The anterior region of the body trunk of animals. In vertebrates it contains the heart and lungs within the rib cage. It is particularly well-defined in mammals, being separated from the abdomen by the diaphragm. In insects the thorax is divided into an anterior prothorax, a middle mesothorax, and a posterior metathorax, each of which bears a pair of legs; the hindmost two segments also both carry a pair of wings. In other arthropods, especially crustaceans and arachnids, the thorax is fused with the head to form a cephalothorax.