Beetles are a group of insects that form the order Coleoptera /koʊliːˈɒptərə/. The word "coleoptera" is from the Greek κολεός, koleos, meaning "sheath"; and πτερόν, pteron, meaning "wing", thus "sheathed wing", because most beetles have two pairs of wings, the front pair, the "elytra", being hardened and thickened into a shell-like protection for the rear pair and the beetle's abdomen.
The Coleopterans include more species than any other order, constituting almost 25% of all known types of animal life forms. About 450,000 species of beetles occur – representing about 40% of all known insects. Some estimates put the total number of species, described and undescribed, at as high as 100 million, but a figure of one million is more widely accepted. Such a large number of species poses special problems for classification, with some families consisting of thousands of species and needing further division into subfamilies and tribes. This immense number of species allegedly led evolutionary biologist J. B. S. Haldane to quip, when some theologians asked him what could be inferred about the mind of the Creator from the works of His Creation, that God displayed "an inordinate fondness for beetles".