Spiders have eight legs
Every spider has eight legs, arranged as four pairs. They attach to the cephalothorax, the front section that fuses the head and chest. This is one of the clearest identifying traits of a spider. It is also a key reason spiders are classified as arachnids rather than insects. The legs are multipurpose tools. Spiders use them for walking, climbing, sensing vibration, gripping silk, and, in hunting species, seizing prey.
How do the eight legs help identify a spider?
Counting legs and body segments is the fastest way to separate spiders from look-alikes:
- Insects such as ants, beetles, and flies have six legs and three body segments, and many have wings and antennae.
- Spiders have eight legs, two body segments (the cephalothorax and the abdomen), no wings, and no antennae.
- Harvestmen, the true daddy long legs, also have eight legs but a single rounded body, and they are not spiders.
What are spider legs actually made of?
A spider leg has seven segments and is covered in fine sensory hairs. Spiders have no muscles to extend their legs outward the way we straighten an arm. Instead, they pump body fluid (hemolymph) into the legs under pressure to push them out. Then they use muscles to pull them back. This hydraulic system is why a dead or dehydrated spider curls its legs inward. It is also why spiders need enough body fluid pressure to move normally.
Why do the sensory hairs matter?
The tiny hairs and slit-like sensors on the legs detect air movement, touch, and vibration with remarkable sensitivity. Many web-building spiders see poorly. For them, these leg sensors are the main way to locate trapped prey. They judge the prey's size and position from the way the silk shakes. Hunting species use the same sense to feel an approaching insect or predator. In effect, a spider "listens" and "feels" through its legs far more than it sees.
Can a spider regrow a lost leg?
Often, yes. A young spider that loses a leg can regenerate it over successive molts. The new leg starts smaller and reaches more typical size as the spider keeps growing. Spiders past their final molt cannot regrow legs. That is why some adults are seen getting by on seven. Some species can also deliberately drop a leg to escape a predator's grip, a trade-off that favors survival.
Do jumping spiders use their legs differently?
Yes. Jumping spiders are famous for launching themselves several times their body length. They do it to pounce on prey or cross a gap. They power those leaps with a quick surge of internal fluid pressure into the back legs, rather than large jumping muscles. They also trail a silk safety line as they go. Wolf spiders, by contrast, are fast ground runners. They chase prey down on foot instead of jumping.
Are eight-legged visitors a problem at home?
Almost never. Most spiders you find indoors are harmless insect-eaters, from long-legged cellar spiders to quick wolf spiders. They help keep other pests in check. If you would rather not host them, seal cracks, reduce clutter, and clear away webs. That discourages them without any chemicals. To identify a specific spider by its features, our pest library lets you compare common species. Only a small number of U.S. spiders are considered medically significant enough to warrant extra caution.
