What is the outer skeleton of organisms such as spiders and insects called?

The outer skeleton of organisms such as spiders and insects is called an exoskeleton. It is a hardened external body covering that supports the animal, protects internal organs, and provides attachment points for muscles.

In arthropods, the exoskeleton is usually called the cuticle and is built largely from chitin, a tough natural polymer. In insects and spiders, proteins harden and strengthen the chitin through a process called sclerotization. Crustaceans such as crabs and lobsters often reinforce their exoskeletons with calcium carbonate, making them especially rigid.

An exoskeleton cannot expand continuously as an animal grows. Instead, the arthropod periodically sheds its old covering in a process called molting, or ecdysis. A new, initially soft exoskeleton forms underneath; after shedding, it expands and hardens. This vulnerable interval explains why molting can be dangerous.

“Exoskeleton” is broader than “shell”: shells are one kind of external hard covering, while arthropod exoskeletons are jointed and include specialized plates, limbs, sensory structures, and muscle attachments. Humans and other vertebrates generally have an internal skeleton, called an endoskeleton.