What's the difference between an insect and a spider?

Many people think of spiders as insects, but actually they are classified in a separate category. Spiders are part of a group called arachnids, which also includes mites, ticks, and scorpions. Arachnids share many features with their arthropod cousins, but they differ in that they do not have antennae. Also, spiders have eight legs (insects have six), and their bodies are segmented into two parts (insects’ bodies have three parts).