Arachnids are any of the invertebrate animals in the subphylum Chelicerata, which is in the phylum Arthropoda. They are differentiated from other arthropods because of their eight jointed legs and eight eyes, as well as a two-sectioned body structure and two sets of specialized appendages used for eating and defense.
Arachnids are commonly called Spiders, and in fact the root word of Arachnid is Arachne, or Spider in Greek. Spiders are found all over the world in every environment except for the extreme cold of the North and South Poles; they have adapted to survive in most land environments and even in some marine environments. Land spiders are almost all predators, eating smaller arthropods to survive, and some of the larger species catch and eat larger animals and birds, having adapted to fill a predatory niche. Parasitic spiders, like ticks, can spread disease in animals and humans. Most spiders prefer a closed, mostly dark environment, where they can spin webs or set traps for smaller arthropods, and are commonly found in corners and under objects in the home.
The Giant Huntsman Spider is largest in the world by legspan, and is a poisonous cave-dweller, recently discovered in Laos. The smallest spider in the world is the Patu digua, which is smaller than the head of a pin, and lives only in Columbia.