Spiders possess a unique anatomy that allows for their distinct movements. Many people wonder if these eight-legged creatures, like humans, have a spine. The answer is straightforward: spiders do not have a spine or an internal bony skeleton. They belong to a different group of animals entirely, relying on alternative structural and locomotive systems.
What Makes a Vertebrate?
Animals with a backbone, or vertebral column, are known as vertebrates. This defining characteristic provides internal support and protects the delicate spinal cord, a bundle of nerves extending from the brain. The vertebral column is a segmented structure, allowing for flexibility and movement while also serving as an attachment point for muscles. Mammals, birds, fish, amphibians, and reptiles all fall under the category of vertebrates. Spiders, however, are classified as invertebrates, a broad group of animals that inherently lack a vertebral column and internal bony framework.
Spiders’ External Support System
Instead of an internal skeleton, spiders possess a rigid outer covering called an exoskeleton. This external structure is primarily composed of chitin, a tough carbohydrate, along with various proteins. The exoskeleton serves several important functions for the spider. It provides structural support, giving the spider its defined shape. It also acts as a protective shield against predators and physical damage. Furthermore, the exoskeleton prevents water loss. Unlike internal skeletons that grow with an animal, the exoskeleton does not expand, necessitating periodic molting, where the spider sheds its old skin to grow larger.
Movement Without Bones
Spiders achieve movement through a combination of muscle action and hydraulic pressure. Their muscles attach directly to the inside of the exoskeleton, allowing them to flex their legs inward. However, spiders lack muscles that extend their legs outward. Instead, they utilize a hydraulic system to extend their limbs.
This hydraulic system relies on the internal fluid pressure of hemolymph, which is the spider’s equivalent of blood. The cephalothorax, the fused head and chest section of a spider, can regulate this fluid pressure, forcing hemolymph into the legs. This increase in pressure causes the legs to extend, as seen when a spider jumps by rapidly increasing hemolymph pressure to propel itself forward. If a spider loses too much body fluid or experiences a drop in internal pressure, its legs will curl inward, which is why dead spiders often appear with their legs tucked.