Most insects are small-sized animals. The largest ones are Phasmid, with a body length of 30 cm, and the fist-sized Atlas andFlower beetles. When Arthropod, their ancestor, transitioned from underwater to land habitats there was a limit to the strength of their exoskeleton. In addition, due tocirculatory and respiratory organ mechanisms, there occurred difficulty in circulating blood and oxygen to every corner of the body. These are considered to be key factors asto why insects did not grow in size.