Beetles
Beetles
Beetles make up the large, extremely diverse order Coleoptera of the class Insecta, and comprise the largest single group of animals on Earth. There are at least 250,000 species of beetles, compared to the 5,000 known species of mammals. The weevil family of beetles alone contains about 50,000 species, and is the largest family in the animal kingdom. Thus, the order Coleoptera, representing about 40% of the known insect species, contributes greatly to making the insects the largest class of the largest phylum—Arthropoda. Arthropods are thought to have evolved as much as 500 million years ago during the Precambrian, while the most primitive insect fossils date to the rocks of the Middle Devonian period about 350 million years ago. Coleoptera are thought to have evolved in the early Permian about 225-280 million years ago, and were common even before the age of reptiles.
Beetles are found in virtually all climates and latitudes throughout the world except at very high altitudes or in regions with extreme temperatures, e.g. the Antarctic. Most species of beetles occur in the tropics, but fewer individuals of a particular species are generally found in tropical regions rather than in temperate areas.
Beetles’ success is due to at least three important characteristics. First, they undergo complete metamorphosis (egg, larva, pupa, adult), with larval and adult stages usually living in different places and eating different food. This division greatly expands the number of ecological niches and food available to them. Second, the front pair of wings is modified into a hard cover (elytra) that protects the soft body underneath. Third, most beetles have mouth parts capable of chewing a wide variety of solid foods.
Some, however, have mouth parts modified for sipping sap and nectar.
The front pair of wings, modified into horny covers (elytra), hide the rear pair of wings and abdomen, and their inner edges appose each other, creating a straight line down the back of the insect. The elytra form a rigid, closely interlocking sheath that covers the mesothorax and metathorax, and most of the abdomen. (The name Coleoptera is derived from the Greek word koleos, meaning sheath.) The perfect alignment of the edges of the elytra form the characteristic, straight line that seems to split the back of the beetle, and gives these insects their common name (beetle from the German word bheid, meaning to split).
Beetles are found on vegetation, under bark, stones, and other objects, as well as almost anywhere on or in the soil, rotting vegetation, dung, and carrion. They vary widely in size and appearance, and many have noteworthy behavior. Some beetles (e.g., Lampyridae) produce light, while others (Cerambycidae) can stridulate, that is, they can produce sound. Large beetles usually make a loud noise during flight, and some, such as the scarab beetles, have a bizarre physical form.
Varieties of beetles
The Coleoptera includes the largest and smallest insects in the world, ranging from the giant, 6.3-in (16-cm) Longhorn beetle (Titanus giganteus ) of the Amazon region to the dot-sized, fringed ant beetle (Nanosella fungi ) of North America, which reaches only 1 inch (0.25 mm) in length—smaller than a large protozoan.
Beetles are economically important in agriculture, either feeding directly on crops and trees, or preying on other species that harm plant crops. For example, the ground beetles (Carabidae) and the rove beetles (Staphylinidae) feed on caterpillars and other larvae as well as on many soft-bodied insects and insect eggs. Many of the adult and larval forms of the ladybugs or ladybird beetles (Coccinellidae) feed on plant-sucking insects (Homoptera) such as aphids and scale insects, while only a few of the Coccinellidae themselves (e.g., Epilachna ) feed on plants.
Many other beetles, however, do feed on plants. Among the most important of these beetles are the leaf beetles (Chrysomelidae) and the weevils and their relatives (Curculionoidea). The larvae of leaf beetles feed on leaves, stems, or roots, while most adults chew on leaves; the larvae of weevils feed on almost all parts of plants. For example, larvae and adult forms of bark beetles (Scolytidae) attack treetissue beneath the bark.
Scarab beetles (Scarabaeidae) are particular pests of crops, lawns, and pastures. One of these insects, the dung beetle, was an important religious symbol to ancient Egyptians, who considered its life cycle to be a reflection of the cyclical processes of nature, especially the “rebirth” of the sun each morning. Glazed steatite (soapstone) and other ancient Egyptian ceramic or stone representations of the beetle, called scarabs, were a symbol of the soul and used as talismans.
Many beetles act as scavengers, breaking down organic material such as wood and dead plant and animal matter. The larvae of some beetles, such as the wedge-shaped beetles, are parasitic on wasps, bees, and cockroaches. The European elm bark beetle (Scolytus multistriatus ) transmits the fungus that causes Dutch elm disease.
