The mandibles of the larval jewel beetle are as hard as some stainless steels by sheathing chitin fibers in protein and cross-linking.

Larval jewel beetles spend up to five years boring through hardwood before metamorphosizing into adult beetles and emerging. The mandibles they produce must be strong enough to chew through the tough acacia wood. Most arthropods and invertebrates incorporate minerals and transition metals into such structures that demand extreme strength and hardness (e.g., beaks, jaws, shells) and this addition has been long considered crucial to their physical properties. However, the larval jewel beetle's mandible is stronger than most metal-laden biomaterials yet contains only carbon-based, organic materials. Amazingly, this material is composed of fibers of crystalline chitin sheathed in proteins that cross-link and harden.

References

"The arthropod cuticle is a remarkable and versatile biological material commonly composed of chitin and proteins…The larval jewel beetle, Pseudotaenia frenchi, [produces a mandible that]...is a form of cuticle which is devoid of transition metals or mineralization...Arthropod cuticle is a biological material that can be soft and flexible, allowing expansion. Alternatively, it can be tanned to a darker, harder and stiffer product, as found in jaws, claws and other regions...Where the cuticle is composed of chitin and proteins, chitin is present as a fibrous phase of crystalline nanofibrils with a variety of proteins present that sleeve and cross-link these fibrils. Sclerotization, or tanning, is the hardening and stiffening of the cuticle...The addition of calcium salts (biomineralization) is well established in the brittle, shell-like carapace of crabs and other crustaceans...When transition metals (e.g. zinc and manganese) and halogens (e.g. chlorine and bromine) are present in non-mineralized cuticle this correlates with an increase in hardness, and often with stiffness. Halogens are integral in systems incorporating transition metals...Transition metals occur in locally hardened cuticle, such as in the cutting or grinding edges of some insect mandibles or the jaws of marine polychaetes...while tanned cuticles of mandibles that lack transition metals perform well, they do not reach levels equivalent to those containing transition metals... [the larval jewel beetle] demonstrates that chitin-based biological materials devoid of biomineralization or metal incorporation can achieve better performance than previously documented." (Cribb et al. 2010:3152)

"The study indicates that a pathway to hardening and sclerotization of cuticle exists which can outperform that achieved through biomineralization or incorporation of transition metals. It demonstrates that arthropod biological materials can achieve better performance than previously considered...sclerotization or tanning of the cuticle is well understood. The amino acid tyrosine is hydroxylated to 3,4-dihydroxyphenylalanine (DOPA) which is decarboxylated to dopamine, which then forms the sclerotization precursors...N-b-alanyldopamine (NBDA)...from excess dopamine...the dark black appearance of the jewel beetle mandibles is more likely to be the result of a high concentration of NBDA taking part in the process of sclerotization...a comparison of larval and adult mandibles from the jewel beetle P. frenchi shows that the manganese-free larval cuticle performs better in terms of hardness than the adult." (Cribb 2010 et al. :3154)

"The larval mandibular cuticle, measured either wet or dry, is harder than many other biological materials noted for their mechanical properties, including the mineralized stone crab carapace. It is also equivalent to the maximal hardness measured for a range of stainless steels." (Cribb et al. 2010:3156)

Journal article
Hardness in arthropod exoskeletons in the absence of transition metalsActa BiomaterialiaFebruary 11, 2010
B.W. Cribb, C.-L. Lin, L. Rintoul, R. Rasch, J. Hasenpusch, H. Huang

Living System/s

Organism
Metallic Wood-boring BeetleBuprestidaeFamily