Molting, also known as ecdysis, is a biological process where arthropods shed their external skeleton, called an exoskeleton. This recurring event is a defining characteristic of insects, arachnids, and crustaceans, essential for their continued life cycle.
The Fundamental Reason: Growth
Arthropods possess a rigid exoskeleton that cannot expand, posing a challenge for growth. Unlike vertebrates, whose internal skeletons grow with them, arthropods must shed their external shell to increase in size.
Before shedding the old exoskeleton, a new, larger, and soft exoskeleton begins to form underneath. The old skeleton splits, allowing the arthropod to pull itself out. Once free, the arthropod inflates its body, often by taking in water or air, to stretch the new cuticle. This expansion creates room for future tissue growth before the new exoskeleton hardens.
Beyond Growth: Other Vital Functions of Molting
While growth is a primary driver, molting also serves other purposes for arthropods. It enables the repair of exoskeleton damage, such as cracks or punctures. A new, intact exoskeleton replaces compromised sections, restoring the animal’s external protection.
Molting facilitates the regeneration of lost appendages, such as legs or antennae. A new limb can develop beneath the old exoskeleton and appear after shedding. This regenerative capacity can occur over successive molts until the part reaches its normal form. Molting is also associated with metamorphosis in many insects, enabling transformations like the change from a larva to a pupa and then to an adult.
The Molting Process and Its Challenges
Molting is a complex and energy-intensive process with inherent dangers for arthropods. Before shedding, the arthropod separates its old exoskeleton from the underlying epidermal cells, a process called apolysis. Enzymes are then secreted to digest parts of the old cuticle, forming the new exoskeleton.
The actual shedding requires the arthropod to perform muscle contractions and often inflate its body to split and emerge from the old shell. During this phase, and immediately afterward, the arthropod is vulnerable because its new exoskeleton is soft and flexible. This “soft-bodied” state leaves it susceptible to predation and environmental hazards.
Arthropods face risks of getting stuck in their old exoskeleton or suffering deformities during exit. Once the new exoskeleton is fully expanded, it undergoes a hardening process called sclerotization, giving the cuticle rigidity and protection. This hardening can take hours or even days, during which the arthropod often seeks hidden, protected locations.