Spiders do not possess bones like humans and other vertebrates. Instead, these eight-legged creatures rely on an exoskeleton, which serves as their external support and protective covering. This rigid outer layer functions distinctly from an internal skeleton, shaping how spiders grow and interact with their environment. Understanding this external framework is key to their unique biology and movement.
Their External Skeleton
A spider’s primary structural support comes from its exoskeleton, a hard outer casing that encases its body. This external skeleton is primarily composed of chitin, a tough carbohydrate, interwoven with various proteins. The chitin and protein molecules are arranged in successive layers, creating a robust and resilient material known as cuticle. This composite material provides both rigidity and flexibility, acting as a protective shield against external threats and a framework for muscle attachment.
Unlike the internal bones of vertebrates, which grow continuously, a spider’s exoskeleton cannot expand. This fixed size necessitates molting, where the spider sheds its old, restrictive exoskeleton to allow for growth. During molting, a new, larger exoskeleton forms underneath the old one, often folded to fit. The spider then breaks free from its old casing, emerging with a soft, new cuticle that hardens over time.
The exoskeleton is not uniform across the spider’s body; it features thickened, hardened areas for structural integrity and thinner, more flexible regions at the joints. This design allows for movement despite the rigid exterior. Muscles attach to the inside of this external shell, enabling the spider to contract its limbs.
The Role of Internal Pressure
While the exoskeleton provides static support and points for muscle attachment, a spider’s movement, particularly the extension of its legs, relies heavily on a hydraulic system. This system functions as a hydrostatic skeleton, where internal fluid pressure is used to manipulate body parts. Spiders circulate a fluid called hemolymph, analogous to blood in vertebrates, throughout their bodies.
Spiders have muscles that can flex their legs inward, but they notably lack extensor muscles in many of their leg joints, which would typically pull limbs outward. Instead, to extend a leg, the spider rapidly increases the pressure of its hemolymph within its cephalothorax, the fused head and chest segment. This increased pressure forces the hemolymph into the leg segments, causing them to straighten and push outward.
This hydraulic mechanism allows spiders to achieve quick movements, jumps, and the ability to fit into tight spaces. When a spider dies, this internal pressure dissipates, causing its legs to curl inward due to the natural contraction of their flexor muscles. This unique combination of a rigid exoskeleton and a flexible hydraulic system shapes how spiders navigate their world.