While crustaceans are the dominant group of arthropods in marine environments, hexapods, including insects, rule the
land. Their dominance among animals is evident in their numbers of species, with over 750,000 described, as well as biomass.
Hexapods include three orders of wingless arthropods (Collembola, Protura, Diplura), as well as the insects. The former orders are soil or litter dwellers. Collembola (springtails) are perhaps the most abundant arthropods on earth. Proturans are very small, pale arthropods that are rarely encountered. Diplurans include a few families of larger, pale arthropods that are frequently found in moist soils. The majority of hexapod species are insects, many of which are winged as adults.
The most distinctive feature of the hexapods is the reduction in walking appendages to six, with three body segments consolidating to form the thorax, which provides much of the locomotory ability of the animals. (This is in contrast to other arthropods, most of which have more than three pairs of legs.)
Insecta - Includes superorders Endopterygota, Exopterygota and Apterygota
Entognatha - Includes orders Collembola, Diplura and Protura.