Spiders possess an exoskeleton, a rigid external covering that acts as their skeleton. This feature is fundamental to arthropods, a group of invertebrates that includes spiders, insects, and crustaceans. Unlike humans who have internal bones, spiders rely on this outer structure for support and protection.
Understanding the Exoskeleton
A spider’s exoskeleton is a complex, multi-layered structure that encases its body. It is primarily composed of chitin, a tough, flexible carbohydrate, combined with various proteins. These components are arranged in successive layers, similar to plywood, which contributes to the exoskeleton’s strength. Spiders harden their exoskeleton through sclerotization, a process involving the cross-bonding of chitin-protein chains.
The exoskeleton is not a single, seamless shell; instead, it is segmented and articulated, especially at the joints, to allow for movement. The outer layer, called the epicuticle, also contains waxes in terrestrial arthropods, which plays a role in reducing water loss.
How Spiders Benefit from Their Exoskeleton
It offers structural support, allowing spiders to maintain their body shape and counteract gravity, particularly important for their eight legs. This rigid casing also acts as a robust defense, protecting the spider from physical injury and potential predators.
Muscles attach to the inner surface of the exoskeleton, enabling movement. While muscles contract to pull limbs inward, spiders use hydraulic pressure from their bodily fluids (hemolymph) to extend their legs outward. The exoskeleton also helps prevent desiccation, or water loss, which is important for terrestrial spiders living in dry environments. Specialized sensory hairs and structures embedded within the exoskeleton allow spiders to detect touch, vibrations, and air currents, providing vital information about their surroundings.
The Molting Process
Because the exoskeleton is rigid and cannot expand, spiders must shed it to grow. This process is called molting, or ecdysis. Young spiders molt frequently, while older spiders may continue to molt annually even after reaching maturity.
Before molting, a new, soft exoskeleton begins to form underneath the old one. The spider secretes a molting fluid that helps separate the old and new layers, and some nutrients from the old exoskeleton are reabsorbed. The old exoskeleton then splits, allowing the spider to extract itself. During and immediately after molting, the spider is vulnerable because its new exoskeleton is soft and flexible, making it susceptible to predators and potential injury. The spider then expands its body, often by increasing hemolymph pressure, to stretch the new exoskeleton to a larger size before it hardens, a process that can take hours to days depending on the spider’s size.