An exoskeleton is a rigid external covering that provides support and protection for an animal’s body. Unlike an internal skeleton, this outer structure gives shape and safeguards internal organs. It is a defining feature for many invertebrates, serving as a protective barrier.
Structure and Composition
An exoskeleton is a complex, layered structure that encases the animal. Its primary component is chitin, a tough, flexible polysaccharide. This chitin forms fibrous chains within a matrix of proteins, creating a robust biological composite material. In many arthropods, such as insects and arachnids, these proteins are hardened through a process called sclerotization, providing rigidity.
For crustaceans like crabs and lobsters, the exoskeleton is further strengthened by the incorporation of minerals, primarily calcium carbonate. Biomineralization deposits crystalline calcium carbonate among the chitin and protein molecules, significantly increasing the exoskeleton’s hardness and resistance to compression. While the main body sections are rigid, areas around joints have thinner, more flexible exoskeletons, allowing for movement.
Key Functions
The exoskeleton serves multiple functions. It provides structural support, giving the animal its defined shape. This external framework acts as a sturdy anchor for muscles, which attach to the inner surface of the exoskeleton. These muscle attachments enable efficient locomotion, allowing for movement of appendages and the entire body.
Beyond support and movement, the exoskeleton offers protection. It acts as a physical barrier, shielding delicate internal organs and soft tissues from predators, physical damage, and environmental hazards. For terrestrial species, it is also a barrier against desiccation, or water loss. This waterproofing capability helps these animals retain moisture, contributing to their adaptability across diverse habitats.
Growth and Molting
Because the exoskeleton is a rigid outer covering, it does not grow or expand with the animal’s internal body. Therefore, for an animal to increase in size, it must periodically shed its old exoskeleton and grow a new, larger one. This process is known as molting or ecdysis. During molting, a new, soft exoskeleton begins to form underneath the old one.
The animal separates from the old cuticle, often by taking in air or water to swell its body, causing the old exoskeleton to split. The animal then wriggles out of its old shell, leaving behind the shed skin, called an exuviae. Immediately after molting, the new exoskeleton is soft and pliable, leaving the animal temporarily vulnerable to predators and environmental stresses. The new exoskeleton gradually hardens and darkens over hours or days, allowing the animal to continue its growth until the next molt. Examples of animals that undergo this process include insects, crustaceans, and arachnids.