The vast array of forms and colors of Coleoptera ranges from the black, furry Brazilian beetle, with creamy white and orange spots, to the squat tortoise beetle, the long-snouted Peruvian beetle, the stag beetle with its two threatening “horns,” and the whirligig beetle, often found gyrating rapidly on the surface of ponds.
Click beetles (family Elateridae) are named for the sharp noise they make. When turned onto its back, a click beetle will bend its head and the upper part of its body backward, then suddenly straighten. This movement produces a click and propels the beetle into the air. This maneuver is repeated until the beetle lands right side up.
Lightning bugs or fireflies (family Lampyridae) produce light; some species produce flashes, while others produce continuous luminescence. These insect light shows, common in spring and summer, are a mating ritual through which the sexes find each other.
The classification of beetles established by R. A. Crowson in 1955 (The Natural Classification of the Families of Coleoptera ), divides the order into four suborders: Archostemata (rarely found beetles), Adephaga (the tiger beetles and various water beetles), Myxophaga (the minute bog beetles and skiff beetles), and Polyphaga (the majority of beetles, such as carrion beetles, scarab beetles, ladybugs, and long-horned beetles). The Polyphaga is the largest suborder, with 18 superfamilies. In all, there are about 135 known families of beetles, of which 120 are found in the Western Hemisphere.
Beetle anatomy and physiology
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, 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 beetles fly, 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 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 water beetles species 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 holds a pair of compound eyes, a pair of antennae (feelers), 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 rod-like 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 35 inches, or 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 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 super-family Curculionoidea have a distinct snout that can bore into wood to 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 hemolymph, a liquid analogous to blood and lymphatic fluid, bathes their tissues directly. A tube-like “heart” in the abdomen pumps 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, where they devour foreign microorganisms. Unlike the blood, hemolymph is not involved in oxygen transport; that function is performed by the spiracles.
Life cycle
The mouth parts, which allow beetles to utilize a wide variety of solid foods in their environment, and the elytra, which protect the hindwings, give beetles great survival advantages. Another factor that contributes to the enormous success of beetles is the fact that they undergo complete metamorphosis. Beetles pass through three distinct developmental stages— egg, larva (grub), and pupa—before becoming adults.
Beetles reproduce sexually, although a few species consist of females only and parthenogenesis (reproduction from unfertilized eggs) sometimes occurs. The male reproductive organ is the aedeagus, a hard, tubelike structure that is inserted into the tip of the female’s abdomen through the bursa copulatrix during mating. The female stores sperm in a saclike structure called the spermatheca until they are used to fertilize eggs.
The beetle larva hatches from an egg and feeds, growing until its burgeoning body splits the skin (cuticle). The larva crawls out of the old skin and forms a new one, a process called molting. This occurs several times, until the larva is mature.
Beetle larvae are always very different from adults in both form and habits. They usually have only chewing mouth parts even if as adults they develop siphoning or piercing mouth parts. Wings develop internally and are not evident until the pupal stage. Because larvae, pupas, and adults live in different places and eat different foods, they do not compete with each other.
The different larval forms of beetles reflect a wide variety of feeding habits and habitats. The predatory larvae of water beetles (dytiscids) and ground beetles (carabids) are slender or have gradually tapered bodies and long legs adapted to chasing prey, and large, slender mandibles for holding food.
The larva of the tiger beetle (Cicindelidae) lives in the ground, digging a burrow up to 2 feet (0.6 m) deep to avoid high temperatures in its tropical and subtropical environments. The head of the tiger beetle is large and is bent at right angles to the body. When the larva is poised vertically within the burrow waiting for passing prey, its lidlike head acts like a living plug flush with the surface. When a potential meal nears the burrow, the beetle springs out like a jack-in-the-box doing a partial back somersault to catch the prey in its jaws. Two barbed spines on the tiger beetle’s back hook into the burrow wall and prevent a strongly struggling victim from pulling the beetle out of its burrow.
The eggs of the European stag beetle (Lucanidae) hatch in the decaying heartwood of old trees, slowly developing into plump larvae, which remain in the tree while they develop into a pupa. A month later, the adult form emerges from the pupa and searches along the forest floor for prey. The large branched jaws of the adult resemble the antlers of a stag. The larvae of ambrosia beetles feed on fungus gardens cultivated by adults in the sapwood of trees.
