The Collaborative International Dictionary of English v.0.48:

Metathorax \Met`a*tho"rax\, n. [NL.: cf. F. m['e]tathorax. See
   Meta-, and Thorax.] (Zool.)
   The last or posterior segment of the thorax in insects. See
   Illust. of Coleoptera.
   [1913 Webster]