There are more than 300 000 named species of Coleoptera in the world. About 40% of all insects and 30% of all animals are beetles; the number of beetle species exceeds that of fungi or of vascular plants and is more than 6 times that of vertebrates. There have been about 20 000 species described from Australia, but several thousand more are known in collections and the true number may exceed 30 000. Curculionidae is the largest family, with more than 6000 Australian species. Scarabaeidae and Chrysomelidae each has about half that number, Carabidae about 2500, and Staphylinidae, Tenebrionidae and Cerambycidae each between 1200 and 1500 species.
It is generally conceded that the single most important factor contributing to the success of Coleoptera is the development of sclerotised fore wings or elytra, resulting in the protection of the folded hind wings when not in use and permitting the occupation of enclosed spaces and cryptic habitats by adults. This was accompanied by a series of other morphological changes, including the flattening of the body, the recession of fore and mid coxae into cavities, and the general reduction of exposed membrane, all of which contributed to increased survival in cryptic habitats by protecting the body from both predation and infection by microorganisms. Another important result of elytral development was the formation of a series of interlocking devices joining the elytra, pterothorax and abdomen, so that an enclosed space, the
, was formed beneath the elytra, enclosing the metathoracic and abdominal spiracles and reducing water loss through transpiration (Cloudsley-Thompson 1965).
Although the order must have originated in the warm, humid environments of the Carboniferous, the first significant radiation of beetles apparently took place in more xeric and temperate environments, as represented by Permian fossils from the Ural Mountains and Siberia. Reviews of the Mesozoic history of beetles may be found in Arnoldi
(1977), Crowson (1975) and Ponomarenko (1969, 1986).
Beetles are generally thought to be most closely related to the neuropteroid complex but not to any single order within that group. The position of the Strepsiptera is questionable, but they are usually considered to be the sister group of Coleoptera on the basis of hind wing dominance, free prothorax, heavily sclerotised abdominal sternites and furcate metendosternite. Similarities based on the 1st instar larvae are certainly homoplasious.
The most recent textbook dealing exclusively with Coleoptera is that of Crowson (1981), which covers in detail most aspects of beetle biology. Basic references on beetle classification include those of Crowson (1955, 1960, 1967) and Lawrence and Newton (1982). Lawrence (1982) gives brief descriptions of families and higher taxa based on both adults and larvae. The Australian fauna has been treated by Britton (1970), and regional manuals on Australian beetles are being produced by Matthews (1980--87) for South Australia and Moore (1980--89) for south-eastern Australia. Hawkeswood (1987a) includes colour photographs and popular accounts of 176 species.
Endopterygote Neoptera, with mesothoracic wings modified into more or less hardened, non-folded, rigid elytra, which usually meet edge to edge in a straight line when at rest and partly or wholly cover the hind wings and abdomen; metathoracic wings, when developed, membranous, folded, and alone used for propulsion in flight; mouth-parts almost always mandibulate (rarely suctorial or reduced); prothorax well developed, almost always free, and forming with the head a distinct fore body, contrasting with hind body, which consists of mesothorax, metathorax and abdomen; body usually more or less depressed, so that coxae and pleural regions lie ventrally; fore coxae variable in shape and usually recessed into cavities formed by the sternum and notum (Polyphaga) or sternum and pleura; mesothorax usually reduced; mid coxae usually globular and more or less recessed into cavities formed by the meso- and metasterna and usually the mesopleura; mesoscutellum usually visible from above between elytral bases; metasternum usually well developed, with single endosternite originating at or near its posterior edge; hind coxae usually transverse, often less deeply recessed and sometimes with posterior excavations which receive the femora; abdominal sternites almost always more heavily sclerotised than tergites, the basal one or two usually reduced and concealed beneath the hind coxae; terminalia usually telescoped into apex of abdomen. Larva with or without thoracic legs; with a distinct head capsule, antennae and mandibulate mouth-parts (rarely suctorial, endognathous or reduced); very rarely with abdominal prolegs. Pupa adecticous and exarate or rarely obtect.