Beetles are the most diverse group of insects. Their order, Coleoptera (meaning "sheathed wing"), has more described species in it than in any other order in the animal kingdom. Forty percent of all described insect species are beetles (about 350,000 species), and new species are regularly discovered. Estimates put the total number of species, described and undescribed, at between 5 and 8 million. For this reason, the Scottish geneticist J. B. S. Haldane, when asked what his studies of nature revealed about God, replied, "An inordinate fondness for beetles."
Beetles can be found in almost all habitats, but are not known to occur in the sea or in the polar regions. They impact the ecosystem in several ways. On the one hand, they feed on plants and fungi, break down animal and plant debris, and eat other invertebrates. On the other hand, they are prey of various animals including birds and mammals. Certain species are agricultural pests, such as the red flour beetle Tribolium castaneum, the Colorado potato beetle Leptinotarsa decemlineata, or the mungbean beetle Callosobruchus maculatus, while others are important controls of agricultural pests. For example, Lady Beetles (family Coccinellidae) consume Aphids, Scale Insects, Thrips, and other plant-sucking insects that damage crops.
The general anatomy of beetles is quite uniform, though specific organs and appendages may vary greatly in appearance and function between the many families in the order. Beetle bodies are divided into three sections: the head, the thorax, and the abdomen. When viewed from below, the thorax is that part from which all three pairs of legs arise. The abdomen, then, is everything posterior to the thorax. When viewed from above, most beetles appear to have three clear sections, but this is deceptive: on the beetle's upper surface, the middle "section" is a hard plate called the pronotum, which is only the front part of the thorax; the back part of the thorax is concealed by the beetle's wings. Like all insects, beetles are [segmented organisms, and all three of the major sections of the body are themselves composed of several further segments, although these are not always readily discernible. This further segmentation can usually best be seen on the abdomen.
Beetles are generally characterised by a particularly hard exoskeleton, hard forewings (elytra), and chewing mouth parts. The beetle's exoskeleton is made up of numerous plates called sclerites, separated by thin sutures. This design creates the armoured defences of the beetle while maintaining flexibility. The elytra are not used for flight, but tend to cover the hind part of the body and protect the second pair of wings (alae). The elytra must be raised in order to move the hind flight wings. A beetle's flight wings are crossed with veins and, after landing, are folded, often along these veins, and stored below the elytra.
In some beetles the ability to fly has been lost, most notably in the Ground Beetles (family Carabidae) and the "true Horticulture/Weevils" (family Curculionidae), but also in some desert and cave-dwelling species of other families. Many of these species have the two elytra fused together, forming a solid shield over the abdomen. In a few families, both the ability to fly and the elytra have been lost, with the best known example being the glowworms of the family Phengodidae, in which the females are larviform throughout their lives.
Beetles have mouthparts similar to those of grasshoppers. Of these parts, the most commonly known are probably the mandibles, which appear as large pincers on the front of some beetles. The mandibles are a pair of hard, often tooth-like structures that move horizontally to grasp, crush, or cut food or enemies (see Defence, below). Two pairs of finger-like appendages are found around the mouth in most beetles, serving to move food into the mouth. These are the maxillary and labial palpi.
The eyes are compound and may display remarkable adaptability, as in the case of whirligig beetles (family Gyrinidae), in which the eyes are split to allow a view both above and below the waterline. Other species also have divided eyes — some longhorn beetles (family Cerambycidae) and weevils — while many beetles have eyes that are notched to some degree. A few beetle genera also possess ocelli, which are small, simple eyes usually situated farther back on the head (on the vertex).
Beetle antennae are primarily organs of smell, but may also be used to feel out a beetle's environment physically. They may also be used in some families during mating, or among a few beetles for defence. Antennae vary greatly in form within the Coleoptera, but are often similar within any given family. In some cases, males and females of the same species will have different antennal forms. Antennae may be clavate (flabellate and lamellate are sub-forms of clavate, or clubbed antennae), filiform, geniculate, moniliform, pectinate, or serrate. For images of these antennal forms, see antenna (biology).
The legs, which are multi-segmented, end in two to five small segments called tarsi, which are vaguely comparable to feet. Like many other insect orders beetles bear claws, usually one pair, on the end of the last tarsal segment of each leg. While most beetles use their legs for walking, legs may be variously modified and adapted for other uses. Among aquatic families — predacious diving beetles (Dytiscidae), crawling water beetles (Haliplidae), many water scavenger beetles (Hydrophilidae), and others — the legs, most notably the hind pair, are modified for swimming and often bear rows of long hairs to aid this purpose. Other beetles have fossorial legs that are widened and often spined for digging. Species with such adaptations are found among the scarabs, ground beetles, and clown beetles (family Histeridae). The hind legs of some beetles, such as Flea Beetles (within Chrysomelidae) and flea weevils (within Curculionidae), are enlarged and designed for jumping.
