Abstract The Mecyclothorax carabid beetle fauna of Haleakalā volcano, Maui Island, Hawai‘i is taxonomically revised, with 116 species precinctive to Haleakalā recognized, 74 newly described. Species are classified into 14 species groups, with the newly described species arrayed as follows: 1, Mecyclothorax constrictus group with Mecyclothorax perseveratus sp. n.; 2, Mecyclothorax obscuricornis group with Mecyclothorax notobscuricornis sp. n., Mecyclothorax mordax sp. n., Mecyclothorax mordicus sp. n., Mecyclothorax manducus sp. n., Mecyclothorax ambulatus sp. n., Mecyclothorax montanus sp. n., Mecyclothorax waikamoi sp. n., Mecyclothorax poouli sp. n., and Mecyclothorax ahulili sp. n.; 3, Mecyclothorax robustus group with Mecyclothorax affinis sp. n., Mecyclothorax anchisteus sp. n., Mecyclothorax consanguineus sp. n., Mecyclothorax antaeus sp. n., Mecyclothorax cymindulus sp. n., and Mecyclothorax haydeni sp. n.; 4, Mecyclothorax interruptus group with Mecyclothorax bradycelloides sp. n., Mecyclothorax anthracinus sp. n., Mecyclothorax arthuri sp. n., Mecyclothorax medeirosi sp. n., Mecyclothorax inconscriptus sp. n., and Mecyclothorax foveolatus sp. n.; 5, Mecyclothorax sobrinus group with Mecyclothorax foveopunctatus sp. n.; 6, Mecyclothorax ovipennis group with Mecyclothorax subtilis Britton & Liebherr, sp. n., Mecyclothorax patulus sp. n., Mecyclothorax patagiatus sp. n., Mecyclothorax strigosus sp. n., Mecyclothorax takumiae sp. n., Mecyclothorax parapicalis sp. n., Mecyclothorax mauiae sp. n., Mecyclothorax subternus sp. n., Mecyclothorax flaviventris sp. n., Mecyclothorax cordaticollaris sp. n., and Mecyclothorax krushelnyckyi sp. n.; 7, Mecyclothorax argutor group with Mecyclothorax ommatoplax sp. n., Mecyclothorax semistriatus sp. n., Mecyclothorax refulgens sp. n., Mecyclothorax argutulus sp. n., Mecyclothorax planipennis sp. n., Mecyclothorax planatus sp. n., and Mecyclothorax argutuloides sp. n.; 8, Mecyclothorax microps group with Mecyclothorax major sp. n., Mecyclothorax xestos sp. n., Mecyclothorax orbiculus sp. n., and Mecyclothorax contractus sp. n.; 9, Mecyclothorax scaritoides group with Mecyclothorax scarites sp. n., Mecyclothorax timberlakei sp. n., Mecyclothorax crassuloides sp. n., Mecyclothorax crassulus sp. n., Mecyclothorax gracilicollis sp. n., and Mecyclothorax dispar sp. n.; 10, Mecyclothorax haleakalae group with Mecyclothorax reiteratus sp. n., Mecyclothorax splendidus sp. n., Mecyclothorax bacrionis sp. n., and Mecyclothorax simpulum sp. n.; 11, Mecyclothorax vitreus group with Mecyclothorax kipwilli sp. n., Mecyclothorax kipahulu sp. n., Mecyclothorax kaumakani sp. n., and Mecyclothorax kuiki sp. n.; 12, Mecyclothorax montivagus group with Mecyclothorax rex sp. n.; 13, Mecyclothorax ducalis group with Mecyclothorax aquilus sp. n., Mecyclothorax invisitatus sp. n., Mecyclothorax longidux sp. n., and Mecyclothorax brevidux sp. n.; and 14, Mecyclothorax palustris group with Mecyclothorax hephaestoides sp. n., Mecyclothorax oculellus sp. n., Mecyclothorax bicoloris sp. n., Mecyclothorax bicoloratus sp. n., Mecyclothorax bilobatus sp. n., Mecyclothorax palustroides sp. n., Mecyclothorax filipoides sp. n., Mecyclothorax nanunctus sp. n., Mecyclothorax tauberorum sp. n., and Mecyclothorax pau sp. n. Mecyclothorax integer Sharp, stat. n. is recognized as a species distinct from Mecyclothorax interruptus Sharp. Because type series for species described by Blackburn, Karsch, and Sharp are most often divided among geographically remote collections, lectotypes are designated to stabilize the nomenclature. The radiation includes numerous cryptic sibling species best diagnosed using male genitalia, and photographs are used to represent the male genitalic variability observed among numerous dissected individuals. The large number of new species is based on substantial new collections made from all quarters of the mountain. The dense geographic sampling allows fine-scale discrimination of species boundaries, elucidating the geographic disjunctions that are associated with speciation within this hyperdiverse radiation. Disjunctions between closely related species precinctive to various areas of the mountain are not congruent across the different lineages of the radiation, indicating differential responses by the various lineages to past geological and geographical events. Of the 62 1’ latitude × 1’ longitude grid cells on Haleakalā that are occupied by Mecyclothorax beetles, 22 house 10 or more species, and 9 house 20 or more species. This substantial level of sympatry, associated with occupation of diverse microhabitats by these beetles, provides ample information useful for monitoring biodiversity of the natural areas of Haleakalā.