Following the larval stage, the beetle enters the pupal stage. The pupa develops beneath the skin of the final larval stage, then emerges when the skin splits. The pupa is a soft, pale image of the adult it is to become. The pre-adult appendages are curled or loosely attached to the body and the wings are in flat bags called wing pads. After the pupa sheds its thin skin, the adult emerges, the wings stretch out to full size, and the outer skeleton hardens. The beetle has undergone complete metamorphosis—from egg to larva, to pupa, to adult.
Defense
Beetles produce a variety of noxious chemicals to protect themselves against predators. For example, members of the genus Meloe release an oily substance from the joints of their legs that can raise blisters on human skin. In addition, members of the genus Eleodes emit an offensive black fluid when disturbed. However, the bombardier beetle displays one of the most dramatic repellent devices. This beetle shoots a boiling hot mixture of liquid and vapor from a “turret” at the rear of its abdomen. Able to fire repeatedly, the beetle has been observed to shoot 29 times within four minutes. The spray protects the beetle from ants, frogs, spiders, and praying mantids.
Parasitic beetles
Throughout the world, beetles have evolved parasitic relationships with a wide variety of animals, feeding on epidermal secretions and the hair of vertebrate hosts. The small beetle Leptinus testaceus of Britain is sometimes found living on the fur of voles and mice, and has also been found in bees’ nests. The American species of this beetle, L. americanus, is also found on small rodents. Another North American member of this genus, L. validus, is an ectoparasite of the common beaver; and L. aplodontiae is an ectoparasite of the mountain beaver. Two South American species, Uroxys gorgon and Trichillium brachyporum, in the Scarabaeidae family live in the fur of the three-toed sloth, while the Australian genus Macropocopris lives in the fur of kangaroos.
Beetles and humans
Beetles have had an important impact on the people that share their environments. Like many other insects, they pose a threat to agriculture, feeding on crops and wood, both harvested and stored. For example, the dermestid beetles of the family Dermestidae are widely distributed and feed on cereal products, grains, stored food, rugs and carpets, upholstery, and fur coats. Although the adults of some species may be destructive, usually it is beetle larvae that do the most damage.
Grain and the rice weevils are particularly destructive, having evolved a snout that can penetrate food plants and bore holes to deposit eggs. The boll weevil (Anthonomus grandis ) is a major cotton crop pest in North America. The boll weevil deposits up to 300 eggs at a time in cotton buds or fruit. The larvae live within the cotton boll, destroying the seeds and surrounding fibers. The Colorado potato beetle (Leptinotarsa decemlineata ) attacks the leaves of potato plants; it became a major pest in the United States during the late nineteenth century.
Beetles, like other insects, also eat a variety of plants that are not of agricultural value, but may be of aesthetic value to humans. For example, two forms of ladybird beetles, the Mexican bean beetle and the squash beetle, are voracious garden pests; and some blister beetles commonly parasitize eggs or larvae of bees.
KEY TERMS
Elytra (singular, elytron)— The front pair of wings in beetles that are modified into horny structures that cover and protect the part of the thorax, most of the abdomen, and the hindwings.
Malpighian tubule— A kidney-like excretory organ found in beetles and most other insects.
Spiracle— An external respiratory opening found in beetles and other insects.
Not all beetle activity is destructive, however. Beetles, along with other insects, also help to pollinate flowers, which then produce fruits and seeds. Beetles crawling over flowers brush up against pollen-bearing organs, and carry the pollen dust to another flower of the same species.
In addition, some beetles keep gardens from being overrun by other plant pests. For example, most species of ladybird beetles feed as adults and larvae on aphids, scale insects, mites, and other pests; the larvae of wedge-shaped beetles are parasitic on cockroaches. These highly predatory beetles are an important factor in keeping populations of plant-feeding pests such as leaf beetle larvae and other insects from reaching plague levels.
An Australian ladybird beetle, the vedalia beetle (Rodolia cardinalis), is used throughout the world to control crop pests, such as the coconut scale, sugar-cane mealy bug, potato aphid, and fir aphid. In addition, certain pollen or sap beetles of the family Nitidulidae prey on the eggs, nymphs, and adult stage of a variety of whitefly and aphid species, among other insects.
Resources
BOOKS
Johnson, Sylvia A. Beetles. Minneapolis: Lerner, 1982. White, Richard E. A Field Guide to Beetles. Boston: Houghton Mifflin, 1983.