Oxygen is obtained via a tracheal system. Air enters a series of tubes along the body through openings called spiracles, and is then taken into increasingly finer fibres. Pumping movements of the body force the air through the system.
Beetles have hemolymph instead of blood, and the open circulatory system of the beetle is powered by a tube-like heart attached to the top inside of the thorax.
Beetles undergo complete metamorphosis, meaning that the larval form does not resemble the adult.
A single female may lay from several dozen to several thousand eggs during her lifetime. Eggs are usually laid according to the substrate the larva will feed on upon hatching. Among others, they can be laid loose in the substrate (e.g. flour beetle), laid in clumps on leaves (e.g. Colorado potato beetle), or individually attached (e.g. mungbean beetle and other seed borers) or buried in the soil (e.g. carrot weevil).
The larva is usually the principal feeding stage of the beetle lifecycle. Larvae tend to feed voraciously once they emerge from their eggs. Some feed externally on plants, such as those of certain leaf beetles, while others feed within their food sources. Examples of internal feeders are most metallic wood-boring beetles (family Buprestidae) and longhorn beetles. The larvae of many beetle families are predatory like the adults (ground beetles, lady beetles, rove beetles). The larval period varies between species but can be as long as several years.
Beetle larvae can be differentiated from other insect larvae by their hardened, often darkened head, the presence of chewing mouthparts, and spiracles along the sides of the body. Like adult beetles, the larvae are varied in appearance, particularly between beetle families. Beetles whose larvae are somewhat flattened and are highly mobile are the ground beetles, some rove beetles, and others; their larvae are described as campodeiform. Some beetle larvae resemble hardened worms with dark head capsules and minute legs. These are elateriform larvae, and are found in the click beetle (Elateridae) and darkling beetle (Tenebrionidae) families. Some elateriform larvae of click beetles are known as wireworms. Beetles in the families of the Scarabaeoidea have short, thick larvae described as scarabaeiform, but more commonly known as grubs.
All beetle larvae go through several instars, which are the developmental stages between each moult. In many species the larvae simply increase in size with each successive instar as more food is consumed. In some cases, however, more dramatic changes occur. Among certain beetle families or genera, particularly those that exhibit parasitic lifestyles, the first instar (the planidium) is highly mobile in order to search out a host, while the following instars are more sedentary and remain on or within their host. This is known as hypermetamorphosis; examples include the blister beetles (family Meloidae) and some rove beetles, particularly those of the genus Aleochara.
As with all endopterygotes, beetle larvae pupate, and from this pupa emerges a fully formed, sexually mature adult beetle, or imago. Adults have an extremely variable lifespan, from weeks to years, depending on the species.
Beetles may display extremely intricate behaviour when mating. Smell is thought to be important in the location of a mate.
Conflict can play a part in the mating rituals of species such as burying beetles (genus Nicrophorus) where conflicts between males and females rage until only one of each is left, thus ensuring reproduction by the strongest and fittest. Many beetles are territorial and will fiercely defend their small patch of territory from intruding males.
Pairing is generally short but in some cases will last for several hours. During pairing sperm cells are transferred to the female to fertilize the eggs.
Parental care varies between species, ranging from the simple laying of eggs under a leaf to certain scarab beetles, which construct impressive underground structures complete with a supply of dung to house and feed their young. Other beetles are leaf rollers, biting sections of leaves to cause them to curl inwards, then laying their eggs, thus protected, inside.
Beetles and their larvae have a variety of strategies to avoid being attacked by predators or parasitoids. These include camouflage, mimicry, toxicity, and active defence.
Camouflage involves the use of colouration or shape to blend into the surrounding environment. Among those that exhibit this defensive strategy are some of the leaf beetles (family Chysomelidae), having green colouring very similar to their habitat on plant leaves. More complex camouflage also occurs, as with some weevils, where various coloured scales or hairs cause the beetle to resemble bird dung.
Another defence that often uses colour or shape to deceive potential enemies is mimicry. A number of longhorn beetles (family Cerambycidae) bear a striking resemblance to wasps, which fools predators into keeping their distance even though the beetles are in fact harmless. This defence can be found to a lesser extent in other beetle families, such as the scarab beetles. Beetles may combine their colour mimicry with behavioural mimicry, acting like the wasps they already closely resemble.
Many beetle species, including Lady Beetles and Blister Beetles, can secrete distasteful or toxic substances to make them unpalatable or even poisonous. These same species often exhibit aposematism, where bright or contrasting colour patterns warn away potential predators.
