Why do arthropods need to molt as they grow?

Arthropods must molt because their exoskeleton—a rigid, protective outer covering—does not grow or stretch as the animal increases in size. To accommodate growth, arthropods undergo a process called molting (or ecdysis), which involves periodically shedding this restrictive shell.1234

During this process, the animal secretes a new, soft, and flexible exoskeleton beneath the old one. Once the old shell splits and is discarded, the arthropod expands its body—often by taking in air or water to increase internal pressure—before the new cuticle hardens through a process known as [sclerotization].15

Because the exoskeleton is nonliving and rigid, this periodic replacement is the only way for an arthropod to grow in length and volume. This requirement also serves as a major limiting factor for how large these animals can become, as the process of molting leaves them soft, vulnerable to predators, and physically challenged by their own size.132