Overview
The order Coleoptera includes more species than any other order, constituting almost 25% of all known life-forms. About 40% of all described insect species are beetles (about 400,000 species), and new species are discovered frequently. Some estimates put the total number of species, described and undescribed, at as high as 100 million, but a figure of 1 million is more widely accepted. The largest taxonomic family, the Curculionidae (the weevils or snout beetles), also belongs to this order.
The diversity of beetles is very wide. They are found in all major habitats, except marine and the polar regions. They have many classes of ecological effects; there are particular species that are adapted to practically every kind of diet. Some are non-specialist detritus feeders, breaking down animal and plant debris; some feed on particular kinds of carrion such as flesh or hide; some feed on wastes such as dung; some feed on fungi, some on particular species of plants, others on a wide range of plants. Some are generalist pollen, flower and fruit eaters. Some are predatory, usually on other invertebrates; some are parasites or parasitoids. Many of the predatory species are important controls of agricultural pests. For example, beetles in the family Coccinellidae ("ladybirds" or "ladybugs") consume aphids, scale insects, thrips, and other plant-sucking insects that damage crops.
Conversely beetles are prey of various invertebrates and vertebrates including insects, fish, reptiles, birds and mammals. The Coleoptera are not generally serious pests, but they include agricultural and industrial pests, such as the Colorado potato beetle Leptinotarsa decemlineata, the boll weevil Anthonomus grandis, the red flour beetle Tribolium castaneum, and the mungbean or cowpea beetle Callosobruchus maculatus,
Species in the Coleoptera have a hard exoskeleton, particularly on their forewings (elytra, singular elytron). These elytra distinguish beetles from most other insect species, except for the Dermaptera. The hemelytra of Heteroptera have a slight resemblance, but are not the same and their function is largely different.
Like all armoured insects, beetles' exoskeletons comprise numerous plates called sclerites, some fused, some separated by thin sutures. This combines armored defenses with maintaining flexibility. The general anatomy of a beetle is superficially uniform, but specific organs and appendages may vary greatly in appearance and function between the many families in the order, and even more so between the suborders (such as Adephaga) that currently seem increasingly like separate orders in their own right. All insects' bodies are divided into three sections: the head, the thorax, and the abdomen, and the Coleoptera are no exception. Their internal morphology and physiology also resemble those of other insects.
Beetles are endopterygotes; they undergo complete metamorphosis, a biological process by which an animal physically develops after a birth or hatching, undergoing a series of conspicuous and relatively abrupt change in its body structure. Males may fight for females in various ways, and in such species there tends to be marked sexual dimorphism.
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