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Beetles are one of the most diverse groups of insects. Their order, Coleoptera (meaning "sheathed wing"), has more 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. Approximately one out of every five animals on our planet is a beetle (20% of all animal life). Estimates put the total number of species, described and undescribed, at between 5 and 8 million. This is why when J. B. S. Haldane, a Scottish geneticist, was asked what his studies of nature revealed about God, he 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 have a major impact on the ecosystem in three ways: feeding on plants and fungi, breaking down animal and plant debris, and eating other invertebrates. Certain species are agricultural pests, such as the Colorado potato beetle, Leptinotarsa decemlineata, while others are important controls of agricultural pests. For example, lady beetles (family Coccinellidae) consume aphids, fruit flies, thrips, and other plant-sucking insects that damage crops.

Anatomy

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. Like all insects, beetles are segmented organisms, and all three of the major sections of the body may themselves be composed of several further segments, although these are not always readily discernable.

Beetles are generally characterised by a particularly hard exoskeleton and hard forewings (elytra). 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). Elytra must generally 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 cases the ability to fly has been lost, most notably in the ground beetles (family Carabidae) and the true 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 likely 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 Predation, 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 some 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 Cerambycidae and Curculionidae), 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 physically feel out a beetle's environment. Further, they may 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 filiform, clavate, flabellate or genticulate.

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 (Dytiscidae, Haliplidae, many 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.

Physiology

There are few things that a beetle somewhere will not eat; even inorganic matter may be consumed. Some beetles are highly specialised in their diet; for example, the Colorado potato beetle (Leptinotarsa decemlineata) almost exclusively colonizes plants of the potato family (Solanaceae). Others are generalists, eating both plants and animals. Ground beetles (family Carabidae) and rove beetles (family Staphylinidae), among others, are entirely carnivorous and will catch and consume other arthropods and small prey such as earthworms and snails. 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).

Various techniques are employed for retaining both air and water supplies. For example, predaceous diving beetles (family Dytiscidae) employ a technique of retaining air, when diving, between the abdomen and the elytra.

Development

Beetles are endopterygotes with complete metamorphosis.

The eggs of beetles are very small. Some may be brightly coloured. A single female may lay from several dozen to several thousand eggs, placing them in clumps.

The larva of a beetle is called a grub, and often represents the principal feeding stage of the life-cycle. Larvae tend to feed voraciously once they emerge from their eggs. Some feed externally on plants, such as those of certain leaf beetles and lady bird beetles, while others feed within their food sources. The larval period varies between species but can be as long as several years. 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. 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 Lepidoptera, beetle larvae pupate for a period of time, and from the 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.

Reproduction

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 fertilise the egg.

Parental care

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.

There are other notable ways of caring for the eggs and young, such as those employed by leaf rollers, who bite sections of leaf causing it to curl inwards and then lay the eggs, thus protected, inside.

Predation

Beetles and their larvae have a variety of strategies to avoid being eaten, for example using camouflage to avoid being spotted by predators. These include the leaf beetles (family Chysomelidae) that have a green colouring very similar to their habitat on tree leaves. More complex camouflage also occurs, as with some weevils (family Curculionidae), where various coloured scales or hairs cause the beetle to resemble bird dung.

A number of longhorn beetles (family Cerambycidae) bear a striking resemblance to wasps. This defense, known as mimicry, can be found to a lesser extent in other beetle families, such as the scarab beetles.

Many species, including lady beetles and blister beetles, can secrete poisonous substances to make them unpalatable. These same species often exhibit aposematism, where bright or contrasting color patterns warn away potential predators.

Large ground beetles will tend to go on the attack, using their strong mandibles to forcibly persuade a predator to seek out easier prey.

Evolutionary history and classification

Beetles entered the fossil record during the Lower Permian, about 265 million years ago.

The four extant suborders of beetle are these:

These suborders diverged in the Permian and Triassic. Their phylogenetic relationship is uncertain, with the most popular hypothesis being that Polyphaga and Myxophaga are most closely related, with Adephaga an outgroup to those two, and Archostemata an outgroup to the other three.

The large number of beetle species poses special problems for classification, with some families consisting of thousands of species and needing further division into subfamilies and tribes.

See the article subgroups of the order Coleoptera for a complete list of families and * for a complete list of World families and subfamilies.

