Entry 1 sense Webster, 1913 Mesothorax /mĕzˌ-əth-ôr'-ăksˌ/ · Mes·o·tho·rax · IPA /mˌɛzəθˈɔrˌæks/ 01 n. The middle segment of the thorax in insects. See Illust. of Coleoptera. 1. The middle segment of the thorax in insects. See Illust. of Coleoptera.(Zool.) See: Coleoptera