As of November 2026, the World Spider Catalog lists 54,250 species of spiders in 139 families. Spiders were among the earliest animals to live on land, probably evolving about 400 million years ago. Fossils from the Devonian Period, about 380 million years ago and more than 150 million years before the dinosaurs, were long thought to be the oldest spiders. Spiders have been around for eons, but there are few fossil records because their soft bodies easily deteriorate, and amber fossils show that a spider fauna basically similar to that of the present day existed more than 30 million years ago.
Fossil animals called Attercopus wove broad sheets of silk from spigots on plates on the underside of the body. Unlike spiders, they had long tails. A spider's silk comes from spinnerets, and silk starts as a liquid stored in internal silk glands and hardens into a solid form before the spider uses it. Spiders use silk to protect their young, catch food, make homes and move around. Ten different types of silk glands and their spigots are known in spiders, and up to eight silk glands may be present in a single species, each producing silk with different properties and uses.
Silk gets its stretchiness from loose, coil-like protein chains and its stiffness and strength from highly ordered protein crystals. Most spider silks are cylindrical, like a rope, but recluse spiders make a flat, ribbon-like silk that works more like a cable. Researchers at William & Mary built a model of recluse spider silk with seven levels of structure, and they found that a single nanofibril inside the silk is about 10,000 times smaller than a human hair. A spider's silk line is typically only about 0.001 mm to 0.004 mm thick. The tensile strength of dragline silk is greater than that of mild steel and almost all artificial fibres, though Kevlar is an exception, and a strand of spider silk is five times stronger than a steel cable of the same weight, said Hannes Schniepp of William & Mary.
A spider has eight legs and two body parts, while an insect has three body parts, so a spider is not an insect. Spiders do not have ears, and they do not hear the way humans do. They hear with hairs on their body that are sensitive to touch and vibration, and they rely on touch, vibration and taste to navigate and find their prey. Most spiders are harmless to people, and spiders very rarely bite people. Leave wild spiders alone, and tell a grown-up if you find one.
Words to know
- Devonian: ancient time period
- fossils: old remains
- nanofibril: tiny fiber
- tensile: pulling strength
