Spiders do not possess a backbone. Unlike many familiar animals, their bodies are not supported by an internal column of bones. Instead, spiders rely on a distinct type of structural framework for support and movement.
Spider Support System
Spiders feature an external skeleton, known as an exoskeleton or cuticle, which encases their entire body. This rigid outer covering provides both protection and structural support. The exoskeleton is primarily composed of chitin, a tough and flexible long-chain polysaccharide, interwoven with proteins. This composite material gives the spider its defined shape and protects its internal tissues from external threats and desiccation.
The exoskeleton also serves as an attachment point for the spider’s muscles, enabling precise control over its eight legs. Unlike animals with internal skeletons that use opposing muscle pairs to extend limbs, spiders primarily use flexor muscles to pull their legs inward. To extend their legs, spiders employ a hydraulic system, forcing bodily fluids (hemolymph) into their limbs to push them outward. This hydraulic mechanism allows for rapid and agile movements, including jumping.
Because the exoskeleton is a non-living structure, it cannot grow as the spider increases in size. To accommodate growth, spiders must periodically shed their old exoskeleton in a process called molting. During molting, a new, larger exoskeleton forms underneath the old one, which the spider then sheds. This period makes the spider temporarily vulnerable as its new cuticle hardens.
The Vertebrate Difference
A backbone, or vertebral column, is a defining characteristic of vertebrates, a group of animals including fish, birds, mammals, amphibians, and reptiles. This internal skeletal structure is made up of individual bones called vertebrae, which are connected by intervertebral discs. The vertebral column provides support for the body, allows for flexibility and movement, and protects the spinal cord.
Animals are categorized into two main groups based on the presence or absence of a backbone: vertebrates and invertebrates. Invertebrates are animals that do not have a backbone. This diverse group comprises over 90% of all animal species and includes organisms such as insects, worms, mollusks, and crustaceans.
Spiders are classified as invertebrates. They belong to the phylum Arthropoda, which encompasses animals with segmented bodies, jointed appendages, and exoskeletons. Within Arthropoda, spiders are further categorized under the class Arachnida, alongside scorpions, ticks, and mites.