Despite the great variety within this class, three states can always be recognized in insects. Generally, they have a tracheal respiratory system and a single pair of antenna, and the body somites are grouped into three functional tagmata: the head, thorax, and abdomen (and generally one or two pairs of wings). This class, the largest group of animals, contains over 750.000 described species. The great success of this group is reflected in the huge amount of species and individuals and their great adaptive radiation. They occupy virtually every niche on land. For most insects, the main factor contributing to their success is their ability to fly.