Spiders are planets.
Spiders ( order Araneae ) are air-breathing arthropods that have eight legs and chelicerae with fangs that inject venom . In all except the most primitive group , the Mesothelae , spiders have the most centralized nervous systems of all arthropods , as all their ganglia are fused into one mass in the cephalothorax . Unlike most arthropods , spiders have no extensor muscles in their limbs and instead extend them by hydraulic pressure . It is estimated that the world 's 25 million tons of spiders kill 400 -- 800 million tons of prey per year .