Beetle

2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and Fish

iBeetles
Colorado potato beetle, Leptinotarsa decemlineata
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Class: Insecta Subclass: Pterygota Infraclass: Neoptera Superorder: Endopterygota Order: Coleoptera Linnaeus, 1758
Suborders
Adephaga Archostemata Myxophaga Polyphaga See subgroups of the order Coleoptera

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. This is why J. B. S. Haldane, a Scottish geneticist, 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 Fabr, 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.

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 beetles 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 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 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 (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.

Although beetle eggs are generally very small, their size, shape, colour, and content vary extensively among species, as is generally the case for most sexually reproducing species. A single female may lay from several dozen to several thousand eggs during its life time. 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 leafs (e.g. Colorado potato beetle), or individually attached (e.g. mungbean beetle and other seed borer) or buried in the medium (e.g. carrot weevil).

The larvae of beetles are usually the principal feeding stage of the 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 (most metallic wood-boring beetles 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. 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 endopterygote insects, 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 colour 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:

• Polyphaga is the largest suborder, containing more than 300,000 described species in more than 170 families, including rove beetles (Staphylinidae), scarab beetles (Scarabaeidae), blister beetles (Meloidae), stag beetles (Lucanidae), and true weevils (Curculionidae). These beetles can be identified by the cervical sclerites (hardened parts of the head used as points of attachment for muscles) absent in the other suborders.
• Adephaga contains about 10 families of predatory beetles, includes ground beetles (Carabidae), predacious diving beetles (Dytiscidae) and whirligig beetles (Gyrinidae). In these beetles the testes are tubular and the first abdominal sternum (a plate of the exoskeleton) is divided by the hind coxae (the basal joints of the beetle's legs).
• Archostemata contains four families of mainly wood-eating beetles, including reticulated beetles (Cupedidae) and telephone-pole beetles (Micromalthidae).
• Myxophaga contains about 100 described species in four families, mostly very small, including skiff beetles (Hydroscaphidae) and minute bog beetles (Sphaeriusidae).

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. 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, and the elm leaf beetle, Pyrrhalta luteola, 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.
• Asian long-horned beetle
• Citrus long-horned beetle

Beneficial organisms

• Both the larvae and adults of some lady beetles (family Coccinellidae) are found in aphid colonies. Other lady beetles feed on scale insects. 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, 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:

• The Coleopterist (United Kingdom beetle fauna)
• The Coleopterists Bulletin (published by The Coleopterists Society)

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

Varied carpet beetle	Lady beetle	Tiger beetle	South African dung beetle
Emerald ash borer	Flea beetle	Japanese beetle	Colorado potato beetle larvae
Spotted flower chafer	Prionus californicus	Pelidnota punctata	Bess beetle
Longhorn beetle Parandra brunnea	Acrocinus longimanus	Drugstore beetle Stegobium paniceum	A weevil
A Hispid	Rhinoceros beetle and a coin	Stag beetle	A shiny blue scarab beetle
Anthia sexguttata a carabid beetle