Spiders shed their skin through a process called molting or ecdysis. This biological event is necessary for their growth and development. Unlike humans, spiders possess a rigid exoskeleton that cannot expand. Molting allows them to periodically replace this non-living shell with a larger one, enabling them to increase in size.
The Molting Process
The molting process begins before the old exoskeleton is shed. The spider’s body prepares by absorbing part of the old cuticle and secreting a new, soft exoskeleton underneath. This new exoskeleton is folded, allowing for future expansion. A molting fluid is also secreted between the old and new layers, creating a gap that aids separation.
To shed the old skin, spiders increase their heart rate, pumping hemolymph (their blood-like fluid) from the abdomen into the cephalothorax, the fused head and thorax. This hydraulic pressure expands the cephalothorax, causing the old exoskeleton to crack along the back of the carapace. The spider then flexes its muscles and wiggles its chelicerae, palps, and eight legs out of the old shell. The entire process can range from minutes to days, depending on the spider’s size and species.
Why Spiders Shed Their Skin
The spider’s rigid exoskeleton, composed mainly of chitin and protein, provides structural support and protection. Since this external skeleton cannot stretch, molting is essential for a spider to grow from a small spiderling into a larger, mature individual.
Beyond growth, molting serves other functions. It allows spiders to repair damaged limbs or regenerate lost appendages. If a spider loses a leg, a smaller version can form, growing larger with each subsequent molt until it becomes indistinguishable. Molting also replaces worn or damaged parts of the exoskeleton, maintaining the spider’s protective outer layer.
Vulnerability and Recovery After Molting
Immediately after molting, a spider is vulnerable. Its new exoskeleton is soft and flexible, offering little protection against predators. Many species seek a safe, secluded location or hang upside down on a silk line to remain out of reach during this period.
The new exoskeleton hardens through sclerotization, where proteins in the cuticle cross-link, leading to increased stiffness and dehydration. During this recovery phase, which can last from hours to weeks for larger spiders, the spider remains sluggish and does not eat. It pumps its legs to expand the new, soft exoskeleton to its full size before it completely hardens.
Common Molting Issues and Identifying Shed Skin
Molting is a process where complications can arise. An issue is a spider getting stuck in its old skin, which can be fatal. Factors like insufficient humidity, injury, or poor health can contribute to difficult molts. Disturbing a molting spider can also cause it to freeze during extraction, leading to its new exoskeleton hardening before it is fully free.
Shed spider skin, known as an exuvia, is mistaken for a dead spider. However, distinct differences exist. An exuvia is translucent, hollow, and lightweight, easily moved by a gentle puff of air. It appears as a complete, ghost-like replica of the spider, with the carapace (top part of the cephalothorax) popped off.
In contrast, a dead spider is heavier, opaque, and has its legs curled tightly underneath its body due to a loss of hydraulic pressure. Finding an exuvia is a positive sign, indicating the spider has successfully grown.