Chitin

Definition

A tough, semi-transparent structural polysaccharide composed of N-acetylglucosamine units, forming the primary structural component of arthropod exoskeletons, fungal cell walls, and various invertebrate hard parts. In insects and arachnids, chitin is embedded in a protein matrix (sclerotin) and impregnated with waxes and minerals to create the flexible yet rigid cuticle that provides support, protection, and attachment sites for muscles. Chitin is functionally analogous to keratin in vertebrates and cellulose in plants, and is the second most abundant natural polymer after cellulose.

Etymology

From French chitine, coined 1821 by Henri Braconnot from Greek khitōn 'tunic, undergarment', referring to the enveloping nature of the material.

Example

The rigid wing covers (elytra) of beetles contain dense, heavily sclerotized chitin that protects the delicate hindwings beneath, while the intersegmental membranes retain flexible, lightly tanned chitin to permit movement.

Related Terms

• Cuticle
• Exoskeleton
• Sclerotin
• Ecdysis
• Integument
• arthropod

Usage Notes

Chitin occurs throughout the cuticle but varies in density and degree of sclerotization; heavily armored regions (sclerites) contain more cross-linked chitin-protein complexes, while flexible joints (arthrodial membranes) have looser, hydrated chitin. The term is sometimes misused for the entire exoskeleton—specialists reserve it for the polysaccharide component specifically, distinguishing it from the broader cuticle (which includes proteins, lipids, and cement layers). Chitin content decreases with each molt as enzymes degrade old cuticle during ecdysis.