The Lepidoptera represents one of the most successful radiations of plant-feeding insects, which predominantly took place within angiosperms beginning in the Cretaceous period. Angiosperm colonization is thought to underlie the evolutionary success of the Lepidoptera because angiosperms provide an enormous range of niches for ecological speciation to take place. By contrast, the basal lepidopteran lineage, Micropterigidae, remained unassociated with angiosperms since the Jurassic but nevertheless achieved a modest diversity in the Japanese Archipelago. We explored the causes and processes of diversification of the Japanese micropterigid moths by performing molecular phylogenetic analysis and extensive ecological surveying. Phylogenetic analysis recovered a monophyletic group of approximately 25 East Asian endemic species that feed on the liverwort Conocephalum conicum, suggesting that niche shifts hardly played a role in their diversification. Consistent with the weak flying ability of micropterigid moths, the distributions of the Conocephalum feeders are each localized and allopatric, indicating that speciation by geographical isolation has been the major process shaping the diversity of Japanese Micropterigidae. To our knowledge, this is the largest example of radiation of herbivorous insects that does not accompany any apparent host-plant differentiation.