The Simuliidae and the Blepharoceridae are of probable Cretaceous age in New Zealand, but the taxa present are specialised rather than primitive. The distributions of these taxa outside New Zealand, imply considerable conservatism in evolution. The divergence between species is slight in the Simuliidae but some Blepharoceridae are markedly divergent. A large part of the speciation in both families is probably attributable to environmental conditions during the Pleistocene.