Spiders belong to the Family of Arthropods, a group of invertebrate animals with jointed legs and an external skeleton made of chitin (pronounced "KEYE-tin"). This is different from the vertebrate animals, which have internal skeletons usually made of cartilage and bone.
The earliest arthropods arose in the oceans and were already highly developed in the early Cambrian Period (570 million years ago) with the marine trilobites. Modern aquatic arthropods include lobsters, shrimp (also known as prawns), and crayfish.
The arthropods left the oceans by the Carboniferous Period (345 million years ago) and quickly began to colonize the land. Land-dwelling arthropods include millipedes, insects, spiders, scorpions, and mites. As the only predators on land at that time were some early land-dwelling vertebrates, the millipedes, insects, and scorpions were all able to develop some giant species.
Spiders, scorpions, and mites all belong to the Arachnidae. The arachnids get their name from a Greek myth in which a weaver named Arachne was so proud of her weaving ability that she challenged the goddess Athena to a contest. Athena destroyed Arachne's weaving during the contest, and Arachne hanged herself. Athena then changed Arachne into a spider and gave her name to the Arachnida.
The first true spiders appeared in the Carboniferous Period and are grouped into the Ricinulei, a group of plump and primitive spiders. A few species of the Ricinulei survive today as "living fossils" in tropical forests.
Spiders are often mistakenly grouped with the insects. Insects, however, all have three pairs of legs (six total), and their bodies are divided into three main parts: head, thorax, and abdomen.
In contrast, spiders have bodies divided into two main parts, a cephalothorax (a combined "cephalic" (head) and thorax region) and an abdomen, and have five pairs of legs (ten total).
"Wait! Wait!" I hear you say, "I thought that spiders had EIGHT legs?" Well, they do. They have four pairs (eight) of walking legs, and they also have a pair of graspers or chelicerae (pronounced "kil-lis-er-ree") on their cephalothorax. Spiders use these chelicerae to maneuver prey to their mouths.
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