The mechanism of phenolic tanning of insect cuticle, and other extracellular protein structures such as byssus and perisarc, has been supposed to be due to specific covalent cross links. Yet cuticle can be swollen in a strong H-bond breaker, an observation that instantly and irrevocably disproves this theory. Physico-chemical expulsion of water is a much more robust mechanism which accords more closely with experimental observation, and suggests stabilisation mechanisms which would give advantages for technical composite materials.