Spiders are arachnids, not insects, although together with insects, they form a group called arthropods, which are animals with hard external shells and jointed legs. The main difference between spiders and insects is body structure.
A spider has eight legs, simple eyes, jaws with fangs for piercing food and a genital opening near the front of its body, which is comprised of a fused head and thorax with a separate abdomen. Spiders also have organs in their abdomens for spinning silk. Insects have three body parts, six legs, antennae, compound eyes, jaws for chewing and a sexual organ on the abdomen. In addition to spiders, scorpions, ticks and mites are arachnids.