Blackburnia lata sp. n. is described from the western reaches of the Alakai Swamp Trail, Kauaikinana Stream watershed, Kauai. Blackburnia lata is placed phylogenetically among other Kauai species in the subgenus Blackburnia Sharp, 1878. It is sympatric with three closely related species, all distributed largely within the limits of the primeval Kauai caldera, but allopatric with its adelphotaxon, B. atra Liebherr, found on Mt. Kahili, an isolated southern spur of the Mt. Waialeale massif. All five species occur in arboreal mossmats, at the same time exhibiting substantial variation in tarsal configuration. Tarsal evolution across this mossmat-specific lineage suggests that a single tarsal conformation is not required for occupation of this arboreal ecological zone. Historical transformation patterns of tarsal characters are not entirely congruent with the phylogenetic relationships among these species, and inclusion or exclusion of these characters in cladistic analysis is irrelevant to the definition of phylogenetic relationships. This incongruence argues against the necessity of excluding characters from phylogenetic analysis because they are deemed adaptive, while supporting the total evidence approach, wherein all potential synapomorphies are analyzed simultaneously to estimate phylogeny, regardless of their putative adaptive significance.