Hexaminius foliorum n.sp. is described and separated from H. popeiana Foster on differences in adult and larval anatomy, supported by differences in cirral activity, copulatory activity and breeding. Structure, function and reproduction in H. foliorum are more specialised than in H. popeiana. The differences are related adaptively to the occupancy by H. popeiana of a variety of hard substrata, but not mangrove leaves, and the confinement of H. foliorum to the ephemeral habitat of immersible mangrove leaves. Hexaminius foliorum may be a specialised descendant of H. popeiana.