Arthropod evolutionary process. During the evolution of arthropods, the number of legs reduced. Definition of leg: The lower extremity of the animals that is in direct contact with the ground in standing or walking. A limb that used especially for supporting the body and for walking and crawling. Modified legs, is not leg. Mantis is 4-legged not 6-legged. 2-legs arthropods will appear on earth in the future. There are four-legged arthropods now. But all arthropods are not four-legged. Centipedes, spiders and insects there are still. There will be two-legged arthropods in the future and all arthropods will not be two-legged. But over time, their number will be increased gradually. 4-legged and 2-legged arthropods are not insects. They are new classes. New class of arthropods is forming and appearance. This new class derived from the class Insecta. The Praying Mantis, the Mantis-flies and the Ochthera are the first species of this new class. The main factor in this evolutionary process is the increase in earth's gravity.