Spiders do shed their skin, a process known scientifically as molting or ecdysis. This biological event involves the complete removal of the spider’s rigid outer layer, which is called the exoskeleton or cuticle. Molting is not simply a shedding of dead skin cells like in mammals, but the mechanical discarding of the entire external skeletal structure. This process is necessary for the spider to continue its life cycle and is a defining feature of all arthropods.
Why Spiders Must Shed Their Skin
Spiders, like all arachnids, do not possess an internal skeleton but instead rely on a hard, external shell for support and protection. This exoskeleton is primarily composed of chitin and various proteins, forming a stiff armor around the spider’s body. The limitation of this rigid structure is that it cannot stretch or expand to accommodate growth. As the spider consumes food and its internal body mass increases, it quickly outgrows the confines of its current exoskeleton. Without the ability to shed this restrictive outer layer, the spider would be unable to increase in size.
Young spiders, or spiderlings, undergo this process frequently because their growth rate is rapid. The frequency of molting slows down as the spider approaches its mature size. While juveniles may molt every few weeks or months, many adult spiders, particularly the smaller-bodied species (Araneomorphs), cease molting once they reach sexual maturity. Larger, long-lived spiders, like tarantulas (Mygalomorphs), may continue to molt annually throughout their lives to replace damaged tissue or regenerate lost limbs.
The Stages of Ecdysis
The entire molting cycle is a sequence of events initiated by hormonal changes within the spider’s body. The first internal stage is called apolysis, where the spider’s epidermis begins to detach from the old exoskeleton. During this pre-molt period, a new, soft cuticle starts to form directly underneath the old one, and a specialized molting fluid is secreted into the space between the two layers.
The spider then begins to reabsorb nutrients from the inner layer of the old cuticle, while the molting fluid works to separate the remaining hard shell. A spider in this phase often becomes lethargic and stops eating, sometimes retreating to a secure, silken chamber for protection. This preparatory phase can last for days or weeks, depending on the spider’s size and age.
The shedding stage itself, known as ecdysis, relies on internal hydraulic power. The spider increases its blood pressure by pumping hemolymph—the arachnid equivalent of blood—from its abdomen into its cephalothorax. This internal pressure causes the old exoskeleton to split, usually along the sides of the carapace near the head.
Once the split occurs, the spider methodically pulls its legs, pedipalps, and abdomen free from the old casing. This extraction process is slow and often requires the spider to lie on its back or hang from a strand of silk to use gravity effectively.
When the spider finally emerges, its new exoskeleton is noticeably soft, pale, and highly susceptible to damage. This post-molt period requires the new cuticle to harden through a process called sclerotization. The spider uses this brief window of softness to expand its body size by absorbing water or air, stretching the new cuticle to its maximum size before it solidifies. Until the new armor is fully hardened—which can take several hours to a few days—the spider remains immobile, unable to hunt or defend itself effectively.
Identifying the Exuvia
The discarded, shed exoskeleton is technically referred to as the exuvia. This material is often mistaken by observers for the remains of a dead spider, as it retains the exact shape and detailed structure of the original animal. Finding an exuvia is definitive proof that a spider has successfully molted and grown larger.
The exuvia is typically translucent, hollow, and intact, often appearing to be a complete, empty shell of the spider. It includes the casings of all eight legs, the pedipalps, and even the delicate coverings of the fangs (chelicerae) and the eye lenses.
The appearance of the exuvia, particularly its preserved detail, is due to the molting fluid’s action, which digests only the inner layers of the old cuticle. The tough outer layers resist this digestion and are shed completely, maintaining their form. The presence of this formed husk indicates that the spider has successfully navigated this process and is now a larger, newly armored individual.