The disharmonic and depauperate nature of biotic communities on oceanic islands has long been recognized as a result of constraints posed by dispersal/establishment limitation and restricted ecological opportunity. Establishment limitation is expected to pose a particular constraint on the assembly of phytophagous insect communities, given that a large proportion of the colonist pool is likely to consist of dietary specialists. Here, I use the results of comparative surveys of the internally-feeding insect fauna associated with Glochidion trees (Phyllanthaceae) in Asia and Polynesia. I find that 1) the most abundant herbivorous insects in Polynesia are lineages which on continents are Phyllanthaceae-specialists, thus representing examples of niche conservatism; 2) some community members (parasitoids) are absent from southeastern Polynesia; 3) the diversification of different insect lineages in southeastern Polynesia has been idiosyncratic, despite sharing the same host plants across the same set of archipelagoes; 4) specialization in pollinating Epicephala moths is reduced on some islands relative to continental regions. I conclude by discussing some hypotheses for understanding the role of niche conservatism and adaptive radiation in assembling communities on oceanic islands.