Beetles may colonize islands by means of active transport, such as flying. Passive transport, such as rafting on logs, is primarily responsible for dispersing wood-boring species throughout the islands of the Caribbean. Larger beetles attracted to the lights may stow away on seafaring vessels and are carried considerable distances to remote islands, while wind currents generated by fierce storms sweep up smaller species.

Countless microhabitats also support beetles. Several families live in the nests of birds, reptiles, mammals, and social insects. Interstitial spaces, the nooks and crannies between sand grains at the seashore, adjacent dunes, and along watercourses, provide ample food and shelter to support numerous species. Leaf-mining species complete their entire life cycle between the upper and lower epidermal layers of plant leaves, whose tissues provide both food and shelter.

Behavior

Beetles communicate primarily to locate mates, using mechanical, visual, and chemical means. Several families of beetles use sound to locate one another. Bess beetles (Passalidae), longhorns, and bark beetles stridulate by rubbing parts of their bodies together. Male death watch beetles (Anobiidae) bang

Order: Coleoptera

their heads against the walls of their wooden galleries to lure females, whereas the South African tok-tokkies, Psammodes (Tenebrionidae), drum their abdomens against soil and rocks to attract mates.

Bioluminescence in fireflies (Lampyridae) is the bestknown form of visual communication. To attract and locate mates, each species has its own pattern and method of presentation. Males typically fly at night, emitting a speciesspecific flashing pattern. The number of flashes and the rate and duration of the signal are critical to mate recognition, as is the delay in and length of the female’s response. The male repeats his signal at regular intervals until he picks up an answer from a receptive female. Upon locating a female, he continues flashing and flies toward the female’s signal.

While beetles infesting stored products are literally wallowing about in mate-rich habitats, others must depend on chemical communication to find distant mates or those hidden nearby among tangles of vegetation. Females usually emit pheromones, chemical messengers that work outside the body, to elicit sexual excitement in males. Receptive males often are capable of detecting pheromones with just a few molecules in the air from considerable distances. Although it is the females that typically produce pheromones to attract males, some males produce pheromones that affect egg production in the female or attract other males. Large numbers of males and females may gather in these mating swarms, or leks, increasing the likelihood that each will find a mate. Some beetles are mutually drawn to odiferous food sources, such as carrion, dung, or sap flows, whereas others gather around prominent features in the landscape, such as open patches of ground, rocky outcrops, or lone sign posts. Horned males stake out an attractive feeding spot, such as a sapping wound on a tree trunk, to await the arrival of a sexually receptive female. They use their armament to defend the site against other insects or rival males.

Elaborate courtship behavior is uncommon among the Coleoptera. Male ground, tiger, and rove beetles may grasp the female’s prothorax with their mandibles before mating, and some male blister beetles tug on the female’s antennae before copulation. In most species the male simply mounts the female from behind and may remain in contact with her afterward, to block the advances of other males.

Beetles are capable of defending themselves with an array of structural, behavioral, and chemical defenses. Large tropical scarabs, stag beetles, and longhorns avoid predators simply through their size and frightening appearance, not to mention their sharp horns and powerful mandibles. Flattened species retreat into tight spaces, where they are out of reach of predators.

Mimicry occurs in the form of shape, color, or behavioral patterns that resemble those of stinging wasps, bees, or ants. Fast-moving, scarlet red and black–checkered beetles (Cleridae) resemble female velvet ants, which actually are wingless stinging wasps. Slender and quick, some rove beetles and longhorns also mimic wasps, while stout, hairy, flower-visiting scarabs of the European genera Trichius and the North American Trichiotinus seem to imitate bees. Another North Amer-

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ican beetle, Ulochaetes leoninus (Cerambycidae) not only looks like a bumblebee, it buzzes like one too and even attempts to “sting” when captured.

The most common type of mimicry in beetles involves an unpalatable or pugnacious model and one or more palatable or docile mimics, a form of Batesian mimicry. Experienced predators quickly associate the color or behavioral patterns of the model with undesirable prey. Well-known Batesian mimicry complexes are found in the families Cantharidae, Lampyridae, Lycidae, Meloidae, Tenebrionidae, and Cerambycidae. In Müllerian mimicry complexes, all species share common aposematic patterns and are unpalatable. Net-wing beetles probably are the best-known Müllerian mimics in the Coleoptera, since all of them are probably unpalatable. Distinct color patterns also may be disruptive in nature, helping many tropical weevils and metallic jewel scarabs disappear by making them look less beetle-like to predators. By contrast, blotchy, somber colored patchworks of browns, blacks, and grays render many longhorns and weevils virtually invisible on a background of bark.

