Chitin armours insects, colours butterflies, and makes a beetle unusually white
About a billion tonnes of chitin are made in the living world every year, second only to cellulose among natural polysaccharides. It forms crab shells and mushroom cell walls, and when built into microscopic lattices it creates the shimmering colours of butterfly wings.
Chitin is essentially a variant of cellulose. Its building block is N-acetylglucosamine, a glucose derivative carrying nitrogen, and the units link in the same way as cellulose's glucose. The swap of one hydroxyl group for an acetyl-amine group allows extra hydrogen bonding between neighbouring chains, making the material tougher. The name comes via French from the Greek khiton, a covering, and Albert Hofmann determined its structure in 1929 using an enzyme extracted from a snail.
Pure chitin is translucent, flexible and tough, as in a caterpillar's soft body wall. Nature usually reinforces it. In insects it combines with a tanned protein called sclerotin to make stiff, light structures like a beetle's wing case. In crustacean and mollusc shells it pairs with calcium carbonate, giving a composite harder than chitin alone and less brittle than pure mineral. Chitin also appears in squid beaks, the rasping tongues of snails, many fungi and even some fish and amphibians.
Its optical tricks are remarkable. Butterfly wing scales stack chitin into gyroid photonic crystals that produce iridescent signals used in mating and foraging, a design engineers study for optical devices. Scarab beetles of the genus Cyphochilus grow scales just five to fifteen micrometres thick, packed with randomly tangled chitin filaments that scatter light so thoroughly they look brilliantly white.
Living things watch for it. Mammals have chitin-degrading enzymes and immune receptors that detect it, especially in the lungs and gut, and plants use dedicated receptors, first cloned in 2006, to switch on defences. Commercially it comes from crab, shrimp and lobster shells, and treating it with sodium hydroxide produces chitosan, explored for wound healing, drug delivery and biodegradable plastic. The oldest intact chitin found so far, preserved in amber, is about 25 million years old.
Source: Chitin
