Spiders exhibit a unique method of growth. Unlike many creatures, they lack an internal skeleton that expands with their bodies. Instead, their development relies on molting, an adaptation that allows them to increase in size and thrive in diverse environments.
The Fundamental Reason for Shedding
Spiders, like all arthropods, are encased in a rigid external skeleton called an exoskeleton. This outer covering provides structural support and protection, much like a suit of armor. However, this hard shell does not grow as the spider’s body increases in size. Therefore, to accommodate growth, spiders must periodically shed their old exoskeletons in a process known as molting, or ecdysis.
Molting serves several other purposes beyond enabling an increase in size. It allows spiders to repair damage to their existing exoskeleton, which can become worn or injured. Spiders can also regenerate lost limbs, such as legs or pedipalps, during the molting process. A lost limb will often appear smaller and paler initially, gradually regaining full size and color over subsequent molts. The shedding also renews sensory hairs and other external structures, ensuring the spider’s senses remain acute.
The Molting Process Explained
The molting process begins with internal changes. Hormones signal the start of molting, and a new, soft exoskeleton begins to form beneath the old one. This new layer is larger but folded to fit within the existing shell. During this pre-molt stage, spiders often become less active, may stop eating, and sometimes construct a silk mat or hammock for support.
Once the new exoskeleton is ready, the spider secretes a fluid between the old and new layers to separate them. The old exoskeleton then typically splits, often near the cephalothorax, the fused head and thorax region. The spider uses internal pressure, often by pumping hemolymph (spider “blood”) into its cephalothorax, to crack open the old shell and extract itself. This physically demanding process involves the spider slowly wiggling its legs, palps, and fangs free from the old casing, which is left behind as a hollow replica called an exuvia.
Life After the Shed: Vulnerability and Recovery
After shedding, the spider is in a soft, pliable state. The newly revealed exoskeleton is flexible, allowing the spider to expand its body, often by taking in air or fluids, to achieve a larger size. During this period, the spider is highly vulnerable to predators and desiccation because its outer layer has not yet hardened. Many spiders remain hidden in a safe location, such as a burrow or a dense web, until their new exoskeleton stiffens.
The hardening process, known as sclerotization, involves biochemical changes where proteins in the new cuticle become cross-linked, making the shell rigid and durable. This can take anywhere from a few hours to several days or weeks, depending on the spider’s size and species. The fangs are also soft after molting and need time to harden before the spider can feed effectively. Once the new exoskeleton has fully hardened, the spider is larger, better protected, and ready to resume its normal activities, including hunting and movement.