Species of the genus Mecyclothorax Sharp, 1903 from the island of Molokai, Hawaii are taxonomically revised. The Molokai fauna constitutes 43 recognized species, 24 newly described: M. polhemusi, new species; M. dunbarorum, new species; M. impunctatus, new species; M. debiliceps, new species; M. convexus, new species; M. subsinuatus, new species; M. subater, new species; M. trisetifer, new species; M. annae, new species; M. joni, new species; M. lisae, new species; M. lissopterus, new species; M. arcuatus, new species; M. latus, new species; M. abax, new species, M. ewingi, new species; M. latissimus, new species; M. punctatostriatus, new species; M. flavolateralis, new species; M. obscurus, new species; M. comma, new species; M. stenolophinus, new species; M. granulatus, new species; and M. cymindoides, new species. Mecyclothorax quadratus Britton is newly synonymized with M. argutor (Sharp). Various species are congruently distributed on Kamakou Volcano, allowing recognition of two areas of endemism, one to the west encompassing Kamakou and Olokui peaks, the second to the east spanning the areas from Wailau and Puu Lua to Puu Ohelo and Kainalu Gulch. Representatives of several hypothesized species pairs mutually occupy the two areas of endemism adding rigor to definition of the areas. Whereas the western Kamakou-Olokui area of endemism encompasses the State of Hawaii's Puu Alii and Olokui Natural Area Reserves and the Kamakou Preserve of The Nature Conservancy of Hawaii, the eastern Wailau-Puu Ohelo area of endemism is not comprehensively managed for conservation. Twelve of the 14 species residing in the eastern area of endemism are newly described, indicating the preliminary state of knowledge regarding the fauna of this area. Of those 12 new species, 9 are geographically restricted to the eastern area. This level of endemic richness is proposed as a criterion that justifies establishment of a comprehensive conservation plan for eastern Molokai, thereby ensuring sustainability of its distinct native forest communities. Two of the species, M. lissopterus and M. arcuatus, are cryptic sibling species, diagnosable only by configurations of the male aedeagal median lobe and female bursa copulatrix. The magnitude of the male median lobe in the two species is positively associated with size of the bursa copulatrix in females of the two species. Aedeagal and bursal configurations do not vary infraspecifically. Available evidence points most parsimoniously to origin of derived states in one of these species via developmental changes in genitalic anlagen common to the sexes, differentially expressed in males and females. Such a mechanism assumes homologous transformation of the male aedeagal median lobe and female bursa copulatrix, and removes the need to invoke more complicated hypotheses involving sexual selection.