The cuticle of the larva and the cuticle of the puparium are the same cuticle, before and after modification. Part of the stiffness is due to loss of water. Larval cuticle is about 60% water. Puparium cuticle is 12 percent water. Insect cuticle consists primarily of long protein and chitin chains that can form hydrogen bonds. Hydration allows the protein and chitin chains to move relative to each other in response to stress on the cuticle (flexibility). In the larva, the chains can move relatively freely providing maximum flexibility. In the puparium cuticle, the protein and chitin chains are chemically linked by covalent bonds that do not dissociate in the presence of water. The covalent bonds provide resistance to movement by protein and chitin chains but do not limit movement altogether. The water that is present allows some movement of the chains relative to each other. In the absence of water these chains are frozen in place and cannot move relative to each other. Dried cuticle is brittle and inflexible incapable of many small movements at the molecular scale that can accommodate stress on the cuticle. In dried cuticle stress causes the cuticle to break at its weak points.
*G. Fraenkel and K. M. Rudall. 1940. A Study of the Physical and Chemical Properties of the Insect Cuticle. Proc. R. Soc. Lond. B 1940 129: 1-35.