Spiders are invertebrates. They have no backbone, no internal skeleton of any kind, and no evolutionary relationship to the vertebrate lineage. Spiders belong to the class Arachnida within the phylum Arthropoda, making them relatives of scorpions, ticks, and mites rather than of fish, reptiles, or mammals. Yet their bodies are far from simple, and the ways they have solved the engineering problems of support, movement, and sensing without a single bone are worth understanding on their own terms.
Where Spiders Sit in the Animal Kingdom
The animal kingdom splits early into two broad camps based on body plan: vertebrates, which have an internal skeleton built around a spinal column, and invertebrates, which do not. Vertebrates make up a single subphylum (Vertebrata) within the phylum Chordata, and they include all fish, amphibians, reptiles, birds, and mammals. Invertebrates, by contrast, are not one group but many: insects, crustaceans, mollusks, worms, jellyfish, and arachnids all fall under this umbrella. The label “invertebrate” simply means “lacking a vertebral column,” so it covers an enormous range of body plans.
Spiders specifically belong to the phylum Arthropoda, the largest animal phylum on Earth, and within that to the subphylum Chelicerata. Chelicerates are defined by their mouthparts, called chelicerae, and by the complete absence of antennae. The class Arachnida, which sits inside Chelicerata, contains roughly 100,000 described species, including spiders (order Araneae), scorpions, harvestmen, ticks, and mites.1ScienceDirect. Arachnida So when someone asks whether a spider is a vertebrate or invertebrate, the answer is not a close call. Spiders sit several major branches away from vertebrates on the tree of life.
The Exoskeleton Does What Bones Do
Vertebrates carry their support on the inside: a rigid skeleton of bone or cartilage surrounded by soft tissue. Spiders, like all arthropods, flip this arrangement. Their structural support is an exoskeleton, a tough outer shell made primarily of a material called cuticle. This cuticle is a layered composite of chitin fibers embedded in a protein matrix, and in spiders it varies in thickness, stiffness, and composition depending on where on the body it appears. The exoskeleton protects internal organs, anchors muscles, and provides the rigid segments that limbs need to function as levers.2PubMed Central. The spider cuticle: a remarkable material toolbox for functional diversity
This arrangement comes with a trade-off that vertebrates avoid. Because the skeleton is external and rigid, a spider cannot simply grow larger the way a puppy does. Instead, spiders must periodically shed their entire exoskeleton in a process called molting. The animal builds a new, soft cuticle beneath the old one, splits the old shell, crawls out, and then inflates the new exoskeleton with body fluid before it hardens. During those few hours, the spider is extremely vulnerable. Molting is one of the defining features of arthropod life and one of the clearest contrasts with vertebrate biology.
Hydraulic Legs Instead of Extensor Muscles
Vertebrate limbs extend and flex using opposing sets of muscles attached to internal bones. Spiders have muscles for flexing (bending) their legs inward, but they lack extensor muscles in most of their major leg joints. Instead, they extend their legs by pumping pressurized fluid into them. The spider’s body cavity, called the prosoma, generates hydraulic pressure that pushes the legs outward.3PubMed. Hydraulic leg extension is not necessarily the main drive in large spiders This is why a dead spider’s legs curl inward: with no hydraulic pressure to hold them extended, the flexor muscles pull unopposed.
This hydraulic system is remarkably effective. Jumping spiders can launch themselves many times their own body length using a rapid surge of fluid pressure in their hind legs. The system does, however, link locomotion tightly to overall body-fluid pressure, which means that a spider suffering from dehydration or injury that causes fluid loss can lose the ability to move normally. It is a fundamentally different engineering solution from the vertebrate approach, and it works within the constraints of having no internal skeleton to anchor opposing muscle pairs.
Open Circulation and a Different Kind of Blood
Vertebrates have a closed circulatory system: blood stays inside vessels (arteries, veins, capillaries) at all times, pushed by a chambered heart. Spiders have an open circulatory system, meaning their blood, called hemolymph, is not confined to vessels for most of its journey. The heart pumps hemolymph into a few arteries, but it then flows freely through the body cavity, bathing the organs directly before returning to the heart through openings called ostia.
