Spiders (order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs that inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species diversity among all other orders of organisms. Spiders occur in a wide range of habitats throughout the world. Most spiders build webs to capture prey, although some do not.
All spiders produce silk, a protein liquid spun out by their spinnerets onto silken threads. The tensile strength of spider silk is greater than that of steel and has been used to make bulletproof vests and other high-strength material. Silk also serves as a safety line for many climbing spiders, allowing them to escape if they fall.
The vast majority of spiders are harmless to humans and can even be beneficial by eating insects that damage crops or spread disease. However, a few species possess powerful venoms capable of causing serious injury or death in humans; these include the black widow spider and the brown recluse spider. Bites from these spiders usually require medical attention and can result in serious health complications such as necrosis (tissue death) or seizures.