Source : Webster's Revised Unabridged Dictionary (1913)

Prothorax \Pro*tho"rax\, n. [Pref. pro- + thorax.] (Zo["o]l.)
   The first or anterior segment of the thorax in insects. See
   Illusts. of {Butterfly} and {Coleoptera}.

Source : WordNet®

prothorax
     n : the anterior part of an insect's thorax; bears the first
         pair of legs
     [also: {prothoraces} (pl)]