mesothorax Syllabification: (mes·o·tho·rax)Pronunciation: /ˌmezəˈTHôrˌaks/Definition of mesothorax noun (plural mesothoraxes or mesothoraces /-ˈTHôrəˌsēz/) Entomology the middle segment of the thorax of an insect, bearing the forewings or elytra. Derivatives mesothoracicPronunciation: /-THəˈrasik/ adjective mesothorax in other Oxford dictionaries Definition of mesothorax in the British & World English dictionary