Two species of the Mecyclothorax argutor (Sharp, 1903) species-group, M. neonomas sp. nov. and M. brunneonubiger sp. nov., are newly described from Mauna Loa volcano on the Island of Hawai‵i. Mecyclothorax neonomas is most similar morphologically to M. swezeyi Liebherr, 2008 of Mauna Kea and is therefore proposed as its adelphotaxon. Conversely, the adelphotaxon proposed for M. brunneonubiger is M. blackburnianus Liebherr, 2008, precinctive to Kohala volcano, Island of Hawai‵i. Based on the younger geological age of origin of Mauna Loa relative to Mauna Kea and Kohala, it is hypothesized these two newly described, sympatric species arose via progressive colonization of Mauna Loa during development of the geologically dynamic Island of Hawai‵i. A revised key to all Mecyclothorax species known from Hawai‵i Island is provided.