Elastic Exoskeletons

Some small exoskeletal regions are characterized by a rubberlike elasticity; they can undergo considerable deformation when exposed to mechanical stresses and return to their original shape when unstressed. The amount of energy used for deformation is almost completely recovered during relaxation. Its elasticity is the result of the matrix protein resilin. Resilin-containing ligaments are used for energy storage when a fast release of mechanical energy is needed: for example, in the flight system of insects and in the jumping systems of fleas and click beetles. Most resilin-containing ligaments contain chitin microfibrils, making them inextensible, but readily flexible, but there are some ligaments that consist of nearly pure resilin and are devoid of chitin. Such ligaments can be reversibly stretched to three to four times their unstrained length before breaking. The protein chains in resilin are cross-linked by a mechanism different from that used for the solid cuticle; the chains are linked together by covalent bonds formed between side chains of tyrosine residues during the secretion of soluble resilin from the epidermal cells. The elastic properties of the cross-linked material are due to the flexibility and random coiling of the chain segments between cross-links.

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