OTHER
Tree of Life Web Project. “Beetles” <http://www.tolweb.org/Coleoptera> (accessed October 31, 2006). Wayne’s World: An On-Line Textbook of Natural History.
“Beetles” <http://waynesword.palomar.edu/ww0502.htm> (accessed October 31, 2006).
Marc Kusinitz
Beetles
Beetles
Beetles make up the large, extremely diverse order Coleoptera of the class Insecta, and comprise the largest single group of animals on Earth . There are at least 250,000 species of beetles, compared to the 5,000 known species of mammals . The weevil family of beetles alone contains about 50,000 species, and is the largest family in the animal kingdom. Thus, the order Coleoptera, representing about 40% of the known insect species, contributes greatly to making the insects the largest class of the largest phylum—Arthropoda. Arthropods are thought to have first evolved as long as 500 million years ago in Precambrian times, while the most primitive insect fossils date to the rocks of the Middle Devonian period about 350 million years ago. Coleoptera are thought to have evolved in the early Permian period about 225-280 million years ago, and were common even before the age of reptiles .
Beetles are found in virtually all climates and latitudes throughout the world except at very high altitudes or in regions with extreme temperatures, e.g. the Antarctic. Most species of beetles occur in the tropics, but fewer individuals of a particular species are generally found in tropical regions rather than in temperate areas.
The success of the beetles is due to at least three important characteristics. First, they undergo complete metamorphosis (egg, larva, pupa, adult), with larval and adult stages usually living in different places and eating different food. This division greatly expands the number of ecological niches and food available to these insects. Second, the front pair of wings is modified into a hard cover (the elytra) that protects the soft body underneath. Third, most beetles have mouth parts capable of chewing a wide variety of solid foods. Some beetles, however, have mouth parts modified for sipping sap and nectar .
The front pair of wings, modified into horny covers (elytra), hide the rear pair of wings and abdomen, and their inner edges appose each other, creating a straight line down the back of the insect. The elytra form a rigid, closely interlocking sheath that covers the mesothorax and metathorax, and most of the abdomen. (The name Coleoptera is derived from the Greek word koleos, meaning sheath.) The perfect alignment of the edges of the elytra form the characteristic, straight line that seems to split the back of the beetle, and gives these insects their common name (beetle from the German word bheid, meaning to split).
Beetles are found on vegetation, under bark , stones, and other objects, as well as almost anywhere on or in the soil , rotting vegetation, dung, and carrion. They vary widely in size and appearance, and many have noteworthy behavior . Some beetles (e.g., Lampyridae) produce light , while others (Cerambycidae) can stridulate, that is, they can produce sound. Large beetles usually make a loud noise during flight, and some, such as the scarab beetles, have a bizarre physical form.
Varieties of beetles
The Coleoptera includes the largest and smallest insects in the world, ranging from the giant, 6.3-in (16-cm) Longhorn beetle (Titanus giganteus) of the Amazon region to the dot-sized, fringed ant beetle (Nanosella fungi) of North America , which reaches only 0.25 mm in length—smaller than a large protozoan.
Beetles are economically important in agriculture, either feeding directly on crops and trees, or preying on other species that harm plant crops. For example, the ground beetles (Carabidae) and the rove beetles (Staphylinidae) feed on caterpillars and other larvae as well as on many soft-bodied insects and insect eggs. Many of the adult and larval forms of the ladybugs or ladybird beetles (Coccinellidae) feed on plant-sucking insects (Homoptera) such as aphids and scale insects , while only a few of the Coccinellidae themselves (e.g., Epilachna) feed on plants.
Many other beetles, however, do feed on plants. Among the most important of these beetles are the leaf beetles (Chrysomelidae) and the weevils and their relatives (Curculionoidea). The larvae of leaf beetles feed on leaves, stems, or roots, while most adults chew on leaves; the larvae of weevils feed on almost every part of plants. For example, larvae and adult forms of bark beetles (Scolytidae) attack tree tissue beneath the bark.
The scarab beetles (Scarabaeidae) are important pests of crops, lawns, and pastures. One of these insects, the dung beetle, was an important religious symbol to ancient Egyptians, who considered its life cycle to be a reflection of the cyclical processes of nature, especially the "rebirth" of the sun each morning. Glazed steatite (soapstone) and other ancient Egyptian ceramic or stone representations of the beetle, called scarabs, were a symbol of the soul and used as talismans.
