The process of skin shedding, or molting, is a common biological necessity primarily observed in the phylum Arthropoda, which includes insects, spiders, and crustaceans. This shedding is not merely a repair mechanism but a programmed life stage required for a creature to increase its body size. The technical term for this biological event is ecdysis, and it represents a transformation in the life cycle of these animals.
Why Arthropods Must Shed Their Skin
Arthropods possess an external skeleton, known as an exoskeleton or cuticle, which serves as both armor and an anchor for muscles. This protective layer is made primarily of chitin and cannot stretch or expand. Because the exoskeleton is a rigid, non-living structure, it imposes a limit on the animal’s potential for growth. To increase in size, the arthropod must periodically cast off this restrictive outer casing.
Molting also provides an opportunity to repair damaged structures, such as lost or injured limbs, which can be regenerated over successive sheds. By shedding the old cuticle, the animal forms a new, unblemished layer. This process is the only way for these creatures to achieve the necessary increase in body volume for development.
The Step-by-Step Process of Ecdysis
The process of ecdysis is controlled by hormones, beginning with a surge of the steroid hormone ecdysone. This chemical signal triggers the underlying epidermal cells to separate from the old exoskeleton, a step called apolysis. The epidermal cells then secrete a new, soft cuticle beneath the old one, followed by a specialized molting fluid into the space between the two layers.
The molting fluid contains inactive enzymes that activate once the new outer layer is partially formed, preventing the digestion of the new skin. These enzymes chemically digest and recycle the inner layers of the old cuticle, which are absorbed by the animal’s body. For the final physical release, the arthropod often swallows air or water to increase internal body pressure, causing the old exoskeleton to split along pre-determined lines. The creature then struggles to pull its body and appendages free, leaving the empty shell, or exuviae, behind.
Common Examples of Molting Arthropods
In the class Insecta, examples include the nymphs of cicadas and dragonflies, which must molt many times before reaching their winged adult form. Larval insects, such as caterpillars, also undergo multiple molts as they grow before pupating into a completely different body plan.
Crustaceans, like crabs, lobsters, and crayfish, are well-known for their molting, as their exoskeletons are highly calcified and hard. They often absorb calcium from the old shell before shedding to store it for the rapid hardening of the new shell. Among Arachnids, spiders and scorpions must also molt, typically doing so while hanging upside down or lying on their backs.
The Vulnerable State After Shedding
Immediately after ecdysis, the newly emerged arthropod is in a temporary state known as teneral. The animal is soft-bodied, pale, and susceptible to injury and predation. Its new cuticle is still pliable and has not yet achieved defensive hardness.
The new skin must then undergo sclerotization, often referred to as tanning, which involves the formation of chemical cross-links between protein and chitin molecules. During this period, which can last from minutes to several days depending on the species, the creature must remain hidden. This results in a hardened, fully pigmented, and functional exoskeleton that provides the necessary support and protection for the next stage of life.