The exoskeleton is the reason that
insects have such a
diversity of shapes. The exoskeleton can be molded into any shape required, from the
horns of some
beetles, to the
wings of butterflies. (Yes, wings are in fact exoskeleton, made into that shape).
However, while creatures with an endoskeleton can grow, the rigorousness of the exoskeleton does not allow any bodyily change while it is rigid. That is why ecdysis (moulting) takes place. The insect sheds its outer exoskeleton, only to find a more supple one underneath, into which it grows.