The subphylum Hexapoda (from the Greek for six legs) constitutes the largest (in terms of number of species) grouping of arthropods and includes the insects as well as a few much smaller groups of wingless arthropods closely related to insects: Collembola, Protura, and Diplura. The Collembola (or springtails) are very abundant in terrestrial environments. Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs.