Molting, also known as ecdysis, allows arthropods to grow. This shedding of their rigid outer covering is necessary because their external skeleton cannot expand with increasing size. Without this periodic process, an arthropod’s growth would be limited. Molting is a recurring event throughout an arthropod’s life cycle, enabling it to reach maturity and larger dimensions.
The Arthropod Exoskeleton
The arthropod exoskeleton is a tough, external structure that encases the animal’s body and appendages. This outer layer provides structural support, points of attachment for muscles, and protection against predators, physical damage, and water loss. The exoskeleton is primarily composed of chitin, a durable polysaccharide, embedded within a matrix of proteins. In some arthropods, particularly crustaceans, this composite material is further strengthened by the incorporation of calcium carbonate, which adds rigidity and durability.
Despite its many advantages, the exoskeleton is a fixed size and cannot stretch. As an arthropod grows internally, its outer casing becomes a restrictive barrier. To increase in size, the arthropod must periodically shed this unyielding outer layer. This necessity for shedding makes molting an integral and recurring event in the life of all arthropods.
The Molting Process
The molting process is initiated by hormonal signals. During the initial preparation phase, known as pre-molt or apolysis, the arthropod’s epidermis begins to detach from the inner surface of the old exoskeleton. A specialized molting fluid, containing digestive enzymes, is secreted into the space created between the old cuticle and the epidermis. These enzymes dissolve the inner layers of the old exoskeleton, allowing dissolved materials, such as chitin and proteins, to be reabsorbed. Concurrently, the epidermis secretes a new, soft, and flexible cuticle underneath, often folded and pleated, preparing it for future expansion.
Following this preparatory phase is the physical act of shedding, called ecdysis. To facilitate its escape, the arthropod increases its internal body pressure by taking in air or water. This internal pressure, combined with muscular contractions, causes the old exoskeleton to split along specific lines of weakness, characteristic for each species, often along the back or head. The arthropod then extracts itself from the discarded shell, a process that can take several minutes to hours.
The entire molting cycle is regulated by hormones. Ecdysone, a steroid hormone, plays a central role by initiating and coordinating the molting process. This hormone triggers the physiological changes needed for the old cuticle to be degraded and a new one to be synthesized. Additionally, juvenile hormone, particularly in insects, influences the developmental outcome of each molt. Its presence ensures that the arthropod continues to develop through juvenile stages, while its absence can signal the transition to an adult form.
Life After Molting
Immediately after ecdysis, the arthropod emerges with a new exoskeleton that is soft, pale, and flexible. In this teneral, or soft-bodied, state, the arthropod is vulnerable to predators and environmental stresses, as its defense and support system is temporarily compromised. During this brief period, many arthropods seek secluded locations to hide until their new covering hardens.
To achieve growth, the arthropod rapidly expands its body size before the new cuticle stiffens. This expansion is accomplished by pumping fluids, such as hemolymph, or air into its body, stretching the pliable new exoskeleton. Once stretched, the new cuticle undergoes sclerotization. This biochemical process involves the cross-linking of proteins within the chitin matrix, causing the exoskeleton to harden and often darken. The time for full hardening varies by species and environmental conditions, ranging from hours to weeks for larger arthropods. Once rigid, the arthropod can resume normal activities, having accommodated its increased body mass.