The chemical arsenals of beetles are synthesized in complex glands or obtained directly from foodstuffs. Dispensed by anal sprays or leaking leg joints, these defensive compounds function as repellents, insecticides, or fungicides. In bombardier beetles (Carabidae), hydroquinones, hydrogen peroxide, peroxidases, and catalases are stored separately. When the beetles are alarmed, these chemicals are combined in another chamber, resulting in a violent chemical reaction that makes an audible “pop,” producing a boiling, acrid stream directed with some accuracy by a flexible abdominal turret. Ladybird, blister, and soldier beetles store defensive chemicals within their blood and release them through their leg joints when attacked, a process known as reflex bleeding. Beetles that feed on toxic plants, such as milkweeds, may incorporate the plant’s chemical defenses as their own, shunting noxious compounds from the digestive tract into their body tissues.

Most beetles have fairly regular life cycles that are influenced heavily by or coincide with changes in temperature and precipitation. In temperate regions the onset of spring and summer, with their requisite combination of increased solar radiation and rain, triggers the emergence of adults from their pupal chambers. In tropical regions, with relatively constant temperatures, seasonal rains trigger emergence. One or more generations are produced per year, although some species may require two or more years to complete their life cycles.

Feeding ecology and diet

Beetles feed on a variety of fungi, plants, and animals, both living and dead. A dead tree branch presents a wealth of feeding niches, providing wood borers with stems of varying sizes and different layers of tissue, from the bark inward. Metallic wood-boring beetles (Buprestidae) may arrive within hours of a fresh cut, while others are drawn to burning wood through the use of special sensory structures that detect infrared radiation. As the branch ages, successive infestations of beetles reduce the wood to dust and are essential for nutrient recycling. Despite their dependence on wood as a food source, beetles

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are incapable of digesting cellulose and must rely on bacteria, yeasts, and fungi found in their digestive tracts. The eggs of wood-feeding beetles are inoculated with these gut symbionts as they pass through a residue lining the ovipositor. The larva’s first meal is it own egg shell, laden with these microorganisms.

All parts of living plants are fodder for both larval and adult beetles. Leaf beetles may defoliate a plant completely or skeletonize the majority of its leaves, causing serious damage to garden plants and agricultural crops. Leaf miners leave in their wake a meandering and ever widening feeding track that traces their development from egg to adult. Floricolous, or flowervisiting, beetles may have tubular mouthparts for sucking up nectar. The mouthparts of pollen feeders sometimes resemble brushes, to facilitate the consumption of fine pollen grains. Beetles can hardly be considered pollinators in the same class as bees, but they can play a significant role in pollen transfer in some plant groups seldom visited by more traditional pollinators.

Carrion and burying beetles (Silphidae), hide beetles (Dermestidae), and others scavenge dead animal tissue, while keratin-feeding skin beetles (Trogidae) consume feathers, fur, horns, and hooves. Industrious dung beetles (Geotrupidae and Scarabaeidae) bury vast amounts of organic waste produced by vertebrates (especially large herbivores) for use as food by their larvae. They employ their membranous mandibles to strain out remnants of undigested food, bacteria, yeasts, and molds as food for themselves. Not all dung beetles feed on dung, preferring instead carrion, fungi, fruit, millipedes, and the slime tracks of snails.

Predatory species seldom hunt long distance, relying instead on habitat preferences to bring them into contact with their prey. The larvae of Lampyridae, however, are specialists, tracking snails by following their slime trails. Checkered (Cleridae) and bark-gnawing (Trogossitidae) beetles are reported to pursue plumes of pheromones to locate their prey, bark beetles (Curculionidae). The ant-loving scarab, Cremastocheilus, also locates its host nests by tracking ant pheromones. Whirligig beetles identify prey by using waves generated by struggling insects, which are transmitted across the surface of the water.

Ground and tiger beetles capture their prey on the run, biting them with their pronounced mandibles and tearing them into small chunks. They attack a broad range of beetles, other insects, and invertebrates, although some prey only on snails. Some rove and clown beetles (Histeridae) hunt for food among the labyrinth of detritus and decaying organic matter, while others live in ant or termite colonies or in the fur of small mammals. Carnivorous larvae of beetles are primarily liquid feeders and must first digest their prey “extraorally,” with digestive enzymes in their saliva. Extraoral digestion also occurs among certain adult ground and rove beetles as well as in some other families.

