A class of the phylum Arthropoda, sometimes called the Hexapoda. In fact, Hexapoda is a superclass consisting of both Insecta and the related class Parainsecta (containing the springtails and proturans). Class Insecta is the most diverse group of organisms, containing about 900,000 described species (see table), but there are possibly as many as 5 million to perhaps 20 million actual species of insects. Like other arthropods, they have an external, chitinous covering. Fossil insects dating as early as the Early Devonian have been found.