A spider has eight legs and two body parts, while an insect has three body parts, so a spider is not an insect. Most spiders have eight eyes, though some have fewer. Daddy-long-legs, also called harvestmen, are not spiders, and they cannot make silk. Most spiders are harmless to people, and spiders very rarely bite people. Do not touch a wild spider, and if you find one, tell a grown-up.

Spiders use silk to protect their young, catch food, make homes and move around. Silk starts as a liquid stored in internal silk glands and hardens into a solid form before the spider uses it. A spider's silk line is typically only about 0.001 mm to 0.004 mm thick. Even so, 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.

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. Cribellate silk and sticky silk are two kinds of catching silk. Cribellate silk tangles the bristles, spines and claws of insect prey, while sticky silk is a glue-like liquid carried on fibrous support lines, such as the spiral line of an orb web.

Orb webs all follow the same general pattern. A spider begins with a disorganized protoweb and replaces it with the frames, spokes and hub of the final web. In a 2021 study of a species of tiny spiders that spin webs at night, scientists showed that web making follows rules that were the same for every spider. Researchers who studied how spiders build webs hope next to learn how the stages of web building are encoded in the spider's brain. Orb-weaving spiders often eat their old silk and reuse it.

Spiders were among the earliest animals to live on land, probably evolving about 400 million years ago. Spiders have been around for eons, but there are few fossil records because their soft bodies easily deteriorate. Amber fossils show that a spider fauna basically similar to that of the present day existed more than 30 million years ago. The greatest threat to spiders is habitat loss, although some spider species are also threatened by the pet trade.

Words to know

• tensile: pulling strength
• protein: body-building molecule
• cribellate: wool-like silk type
• harvestmen: daddy-long-legs