Molting in Arthropods: Why It Happens and How

Molting is the process by which an animal sheds part of its body. For arthropods—a group that includes insects, spiders, and crustaceans—this process is called ecdysis. These creatures possess a rigid exoskeleton that provides both physical protection and structural support. Because this outer shell is unyielding and cannot expand, molting is necessary for growth.

The Purpose of Molting

The most direct reason for molting is to allow for growth. An arthropod’s inflexible exoskeleton cannot expand, so the animal must shed its current shell and form a new, larger one. Without ecdysis, the creature’s size would be permanently limited.

Beyond simple enlargement, molting serves other functions. It is a mechanism for regeneration, allowing an arthropod to replace a lost or damaged limb. A new limb can form in a compressed state underneath the old exoskeleton, emerging and expanding during the shedding process. Molting is also fundamental to metamorphosis, the transformation that some insects undergo from a larval stage to a winged adult.

The Molting Process

The molting cycle is a complex sequence of events that begins long before the old exoskeleton is cast off. The initial preparatory stage is known as proecdysis. It starts with apolysis, the separation of the epidermis from the inner surface of the old exoskeleton. The separated epidermis then begins to secrete a new, soft cuticle that will become the new exoskeleton’s outer layer.

Following apolysis, the space between the old and new cuticle fills with a molting fluid containing inactive enzymes. These enzymes, primarily chitinases and proteases, are activated and begin to digest the inner layers of the old exoskeleton. The arthropod absorbs these digested materials, recycling the nutrients to help form the new cuticle. This digestion thins the old shell, leaving behind the indigestible outer layer.

The act of shedding the weakened exoskeleton is called ecdysis. To break free, the arthropod increases its internal pressure by taking in air or water, causing its body to swell. This pressure and muscular contractions force the old exoskeleton to split along predefined lines of weakness. The animal then pulls its body and appendages out of the old shell, called the exuviae.

Immediately after emerging, the arthropod enters post-ecdysis, a stage where it is soft and enlarged. The new exoskeleton has not yet hardened, so the animal inflates its body to stretch the new cuticle to its maximum size. Over the next few hours to days, sclerotization occurs as protein molecules cross-link, causing the exoskeleton to harden and darken.

Hormonal Regulation

The molting cycle is regulated by hormones. The process begins in the brain, where neurosecretory cells respond to cues like body size or changes in day length. These cells release Prothoracicotropic hormone (PTTH), which travels through the hemolymph, the arthropod equivalent of blood, to a gland in the thorax.

This gland, the prothoracic gland, is stimulated by PTTH to produce and release a steroid hormone called ecdysone. As the principal molting hormone, its rising concentration in the hemolymph signals epidermal cells to begin apolysis and secrete the new cuticle.

Another hormone, Juvenile Hormone (JH), determines the molt’s outcome. When JH levels are high, the ecdysone-induced molt results in a larger larval stage, or instar. As the arthropod matures, JH production decreases. When ecdysone acts with low or absent levels of JH, the molt leads to a pupa or a sexually mature adult, completing metamorphosis.

Vulnerability and Recovery

Immediately following ecdysis, the arthropod is in the teneral stage. Its body is soft and the new exoskeleton offers little protection, making it susceptible to predators and environmental hazards. The animal’s movement is also limited as its muscles are attached to the soft cuticle, often forcing it to remain hidden while it hardens.

This stage carries the risk of dyscdysis, an incomplete or failed molt where an arthropod becomes trapped in its old exoskeleton, leading to deformity or death. This failure can be caused by insufficient humidity, physical injury, or illness.

The shed exuviae is not always abandoned. Many arthropods, from crickets to tarantulas, consume their old shed. This behavior is a practical strategy for reclaiming nutrients like proteins and calcium from the old exoskeleton, which aids in hardening the new shell.

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