Many beetles act as scavengers, breaking down organic material such as wood and dead plant and animal matter . The larvae of some beetles, such as the wedge-shaped beetles, are parasitic on wasps , bees , and cockroaches . The European elm bark beetle (Scolytus multistriatus) transmits the fungus that causes Dutch elm disease .
The vast array of forms and colors of Coleoptera ranges from the black, furry Brazilian beetle, with creamy-white and orange spots, to the squat tortoise beetle, the long-snouted Peruvian beetle, the stag beetle with its two threatening "horns," and the whirligig beetle, often found gyrating rapidly on the surface of ponds.
Click beetles (family Elateridae) are named for the sharp noise they make. When turned onto its back, a click beetle will bend its head and the upper part of its body backward, then suddenly straighten. This movement produces a click and propels the beetle into the air. This maneuver is repeated until the beetle lands right side up.
Lightning bugs or fireflies (family Lampyridae) produce light; some species produce flashes, while others produce continuous luminescence . These insect light shows, common in spring and summer, are a mating ritual through which the opposite sexes find each other.
The classification of beetles established by R. A. Crowson in 1955 (The Natural Classification of the Families of Coleoptera), divides the order into four suborders: Archostemata (rarely found beetles), Adephaga (the tiger beetles and various water beetles), Myxophaga (the minute bog beetles and skiff beetles), and Polyphaga (the majority of beetles, such as carrion beetles, scarab beetles, ladybugs, and long-horned beetles). The Polyphaga is the largest suborder, with 18 superfamilies. In all, there are about 135 known families of beetles, of which 120 are found in the Western Hemisphere.
Beetle anatomy and physiology
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.
Life cycle
The mouth parts, which allow beetles to utilize a wide variety of solid foods in their environment, and the elytra, which protect the hindwings, give beetles great survival advantages. Another factor that contributes to the enormous success of beetles is the fact that they undergo complete metamorphosis. Beetles pass through three distinct developmental stages—egg, larva (grub), and pupa—before becoming adults.
Beetles reproduce sexually, although a few species consist of females only and parthenogenesis sometimes occurs. The male reproductive organ is the aedeagus, a hard, tubelike structure that is inserted into the tip of the female's abdomen through the bursa copulatrix during mating. The female stores sperm in a saclike structure called the spermatheca until they are used to fertilize eggs.
The beetle larva hatches from an egg and feeds, growing until its burgeoning body splits the skin (cuticle). The larva crawls out of the old skin and forms a new one, a process called molting. This occurs several times, until the larva is mature.
Beetle larvae are always very different from adults in both form and habits. They usually have only chewing mouth parts even if as adults they develop siphoning or piercing mouth parts. Wings develop internally and are not evident until the pupal stage. Because larvae, pupas, and adults live in different places and eat different foods, they do not compete with each other.
The different larval forms of beetles reflect a wide variety of feeding habits and habitats. The predatory larvae of water beetles (dytiscids) and ground beetles (carabids) are slender or have gradually tapered bodies and long legs adapted to chasing prey , and large, slender mandibles for holding food.
The larva of the tiger beetle (Cicindelidae) lives in the ground, digging a burrow up to 2 ft (0.6 m) deep to avoid high temperatures in the subtropical and tropical environments. The head of the tiger beetle is large and is bent at right angles to the body. When the larva is poised vertically within the burrow waiting for passing prey, its lidlike head acts like a living plug flush with the surface. When a potential meal nears the burrow, the beetle springs out like a jack-in-the-box doing a partial back somersault to catch the prey in its jaws. Two barbed spines on the tiger beetle's back hook into the burrow wall and prevent a strongly struggling victim from pulling the beetle out of its burrow.
The eggs of the European stag beetle (Lucanidae) hatch in the decaying heartwood of old trees, slowly developing into plump larvae, which remain in the tree while they develop into a pupa. A month later, the adult form emerges from the pupa and searches along the forest floor for prey. The large branched jaws of the adult resemble the antlers of a stag. The larvae of ambrosia beetles feed on fungus gardens cultivated by adults in the sapwood of trees.
Following the larval stage, the beetle enters the pupal stage. The pupa develops beneath the skin of the final larval stage, then emerges when the skin splits. The pupa is a soft, pale image of the adult it is to become. The pre-adult appendages are curled or loosely attached to the body and the wings are in flat bags called wing pads. After the pupa sheds its thin skin, the adult emerges, the wings stretch out to full size, and the outer skeleton hardens. The beetle has undergone complete metamorphosis—from egg to larva, to pupa, to adult.