Reproductive biology

Most beetles engage in sexual reproduction. Parthenogenesis, development from an unfertilized egg, occurs rarely in

Order: Coleoptera

Mating wattle pigs (Leptopius sp.), a type of weevil, in Alice Springs, Northern Territory, Australia. (Photo by David C. Renitz. Bruce Coleman, Inc. Reproduced by permission.)

some families. Males of parthenogenetic species are rare or unknown. In sexually reproducing species, females generally need to mate only once, although several males may inseminate them. Females store sperm in an internal reservoir, or spermatheca. Fertilization takes place as the eggs are laid. Not all beetle eggs hatch outside the mother’s body. In ovoviviparous species (some ground, rove, leaf, and darkling beetles), eggs are retained within the reproductive tract of the female until they hatch; later the larvae are “born.”

The eggs usually are laid singly or in batches. Ground dwellers simply drop them on the ground, scatter them in rich organic soil, or place them in or near piles of dung or carrion. The eggs of herbivores and leaf miners are placed at the base of their food plant, glued to stems and leaves, or inserted in the crevices of bark. Leaf miners actually tear the tissue of the leaf surface to deposit an egg inside. Some female longhorns chew a channel around a branch, killing it by girdling to create a food source for their wood-boring larvae.

Upon hatching, the first-stage, or instar, larva begins its life with a single purpose: to eat. Larvae scavenge carrion and dung, attack roots, mine plant tissues, or tunnel through wood, taking weeks or years to mature. Three or more larval instars are required before the pupal stage. In temperate climates the pupa often is best equipped to survive harsh winter conditions when it is tucked carefully away in soil or humus or within the tissues of plants. Paedogenesis, the retention of immature features, occurs in micromalthids (Micromalthidae), glowworms (Phengodidae), and certain net-winged bee-

Order: Coleoptera

tles and fireflies. In these families adult females are distinguished from the larvae by the presence of compound eyes and reproductive organs.

True social behavior is unknown among the Coleoptera. Subsocial behavior, exhibited by limited interaction between parents and young, is known, however, in at least 10 families. Some ground beetles construct soil depressions in which to lay their eggs and guard and clean them until they hatch. In several species of tortoise beetles (Chrysomelidae), the mother guards the eggs and remains with her larvae through pupation. Adult and larval bess beetles (Passalidae) bore in rotten logs, live in dense colonies, and stridulate continuously. This form of communication may serve to keep groups of adults and larvae together, since larvae seem to fare better ingesting wood that has been chewed, predigested, or converted into frass by the adults.

Female rove beetles of the European genus Bledius maintain and defend their intertidal brood tunnels, providing the larvae with algae for food. Bark and ambrosia beetles (Curculionidae) cultivate a mutualistic fungus for food for themselves and their larvae, much like some ants and termites. Adults carry fungal spores in the mycangia, specialized pits in their heads.

Among the earth-boring beetles, dung scarabs, and burying beetles, males and females cooperate in digging nests for their eggs and provisioning them with dung or carrion. Competition for this nutrient-rich, yet ephemeral resource is intense. They quickly sequester the “spoils” in underground chambers, or nests, to exclude flies, ants, mites, and other competitors and to maintain optimum moisture levels for successful brood development. Dung scarabs have evolved several strategies for nest building. Endocoprids live and breed directly in the dung pad, while paracoprids dig brood chambers underneath or immediately adjacent to the pad and provision them with dung. Telocoprids quickly carve out pieces of dung and shape them into balls that can be easily rolled away for burial later, some distance away from the source.

Burying beetles of the genus Nicrophorus exhibit the most advanced types of parental care known in the Coleoptera. Working in sexual pairs, adults locate and bury a carcass in an underground chamber. The carcass is meticulously prepared as food for both adults and larvae. All feathers or hair are removed, and the body is kneaded into a ball. Fungicides in the saliva of the adults retard decomposition. A conical depression is created on the upper surface of the carcass, to serve as a receptacle into which the adults regurgitate droplets of tissue as food for the newly hatched larvae. The female calls the larvae to the pool of tissue by rubbing a ridge on the elytra against the corresponding abdominal segment. The adults remain in the chamber with their brood until they pupate.

Conservation status

Habitat loss due to fire, urbanization, acid rain, electric lights, overgrazing, agricultural expansion, water impoundment, pollution, deforestation, soil erosion, persistent adverse

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weather, use of off-road recreational vehicles, exotic species, and logging—not collecting—are the greatest threats to beetle populations. The threat of habitat loss is exacerbated further by a steady decline in the number of trained taxonomists who provide critical data for beetle protection and habitat management.

The 2002 IUCN Red List contains 72 species of beetles, primarily from the families Dytiscidae, Carabidae, Lucanidae, Scarabaeidae, and Curculionidae. Listed species are categorized as Lower Risk (three), Vulnerable (27), Endangered (15), Critically Endangered (10), or Extinct (17). Most of the listed extant species have very restricted ranges within sensitive habitats, such as caves or sand dunes. All 14 species of the flightless South African genus Colophon (Lucanidae) are listed by the IUCN and in Appendix I of CITES primarily because of the high prices they command on the market.

