The primary protein used in the exoskeletons of arthropods, such as insects, crustaceans, and spiders, is chitin.

Chitin is a tough, flexible, and resilient polysaccharide that provides structural support and protection. It's similar to cellulose in plants but with a nitrogen-containing group.

While chitin is the main component, other proteins like sclerotin and resilin also play a role in the exoskeleton's structure and function. Sclerotin is a hardened protein that adds rigidity, while resilin is an elastic protein that allows for flexibility.