Large ground beetles and longhorn beetles may go on the attack, using their strong mandibles to forcibly persuade a predator to seek out easier prey. Others, such as bombardier beetles (within Carabidae) spray acidic gas from their abdomen to repel predators.
Symptoms and Signs
Besides being abundant and varied, the Coleoptera are able to exploit the wide diversity of food sources available in their many habitats. Some are generalists, eating both plants and animals. Other beetles are highly specialised in their diet. Many species of leaf beetles, longhorn beetles, and weevils are very host specific, feeding on only a single species of plant. Ground beetles and Rove Beetles (family Staphylinidae), among others, are primarily carnivorous and will catch and consume many other arthropods and small prey such as earthworms and snails. While most predatory beetles are generalists, a few species have more specific prey requirements or preferences.
Decaying organic matter is a primary diet for many species. This can range from dung, which is consumed by coprophagous species such as certain scarab beetles (family Scarabaeidae), to dead animals, which are eaten by necrophagous species such as the carrion beetles (family Silphidae). Some of the beetles found within dung and carrion are in fact predatory, such as the clown beetles, preying on the larvae of coprophagous and necrophagous insects.
Adaptations to the environment
Aquatic beetles use several techniques for retaining air beneath the water's surface. Predaceous diving beetles hold air between the abdomen and the elytra when diving. Water scavenger beetles have hairs on their under surface that retain a layer of air against their bodies. Adult crawling water beetles use both their elytra and their hind coxae (the basal segment of the back legs) in air retention (Roughley 2001) while whirligig beetles simply carry a bubble down with them whenever they dive.
Impact on humans
Many agricultural, forestry, and household insect pests are members of this order. Some of these are:
- The Colorado potato beetle, Leptinotarsa decemlineata, is a notorious pest of potato plants. Crops are destroyed and the beetle can only be treated by employing expensive pesticides, many of which it has begun to develop immunity to. As well as potatoes, suitable hosts can be a number of plants from the potato family (Solanaceae), such as nightshade, tomato, aubergine and capsicum.
- The bark beetles Hylurgopinus rufipes and Scolytus multistriatus, the elm leaf beetle, Pyrrhalta luteola, and other beetles attack elm trees. The bark beetles are important elm pests because they carry Dutch elm disease as they move from infected breeding sites to feed on healthy elm trees. The spread of the fungus by the beetle has led to the devastation of elm trees in many parts of the Northern Hemisphere, notably in Europe and North America.
- The death watch beetle, Xestobium rufovillosum, (family Anobiidae) is of considerable importance as a pest of older wooden buildings in Britain. It attacks hardwoods such as oak and chestnut, always where some fungal decay has taken or is taking place. It is thought that the actual introduction of the pest into buildings takes place at the time of construction.
- Both the larvae and adults of some lady beetles (family Coccinellidae) are found in aphid colonies. Other lady beetles feed on scale insects and mealybugs. If normal food sources are scarce they may feed on other things, such as small caterpillars, young plant bugs, aphid honeydew, and plant nectar.
- Ground beetles (family Carabidae) are common predators of many different insects and other arthropods, including fly eggs, caterpillars, wireworms, many and others.
- Plant-feeding beetles are often important beneficial insects, controlling problem weeds. Some flea beetles of the genus Aphthona feed on leafy spurge, a considerable weed of rangeland in western North America.
Some farmers develop w:beetle banks to foster and provide cover for beneficial beetles.
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Emerald ash borer
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Drugstore beetle Stegobium paniceum
- Poul Beckmann, Living Jewels: The Natural Design of Beetles ISBN 3-7913-2528-0
- Arthur V. Evans, Charles Bellamy, and Lisa Charles Watson, An Inordinate Fondness for Beetles ISBN 0-520-22323-3
- Entomological Society of America, Beetle Larvae of the World ISBN 0-643-05506-1
- David Grimaldi, Michael S. Engel, Evolution of the Insects ISBN 0-521-82149-5
- Ross H. Arnett, Jr. and Michael C. Thomas, American Beetles (CRC Press, 2001-2). ISBN 0-8493-1925-0
- K. W. Harde, A Field Guide in Colour to Beetles ISBN 0-7064-1937-5 Pages 7-24
- White, R.E. 1983. Beetles. Houghton Mifflin Company, New York, NY. ISBN 0-395-91089-7
- Roughley, R.E. 2001. Haliplidae. pp. 138-143 in R.H. Arnett Jr. and M.C. Thomas (Eds.), American Beetles, Volume 1. CRC Press, Boca Raton, FL.