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.

My high school biology teacher differentiated this from endoskeleton using the following: if it goes "crunch squish", it has an exoskeleton. If it goes "squish crunch" it has an endoskeleton. This is perhaps the most memorable fact from my education in biology.

Ex`o*skel"e*ton (?), n. [Exo- + skeleton] Anat.

The hardened parts of the external integument of an animal, including hair, feathers, nails, horns, scales, etc.,as well as the armor of armadillos and many reptiles, and the shells or hardened integument of numerous invertebrates; external skeleton; dermoskeleton.

 

© Webster 1913.