The hemolymph itself is chemically different from vertebrate blood. Instead of hemoglobin (the iron-based molecule that makes your blood red), spiders use hemocyanin, a copper-based protein that carries oxygen and gives hemolymph a faint bluish tint when oxygenated. Research into the spider clotting system has revealed that hemocyanin is the dominant protein in hemolymph clots, suggesting it plays a role not only in oxygen transport but also in wound sealing.4PubMed. The spider hemolymph clot proteome reveals high concentrations of hemocyanin and von Willebrand factor-like proteins Vertebrate clotting relies on an entirely different set of proteins, reflecting how deeply the two lineages diverged.
Breathing With Lungs and Tracheae at the Same Time
Spiders are unusual even among invertebrates when it comes to how they breathe. Most terrestrial arthropods use either book lungs (layered plates of tissue exposed to air) or tracheae (a network of tiny tubes that deliver oxygen directly to tissues). Spiders are the only animal group known to use both systems simultaneously.5PubMed. Respiration in spiders (Araneae) Their book lungs handle the bulk of gas exchange, while the tracheal system supplements delivery to tissues that are farther from the lungs or have higher oxygen demands.
The relative importance of each system varies across spider families. Some small spiders rely heavily on tracheae and have reduced book lungs, while large spiders like tarantulas depend more on their book lungs. Neither system resembles the vertebrate arrangement of paired lungs with internal branching airways and a muscular diaphragm. In a spider, air enters through slits on the underside of the abdomen and passes over the thin tissue of the book lung plates, where oxygen diffuses into hemolymph flowing on the other side. There is no active inhalation; the process is largely passive, driven by the movement of hemolymph and the spider’s own body motion.
A Brain Without a Spinal Cord
In vertebrates, the central nervous system consists of a brain inside the skull connected to a spinal cord running through the vertebral column. Spiders have nothing resembling a spinal cord. Instead, their central nervous system is a fused mass of nerve tissue called a synganglion, located in the prosoma (the front body section). This synganglion is essentially all of the spider’s “brain” and “nerve cord” consolidated into one dense structure. Recent three-dimensional mapping of this structure in orb-weaving spiders has revealed distinct functional regions for processing visual, tactile, and chemical information, all packed into a space smaller than a pinhead.6PubMed Central. A three-dimensional immunofluorescence atlas of the brain of the hackled-orb weaver spider, Uloborus diversus
Despite its miniature size, the spider synganglion supports remarkably complex behavior. Orb weavers build geometrically regular webs. Jumping spiders plan detour routes to reach prey they can no longer see. Portia spiders adjust their hunting strategies based on the species of spider they are stalking. All of this runs on a nervous system without a single vertebra protecting it. The synganglion fills so much of the prosoma in some small spiders that it actually deforms the exoskeleton, pushing into the base of the legs. Brain size relative to body size, and the cognitive performance it supports, is one area where spiders challenge any assumption that invertebrates are necessarily “simpler” than vertebrates.
Thousands of Sensors Embedded in the Shell
Vertebrates rely on sensory organs housed in soft tissue: eyes in sockets, ears with tympanic membranes, nerve endings in skin. Spiders, with their rigid exoskeleton, have evolved sensory systems that work with and through that shell rather than in spite of it. Their bodies are covered with several thousand strain-detecting organs called slit sensilla, which are tiny cracks in the cuticle that deform under mechanical stress. These sensors detect vibrations, body position, and forces acting on the exoskeleton with extraordinary sensitivity.7PubMed. Spider mechanoreceptors
In addition to slit sensilla, spiders have hair-shaped sensors called trichobothria that detect air currents. These hairs are so sensitive that they can pick up the air displacement caused by a flying insect from several body lengths away. The refinement of these sensors at the level of stimulus uptake and signal conversion reflects the physics of what they need to detect: web-borne vibrations, approaching predators, wind direction, and the footsteps of prey.7PubMed. Spider mechanoreceptors A vertebrate achieves similar awareness through a fundamentally different toolkit, but the end result in terms of environmental sensitivity is comparable.
