Beetle

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Beetle
Colorado potato beetle
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 species in it than any other order in the entire animal kingdom. Nearly half of all described insect species are classified as beetles, and overall there are about 400,000 known species of beetles—or about one-quarter of all named species in the plant and animal kingdoms. In addition, new species are regularly discovered. Estimates put the total number of beetle species—described and undescribed—at between 5 and 8 million.

The vast numbers of beetles led to the famous quip, perhaps apocryphal, from British geneticist J. B. S. Haldane, who, when asked what one could conclude as to the nature of God from a study of his creation, replied: "An inordinate fondness for beetles" (Gould 1996). Haldane himself was a noted atheist and this quote reflects not only the vast numbers of beetles but also Haldane's skeptical perspective on natural theology.

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 can be agricultural pests, for example the Colorado potato beetle (Leptinotarsa decemlineata), while other species are important controls of agricultural pests, for example the ladybirds (family Coccinellidae) consume aphids, fruit flies, thrips, and other plant-sucking insects that damage crops.

The study of beetles is called coleopterology; its practitioners are coleopterists. There is a thriving industry in the collection of wild caught species by amateur and professional collectors.


Anatomy

The anatomy of beetles is quite uniform. Beetles are generally characterized by a particularly hard exoskeleton, and the hard wing-cases (elytra) that tend to cover the hind part of the body and protect the second wings, the alae. The elytra are not used in flying, but generally must be raised in order to move the hind wings. In some cases, the ability to fly has been lost, characteristically in families such as Carabidae (ground beetles) and Curculionidae (snout beetles and true weevils). After landing, the hind wings are folded below the elytra.

In a few families, both the ability to fly and the wing-cases are absent, with the best known example being the "glowworms" of the family Phengodidae, in which the females are larviform throughout their lives.

The bodies of beetles are divided into three sections, the head, the thorax, and the abdomen, and these in themselves may be composed of several further segments.

The eyes are compound, and may display some remarkable adaptability, as in the case of the Whirligig beetles (family Gyrinidae), in which the eyes are split to allow a view both above and below the waterline. The dorsal appendage aids the beetle in stalking prey.

Like all insects, antennae and legs are both jointed.

Oxygen is taken in via a tracheal system: this takes air in through a series of tubes along the body, which is then taken into increasingly finer fibers. 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.

Development

Larva of the cockchafer (Melolontha melolontha)

Beetles are endopterygotes—a superorder of insects of the subclass Pterygota that go through distinctive larval, pupal, and adult stages, or complete metamorphosis. The larva of a beetle is often called a grub and represents the principal feeding stage of the life cycle.

The eggs of beetles are minute, but may be brightly colored. They are laid in clumps and there may be from several dozen to several thousand eggs laid by a single female.

Once the egg hatches, the larvae tend to feed voraciously, whether out in the open such as with ladybird larvae, or within plants such as with leaf beetle larvae.

As with lepidoptera, beetle larvae pupate for a period, and from the pupa emerges a fully formed beetle or imago.

In some cases, there are several transitory larvae stages; this is known as hypermetamorphosis. Examples include the blister beetles (family Meloidae).

The larval period of beetles varies between species, but can be as long as several years. Adults have an extremely variable lifespan of weeks to years.

Reproduction and parental care

Striped love beetle Eudicella gralli from the forests of Central Africa. The iridescent wing cases are used in marriage ceremonies.

Beetles may display some intricate behavior when mating. Smell is thought to be important in the location of a mate.

Conflict can play a part in the mating rituals, for example, in species such as burying beetles (genus Nicrophorus) where localized 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 egg.

Parental care between species varies widely, ranging from the simple laying of eggs under a leaf to 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.

Diet and behavior

There are few things that a beetle somewhere will not eat. Even inorganic matter may be consumed.

Some beetles are highly specialized in their diet; for example, the Colorado potato beetle (Leptinotarsa decemlineata) almost entirely colonizes plants of the potato family (Solanaceae). Others are generalists, eating both plants and animals. Ground beetles (family Carabidae) and rove beetles (family Staphylinidae) are entirely carnivorous and will catch and consume 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 the scarab beetles (family Scarabaeidae), to dead animals, which are eaten by necrophagous species, such as the carrion beetles (family Silphidae).

Various techniques are employed by many species 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.

Beetles and larvae have a variety of strategies for avoiding being eaten. Many employ simple camouflage to avoid being spotted by predators. These include the leaf beetles (family Chysomelidae) that have a green coloring very similar to their habitat on tree leaves. A number of longhorn beetles (family Cerambycidae) bear a striking resemblance to wasps, thus benefitting from a measure of protection. Large ground beetles by contrast will tend to go on the attack, using their strong mandibles to forcibly persuade a predator to seek out easier prey. Many species, including lady beetles and blister beetles, can secrete poisonous substances to make them unpalatable.

Evolutionary history and classification

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

The four extant (living) suborders of beetle are:

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 extraordinary 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.

Impact on humans

Pests

Damage to beans by larvae of the common bean weevil, Acanthoscelides obtectus

There are several agricultural and household pests represented by the order. These include:

Beneficial beetles

To foster and provide cover for beneficial beetles, some farmers introduce beetle banks (a strip of grass or perennials that provide habitat for insects hostile to pests).

Scarab beetles in Egyptian culture

Ancient Egyptian scene depicting a scarab beetle

Several species of the dung beetles, most notably the Scarabaeus sacer (often referred to as "scarab"), enjoyed a sacred status among the Egyptians, as the creature was likened to the god Khepri. Some scholars suggested 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. 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 to, "do not stand as a witness against me."


Taxonomy

This section classifies the subgroups of the order Coleoptera (beetles) down to the level of families, following the system in Arnett and Thomas' (2001, 2002) two volume set American Beetles. A few subfamilies, tribes and synonyms are shown here where a family has been reclassified.

Order Coleoptera (beetles)

Gallery

References
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