Defense
Beetles produce a variety of noxious chemicals to protect themselves against predators. For example, members of the genus Meloe release an oily substance from the joints of their legs that can raise blisters on human skin. In addition, members of the genus Eleodes emit an offensive black fluid when disturbed. However, the bombardier beetle displays one of the most dramatic repellent devices. This beetle shoots a boiling hot mixture of liquid and vapor from a "turret" at the rear of its abdomen. Able to fire repeatedly, the beetle has been observed to shoot 29 times in rapid succession within four minutes. The spray protects the beetle from ants , frogs , spiders, and praying mantids.
Parasitic beetles
Throughout the world, beetles have evolved parasitic relationships with a wide variety of animals, feeding on epidermal secretions and the hair of vertebrate hosts. The small beetle Leptinus testaceus of Britain is sometimes found living on the fur of voles and mice , and has also been found in bees' nests. The American species of this beetle, L. americanus, is also found on small rodents . Another North American member of this genus, L. validus, is an ectoparasite of the common beaver; and L. aplodontiae is an ectoparasite of the mountain beaver. Two South American species, Uroxys gorgon and Trichillium brachyporum, in the Scarabaeidae family live in the fur of the three-toed sloth, while the Australian genus Macropocopris lives in the fur of kangaroos.
Beetles and humans
The vast number and variety of beetles have inevitably had an important impact on the human populations that share environments with these insects. Beetles, like some other insects, pose a threat to agriculture, feeding on crops and wood, both harvested and stored. For example, the dermestid beetles of the family Dermestidae are widely distributed and feed on cereal products, grains, stored food, rugs and carpets, upholstery, and fur coats. Although the adults of some species may be destructive, usually it is beetle larvae that do the most damage.
The grain weevil and the rice weevil are particularly destructive, having evolved a snout that can penetrate food plants and also bore holes to deposit eggs. The boll weevil (Anthonomus grandis) is a major cotton crop pest in North America. The boll weevil deposits up to 300 eggs at a time in cotton buds or fruit. The larvae live within the cotton boll, destroying the seeds and the surrounding fibers. The Colorado potato beetle (Leptinotarsa decemlineata) attacks the leaves of potato plants; it became a major pest in the United States during the late nineteenth century.
Beetles, like other insects, also eat a variety of plants that are not of agricultural value, but may be of aesthetic value to humans. For example, two forms of ladybird beetles, the Mexican bean beetle and the squash beetle, are voracious garden pests; and some blister beetles commonly parasitize eggs or larvae of bees.
Not all beetle activity is destructive, however. Beetles, along with other insects, also help to pollinate flowers, which then produce fruits and seeds. Beetles crawling over flowers brush up against pollen-bearing organs, and carry the pollen dust to another flower of the same species.
In addition, some beetles keep gardens from being overrun by plant pests. For example, most species of ladybird beetles feed as adults and larvae on aphids, scale insects, mites , and other insect pests. These highly predatory beetles are an important factor in keeping populations of plant-feeding pests such as leaf beetle larvae and other insects from reaching plague levels. And the larvae of wedge-shaped beetles are parasitic on cockroaches.
An Australian ladybird beetle, the vedalia beetle (Rodolia cardinalis), is used throughout the world to control crop pests, such as the coconut scale, sugarcane mealy bug, potato aphid, and fir aphid. In addition, certain pollen or sap beetles of the family Nitidulidae prey on the eggs, nymphs, and adult stage of a variety of whitefly and aphid species, among other insects.
Resources
books
Johnson, Sylvia A. Beetles. Minneapolis: Lerner, 1982.
White, Richard E. A Field Guide to Beetles. Boston: Houghton Mifflin, 1983.
Christine Miner Minderovic
Marc Kusinitz
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Elytra (singular, elytron)
—The front pair of wings in beetles that are modified into horny structures that cover and protect the part of the thorax, most of the abdomen, and the hindwings.
- Malpighian tubule
—A kidney-like excretory organ found in beetles and most other insects.
- Spiracle
—An external respiratory opening found in beetles and other insects.
- Stridulation
—Chirping, clicking or other audible sounds made by certain insects by rubbing body parts together.