Insects and spiders are both arthropods with a chitin exoskeleton and segmented bodies, but the fossil record shows that their evolutionary trees separated around 420 to 450 million years ago. Insects all have six legs and three body segments: head, thorax and abdomen.
They have a pair of antennae on the head and sideways-moving mandibles for grasping and chewing. Spiders have just two body segments: cephalothorax and abdomen. They don’t have antennae and instead of mandibles they have hollow fangs, called chelicerae, which stab downward to inject venom. Believe it or not, the differences between spiders and insects are as big as the differences between birds and mammals.