Jumping Spider Eyes That Rival Vertebrate Vision
Most spiders have relatively poor eyesight, relying instead on vibration and touch to navigate the world. Jumping spiders are the dramatic exception. Their principal eyes (the large, forward-facing pair) are camera-type eyes with a cornea, a lens, and a layered retina, structurally similar in concept to vertebrate eyes. More striking, jumping spiders are the only known invertebrate group whose eyes contain a fovea, the small, densely packed region of the retina responsible for sharp central vision. In vertebrates, a fovea is found in some birds, some lizards, and primates, including humans.8PubMed Central. A Comparative Analysis of the Camera-like Eyes of Jumping Spiders and Humans
The resemblance is a case of convergent evolution: two very distantly related lineages arriving at a similar solution to the same optical problem. Jumping spider eyes and human eyes differ in many details. The spider’s retina has only four layers of photoreceptors (compared to our ten), but those layers are stacked at different focal lengths, giving the spider a form of depth perception from a single eye. The spider also moves its retina behind a fixed lens, scanning the visual field by sliding the retina rather than by rotating the eyeball. A comparative study of these two eye types found both similarities and differences in the properties and functions of their basic components, highlighting how visual systems can evolve independently toward high-resolution image formation.8PubMed Central. A Comparative Analysis of the Camera-like Eyes of Jumping Spiders and Humans
Venom Glands and How Spiders Develop
One system that has no real counterpart in vertebrates is the spider venom apparatus. Venom glands develop inside the chelicerae (the fang-bearing mouthparts) during embryonic life and then continue growing after the spider hatches. Research on the common house spider shows that venom gland tissue first becomes detectable as a small infolding of tissue at the tip of the chelicerae during embryonic development. After hatching, the glands elongate and migrate backward into the prosoma, where they eventually occupy considerable space.9PubMed Central. Venom gland organogenesis in the common house spider Almost all spiders are venomous, though very few produce venom potent enough to harm humans.
Spider development also differs sharply from the vertebrate pattern. Vertebrate embryos develop inside a mother or inside an egg with a fixed body plan that grows continuously. Spider embryos develop inside an egg sac, and after hatching they must molt repeatedly to reach adult size. Each molt is a complete replacement of the exoskeleton, and the timing of these molts is regulated by hormones. The process is punctuated rather than continuous: a spider grows in steps, trapped at one size until its next molt, then suddenly larger. This is characteristic of all arthropods and is one of the fundamental biological differences that separates the invertebrate arthropod lineage from the vertebrate one.
How Far Back the Split Goes
Spiders and vertebrates did share a common ancestor, but you would have to go back extraordinarily far to find it. Molecular phylogenetic studies estimate that the arthropod and vertebrate (deuterostome) lineages diverged roughly 900 to 1,100 million years ago, well before the Cambrian Explosion that produced most recognizable animal body plans in the fossil record.10PubMed Central. The colonization of land by animals: molecular phylogeny and divergence times among arthropods That common ancestor was almost certainly a small, soft-bodied marine organism with no skeleton of any kind, neither internal nor external. Every structural feature that defines a spider (exoskeleton, chelicerae, book lungs, hydraulic limbs, silk glands, synganglion) evolved independently along the arthropod branch after that split.
Both lineages eventually colonized land, but they did so separately and using completely different body architectures. Vertebrates carried their support inside and developed lungs from outpouchings of the gut. Arthropods carried their support outside and developed breathing organs from modified gill-like structures. The result is that a spider and a lizard sitting on the same rock are both land animals with legs, eyes, and predatory lifestyles, but their bodies share almost no structural design in common beyond the most basic features inherited from that ancient marine ancestor.
Why the Question Comes Up
The confusion between vertebrate and invertebrate sometimes arises because spiders are large enough, active enough, and behaviorally complex enough to “feel” more advanced than, say, a worm or a jellyfish. People tend to associate invertebrates with soft, simple creatures and vertebrates with complex, active ones. But invertebrates vastly outnumber vertebrates in species count and occupy every ecological niche on the planet. Octopuses solve puzzles, bees communicate through dance, and spiders build architecturally precise webs. Complexity of behavior and sophistication of body systems do not require a backbone.
Another source of confusion involves the word “spider” being applied loosely. Sea spiders (Pycnogonida), for example, are marine arthropods that are not true spiders at all, and their exact relationship to other chelicerates has been debated for decades.11ScienceDirect. Morphological phylogenetics of the sea spiders (Arthropoda: Pycnogonida) They are still invertebrates, but their body plan is so different from a true spider’s that grouping them together can muddy the picture. True spiders, order Araneae, are arachnids, are arthropods, and are unambiguously invertebrates. No spider, whether a tiny money spider on your windowsill or a dinner-plate-sized tarantula in the rainforest, has anything resembling a vertebral column or any other internal bony structure.