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.
"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)