ecdysis

Ecdysis is the process by which ecdysozoan animals shed their outer covering, such as an arthropod’s cuticle or exoskeleton, to grow and remodel the body. The cycle is tightly coordinated and essential because most of these animals cannot increase in size while their exoskeleton remains intact.

In arthropods, a new cuticle forms beneath the old one. Hormonal signals initiate the molt: a rise in ecdysone (often 20-hydroxyecdysone) triggers the molting program and stimulates epidermal cells to separate from the old cuticle (apolysis) and to digest interior layers to soften the shell. The old cuticle is shed during ecdysis, a phase driven by rhythmic body movements and regulated by hormones such as ecdysis triggering hormone (ETH) and eclosion hormone (EH). After shedding, the animal inflates and the new cuticle expands and hardens through sclerotization.

The molting cycle is commonly described as three phases: pre-ecdysis (pre-molt behavior that loosens the old shell), ecdysis (shedding of the cuticle), and post-ecdysis (expansion and hardening of the new cuticle). Timing is influenced by environmental cues and hormonal signals; in insects, juvenile hormone modulates whether molts produce larval, pupal, or adult forms.

Cuticle composition includes chitin and various proteins; after molting, the new cuticle may begin soft and then rapidly harden. Ecdysis is essential for growth in most arthropods and is a fundamental aspect of ecdysozoan biology.