One subclass of the class Insecta, the other being Pterygota. They are wingless insects. There is little or no metamorphosis; the young stages, which are usually called nymphs, resemble the adults. The subclass Apterygota contains four orders: the most often seen are bristletails (Thysanura) and springtails (Collembola), the others are Diplura and Protura.