(1):

(n.) The second, or middle, region of the body of a crustacean, arachnid, or other articulate animal. In the case of decapod Crustacea, some writers include under the term thorax only the three segments bearing the maxillipeds; others include also the five segments bearing the legs. See Illust. in Appendix.

(2):

(n.) The middle region of the body of an insect, or that region which bears the legs and wings. It is composed of three united somites, each of which is composed of several distinct parts. See Illust. in Appendix. and Illust. of Coleoptera.

(3):

(n.) The part of the trunk between the neck and the abdomen, containing that part of the body cavity the walls of which are supported by the dorsal vertebrae, the ribs, and the sternum, and which the heart and lungs are situated; the chest.

(4):

(n.) A breastplate, cuirass, or corselet; especially, the breastplate worn by the ancient Greeks.