Arthropods, such as insects, spiders, and crustaceans, have a rigid exoskeleton. This external skeleton provides support and protection but does not grow with the animal. To increase in size and undergo biological changes, arthropods must periodically shed their old exoskeleton in a process known as molting, or ecdysis. This process enables growth, repairs damaged structures, and maintains hygiene.
Facilitating Growth
The primary reason arthropods molt is to allow for growth. Their exoskeleton, made of chitin and proteins, is a stiff, unyielding external armor. Unlike the internal skeletons of vertebrates, this outer shell cannot expand as the animal’s body increases in volume. Without molting, growth would be severely restricted, potentially crushing internal organs within the unyielding casing.
During molting, a new, soft cuticle forms underneath the old one. Enzymes digest parts of the old exoskeleton, separating it from the underlying skin. The arthropod then inflates its body, often by taking in air or water, which helps split the old exoskeleton along pre-determined lines. Once free, the arthropod rapidly expands its new body before the fresh exoskeleton hardens and darkens, a process that can take hours to days depending on the species. This allows for a significant increase in size between molts, repeating several times until the arthropod reaches adult size.
Repairing and Renewing Structures
Molting offers arthropods a unique opportunity to repair or regenerate damaged body parts. Lost limbs, antennae, or other appendages can be regrown during the molting process. For instance, a crab that loses a claw might regenerate a new, smaller one with subsequent molts, gradually restoring it to full size over several shedding cycles.
This regenerative capacity is linked directly to the molting cycle, as new appendages develop beneath the old exoskeleton and emerge when it is shed. Beyond large structures, molting also renews worn or damaged sensory structures, such as bristles or eye lenses, which are integral parts of the exoskeleton. The process ensures functional integrity, replacing compromised external components with fresh ones.
Shedding and Defense
Molting also serves a hygienic and defensive purpose. Shedding the old exoskeleton allows the animal to rid itself of parasites, fungi, bacteria, and other microorganisms that may have attached to its outer surface. Accumulated debris or pollutants clinging to the exoskeleton are also removed.
By shedding these burdens, arthropods can maintain a cleaner, healthier body surface. While the newly molted arthropod is temporarily soft and vulnerable to predators, this period is short. Some arthropods may also undergo subtle changes in color or pattern with each molt, offering improved camouflage or playing a role in species recognition and display behaviors.