The Endangered Species Act of the United States lists 16 species of American beetles, four as threatened and 12 as endangered. Of these species, the American burying beetle, Nicrophorus americanus (Silphidae), is probably one of the best documented, with a recovery plan in place for captive breeding and release. Only one species of beetle, Phalacrognathus muelleri (Lucanidae), is protected federally by the Australian Wildlife Protection Act, but in Western Australia collection of the entire family of jewel beetles (Buprestidae) is restricted. New Zealand’s Department of Conservation recognizes 24 species of beetles as endangered.

The countries of the former Soviet Union, Finland, and Sweden also have produced Red Data books that include beetles. State and provincial governments throughout the world have enacted ordinances that prohibit the collection, trading, and export of species protected elsewhere by other conventions. Two European organizations actively promote the conservation of beetles on the basis of their ecological roles. The Water Beetle Specialist Group, part of the Species Survival Commission of the IUCN, recognizes the importance of aquatic beetles as bioindicators in wetland management in Europe and Southeast Asia. The Saproxylic Invertebrates Project focuses on selected groups of invertebrates, including beetles, dependent upon standing or fallen trees or woodinhabiting fungi.

Significance to humans

The best-known example of a beetle in ancient mythology is the sacred scarab, Scarabaeus sacer. The Egyptian sun god Ra was symbolized as a great scarab, and images of scarabs appeared in much funerary art and hieroglyphs. Carved scarabs bore religious inscriptions from the Book of the Dead and were placed in tombs to ensure the immortality of the soul. Heart scarabs were stones placed with the dead that bore inscriptions admonishing the heart not to bear witness against its body on the day of judgment.

Beetles have been depicted in vase paintings, porcelain statuary, precious stones, glass paintings, sculptures, jewelry, coins, and illustrated manuscripts. Fireflies have long held a special fascination for the Chinese and Japanese and appear

Vol. 3: Insects

often in their art. One of the most notable examples of beetles in art is the German Renaissance artist Albrecht Dürer’s 1505 watercolor of the European stag beetle, Lucanus cervus. The French artist E. A. Seguy’s art deco insect portfolios, created in the 1920’s, include several striking examples of Coleoptera. The durable bodies of the beetles themselves also are used in arts and crafts, especially the elytra, horns, mandibles, and legs. South American indigenous artisans use the elytra of the giant Euchroma gigantea (Buprestidae) for necklaces, head ornaments, and other decorative pieces. Today, in parts of Mexico and Central America, a zopherid beetle popularly known as the ma’kech, Zopherus chilensis (Zopheridae), is decorated with brightly colored glass beads, fixed to a short chain tether, and pinned to clothing as a reminder of an ancient legend.

Beetles are an important source of protein and fat for peoples around the world. Throughout the islands of the South Pacific, grubs of the palm weevil, Rhynchophorus palmarum, and rhinoceros scarabs, Oryctes rhinoceros, are roasted and relished. Larval and pupal predaceous diving and jewel beetles are eaten in Southeast Asia. The Chinese collect large aquatic beetles; remove the elytra, wings, legs, and head; and fry them in oil or soak them in brine. The Aborigines of Australia eat longhorn larvae, removing the large, nut-flavored grubs from rotten logs and roasting them like marshmallows over a fire. Even in the United States the mystique of “eating the worm”

Order: Coleoptera

from a bottle of Mexican mescal has resulted in the novelty of a tequila-flavored lollipop stuffed with a larva of the common mealworm, Tenebrio molitor (Tenebrionidae).

A small number of beetles have become important economic pests, as the result of their feeding and egg-laying activities on stored products, pastures, crops, and timber. Beetles feeding on legumes, tomatoes, potatoes, melons, gourds, and grains are some of humanity’s greatest competitors for food. In temperate forests throughout the world, beetles generally attack trees that already are under stress from lack of water and nutrition. Bark beetles mine branches, twigs, cones, or roots, and the most destructive species attack the trunks of living trees.

Predatory beetles, especially ground beetles and ladybirds (Coccinellidae), are used to control insect pests around the world, and many herbivores have been recruited as biological control agents for plant pest projects. In the 1970s the Australians began a program to import exotic dung scarabs and predatory clown beetles (Histeridae) as biological agents to clean up dung pads and eat the maggots of the pestiferous flies breeding in them. The leaf, metallic wood-boring beetles and weevils are utilized to control the spread of noxious weeds throughout the world, as they feed on the leaves, bore into twigs and stems, or destroy the seeds.