Nobody knows exactly how many insect species exist in the world since many species are not yet known. In fact, it is possible that there are more unknown than known insect species! Estimates range from 5 million to 80 million species. While many scientists have estimated that the order Coleoptera includes the most species of any insect order -- no one knows this for sure. In order to make sense of the vast diversity of insects, we must first define what an insect actually is. Insects are animals of the phylum Arthropoda. In order to be considered distinct from other arthropods, adult insects are characterized by one pair of antennae, three body regions (head, thorax, abdomen), and three pairs of legs. Many adult insects have two pairs of wings attached to the thorax. In the fly to the right the second pair of wings is modified to form the haltere.

A species scape showing relative species diversity. As shown by the size of the fly, insects are more diverse than any other group. Figure modified from P. J. Gullan & P. S. Cranston (1994), The Insects: An Outline of Entomology. Chapman and Hall. A diagram of an insect body plan. Drawing courtesy of Lori Grove.