The 101 species of Mecyclothorax Sharp known to inhabit Tahiti Island, French Polynesia are taxonomically revised, including 28 species that are newly described: M. claridgeiae sp. n., M. jeanyvesi sp. n., M. poria sp. n., M. aano sp. n., M. papau sp. n., M. manina sp. n., M. everardi sp. n., M. ramagei sp. n., M. pitohitiensis sp. n., M. curtisi sp. n., M. hoeahiti sp. n., M. ninamu sp. n., M. kokone sp. n., M. paahonu sp. n., M. kayballae sp. n., M. ehu sp. n., M. papuhiti sp. n., M. tuea sp. n., M. taatitore sp. n., M. konemata sp. n., M. arboricola sp. n., M. rahimata sp. n., M. oaoa sp. n., M. maninapopoti sp. n., M. hunapopoti sp. n., M. fefemata sp. n., M. maninamata sp. n., and M. niho sp. n. Mecyclothorax muriauxioides Perrault, 1984 is newly synonymized with M. muriauxi Perrault, 1978. Lectotypes are designated for: Thriscothorax altiusculus Britton, 1938; T. bryobius Britton, 1938; Mecyclothorax globosus Britton, 1948: and M. sabulicola Britton, 1948. Dichotomous identification keys augmented by dorsal habitus and male aedeagal photographs are provided to the various species-groups and all included species. The spermatophore of M. papau sp. n. is described, with the ampulla and collar found to correspond dimensionally to the length of the internal sac flagellar plate. Variation among characters of the female reproductive tract is presented for all newly described plus other representative species comprising the radiation. Taxa are assigned to species groups, modified from the classification of G.G. Perrault, based on derived character states polarized using the Australian outgroup taxon Mecyclothorax punctipennis (MacLeay). Much of the species-level diversity on this small Pacific island is partitioned allopatrically over very small distributional ranges. No species is shared between Tahiti Nui and Tahiti Iti, and nearly all species in Tahiti Nui are geographically restricted to one ridgelike massif of that volcano. Cladistically similar species are often distributed on different massifs suggesting that vicariance associated with erosional valley formation has facilitated speciation, however several instances in which sister species occupy sympatric distributions on the same ridge system demonstrate that speciation may also occur across extremely localized landscapes. Such localized differentiation is facilitated by the low vagility of these small-bodied, flightless predators whose fragmented populations can persist and diverge within spatially limited habitat patches. The intense philopatry of Tahitian Mecyclothorax spp. coupled with the highly dissected landscape has produced the geographically densest adaptive radiation on Earth. This radiation has occurred very rapidly, with species durations averaging 300,000 yr; a speciation rate similar to that observed in Hawaiian Oliarus planthoppers and Laupala crickets, and East African Rift lake cichlid fishes.
