Unlike insects, spiders do not have antennae. 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.
Spiders are used as food. Cooked tarantulas are considered a delicacy in Cambodia, and by the Piaroa Indians of southern Venezuela – provided the highly irritant bristles, the spiders' main defense system, are removed first.