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"The abdomen and cephalothorax of spiders are closely attached, while in ants the equivalent of these body parts are separated by a narrow segment called the petiole," Poinar said.

Spiders have a two-part body that resembles a figure eight: a fused head and thorax called a cephalothorax, separated from an abdomen by a narrow constriction.

The cephalothorax and abdomen together are five feet long.

C, carapace covering the cephalothorax.

In the smallest spiders, Dr. Eberhard and his colleagues found, the central nervous systems filled nearly 80 percent of the cephalothorax, or body cavity, including 25 percent of the legs.