Insects are the most diverse group of organisms, in terms of numbers of species, dominating terrestrial habitats. more than 30 orders of insects have been described, with the order coleoptera being the largest. classification is based on traits such as wings and mouthparts. all insects have a three-part body plan consisting of a head, thorax, and abdomen; three pairs of walking legs; and one or two pairs of wings. the chart below indicates defining characteristics for eight of the more than 30 orders of insects.