Nymphs of Hyalymenus, unlike adults, have a highly differentiated ant-like morphology. Both H. tarsatus and H. limbativentris feed mainly on reproductive parts of composites and solanaceous plants, respectively. Mimetic nymphs were observed on plants, together with ants, both day and night; adult Hyalymenus, however, are predominantly nocturnal. Ant-resemblance in nymphs is achieved by several structural adaptations which, when coupled with the mimic's zig-zag locomotion and constantly agitated antennae, produces a striking visual deception. Experiments in captivity showed that mimetic nymphs, but not adult Hyalymenus, are somewhat protected against attacks from the praying mantid Oxyopsis media found on their host plant. Colour and size changes through different nymphal instars of Hyalymenus allow the immature bugs to resemble, during their development, differently sized and coloured ant models. Similar-looking ant species seem to act as Müllerian mimics toward insectivorous vertebrates and invertebrates that avoid ants. It is suggested that nymphs of Hyalymenus gain Batesian protection by resembling available ant models of different Müllerian complexes. Density-dependent selection is thought to be responsible for the observed differences in mimetic morph proportions between populations of mimics, as well as for the mimetic strategy itself employed by nymphs of Hyalymenus. © 1985.834371384