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science 07-18-10

Beetles are the group of insects with the largest number of known species. They are classified in the order Coleoptera (from Greek κολεός,koleos, “sheath”; andπτερόν, pteron, “wing”, thus “sheathed wing”), which contains more described species than in any other order in the animal kingdom, constituting about 25% of all known life-forms. About 40% of all described insect species are beetles (about 400,000 species), and new species are frequently discovered. Some estimates put the total number of species, described and undescribed, at as high as 100 million, but 1 million is a more likely figure. The largest family also belongs to this order – the weevils, or snout beetles, Curculionidae.

Beetles can be found in almost all habitats, but are not known to occur in the sea or in the polar regions. They interact with their ecosystems in several ways. They often feed on plants and fungi, break down animal and plant debris, and eat other invertebrates. Some species are prey of various animals including birds and mammals. Certain species are agricultural 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, while other species of beetles 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.

Other characters of this group which are believed to be monophyletic include a holometabolous life cycle; having a prothorax that is distinct from and freely articulating with the mesothorax; the meso- and meta-thoracic segments fusing to form a pterothorax; a depressed body shape with the legs on the ventral surface; the coxae of legs recessed into cavities formed by heavily sclerotised thoracic sclerites; the abdominal sternites more sclerotised than the tergites; antennae with 11 or fewer segments; and terminal genitalic appendages retracted into the abdomen and invisible at rest.

The general anatomy of a beetle is quite uniform, although specific organs and appendages may vary greatly in appearance and function between the many families in the order. Beetles’ 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 and both pairs of wings arise. The abdomen is everything behind the thorax.

Beetles are generally characterised by a particularly hard exoskeletonand 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). The elytra must be raised in order to move the hind flight wings. A beetle’s flight wings are crossed with veins and are folded after landing, often along these veins, and are stored below the elytra.

In some beetles, the ability to fly has been lost. These include someground beetles (family Carabidae) and some “true weevils” (familyCurculionidae), but also 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 glow-worms of the family Phengodidae, in which the females are larviform throughout their lives.


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