Impact on humans

Pests

Many agricultural, forestry, and household pests are represented by the order. These include:

  • The Colorado potato beetle, Leptinotarsa decemlineata, is a notorious pest of potato plants. Adults mate before overwintering deep in the soil, so that when they emerge the following spring, females can lay eggs immediately, once a suitable host plant has been found. As well as potatoes, a suitable host can be a number of other plants from the potato family (Solanaceae) such as nightshade, tomato, aubergine and capsicum. Crops are destroyed and the beetle can only be treated by employing expensive pesticides, many of which it has begun to develop immunity to.
  • The elm bark beetles, Hylurgopinus rufipes, elm leaf beetle Pyrrhalta luteola and Scolytus multistriatus (in the family Scolytidae) attack elm trees. They are important elm pests because they carry Dutch elm disease (the fungus Ophiostoma ulmi) 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 North America and Europe.
  • The death watch beetle Xestobium rufovillosum is of some considerable importance as a pest of wooden structures in older buildings in Britain. It attacks hardwoods such as oak and chestnut, and 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.
  • Asian long-horned beetle
  • Citrus long-horned beetle

Beneficial organisms

  • The larvae of lady beetles (family Coccinellidae) are often found in aphid colonies. While both adult and larval lady beetles found on crops prefer aphids, they will, if aphids are scarce, use food from other sources, such as small caterpillars, young plant bugs, aphid honeydew, and plant nectar.
  • Large ground beetles (family Carabidae) are predators of caterpillars and, on occasion, adult weevils, whereas smaller species attack eggs, small caterpillars, and other pest insects.

Some farmers introduce beetle banks to foster and provide cover for beneficial beetles.

Scarab beetles in Egyptian culture

Several species of the dung beetles, most notably Scarabaeus sacer (often referred to as "scarab"), enjoyed a sacred status among the ancient Egyptians, as the creatures were likened to the god Khepri. Some scholars suggest that the people's practice of making mummies was inspired by the brooding process of the beetle.

Many thousands of amulets and stamp seals have been excavated that depict the scarab. In many artifacts, the scarab is depicted pushing the sun along its course in the sky. During and following the New Kingdom, scarab amulets were often placed over the heart of the mummified deceased. The amulets were often inscribed with a spell from the Book of the Dead which entreated the heart, "Do not stand as a witness against me."

Study and Collection

The study of beetles is called coleopterology, and its practitioners are coleopterists. See the list of notable coleopterists for more information.

Coleopterists have formed organizations to facilitate the study of beetles. Among these is The Coleopterists Society, an international organization based in the United States.

Research in this field is often published in peer-reviewed journals specific to the field of coleopterology, though journals dealing with general entomology also publish many papers on various aspects of beetle biology. Some of the journals specific to beetle research are:

There is a thriving industry in the collection of beetle specimens for amateur and professional collectors. Some countries have established laws governing or prohibiting the collection of certain rare (and often much sought after) species.

Gallery

Image:Anthrenus verbasci 1 (aka).jpg|Varied carpet beetle Image:Asian multicolored lady beetle.jpg|Lady beetle Image:Cicindela sexguttata - six-spotted tiger beetle - desc-iridescent in sunlight on ground.jpg|Tiger beetle Image:Dungbeetle.jpg|South African dung beetle Image:Emerald ash beetle.jpg|Emerald ash borer Image:Aphthona flava flea beetle.jpg|Flea beetle Image:Jbeetle.jpg|Japanese beetle Image:Potato beetle larvae.jpg|Colorado potato beetle larvae Image:Spotted flower chafer.jpg|Spotted flower chafer Image:20050702 - Beetle (Cerambycidae Coleoptera) - California Prionus.JPG|Prionus californicus Image:Beetle August 27-2005..png|Pelidnota punctata Image:Beetle-Hissing.jpg|Bess beetle Image:Longhorn Parandra brunnea 1.jpg|Longhorn beetle Parandra brunnea Image:Harlequin.JPG|Acrocinus longimanus Image:Drugstore_beetle_04.jpg| Drugstore beetle Stegobium paniceum Image:Weevil bangalore.jpg |A weevil Image:Hispid beetle.jpg|A Hispid Image:Rhinoceros_beetle_coin.jpg|Rhinoceros beetle and a coin

References

  • Poul Beckmann, Living Jewels: The Natural Design of Beetles ISBN 3791325280
  • Arthur V. Evans, Charles Bellamy, and Lisa Charles Watson, An Inordinate Fondness for Beetles ISBN 0520223233
  • Entomological Society of America, Beetle Larvae of the World ISBN 0643055061
  • David Grimaldi, Michael S. Engel, Evolution of the Insects ISBN 0521821495
  • Ross H. Arnett, Jr. and Michael C. Thomas, American Beetles (CRC Press, 2001-2)
  • K. W. Harde, A Field Guide in Colour to Beetles ISBN 0706419375 Pages 7-24

See also

  • Heteroptera - insect suborder that is superficially similar to beetles

External links

Beetles | Insects | Endopterygota

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This article is licensed under the GNU Free Documentation License. It uses material from the "Beetle".

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