Material from the California Academy of Sciences Arthropods of Madagascar project included an as yet undescribed rhyparochromid bug with enormously swollen second, third and fourth antennal segments. The morphology of these antennae is described and figured with line drawings and scanning electron micrographs. A similar antennal modification in another undescribed rhyparochromid from Oregon, USA, is also described and figured. A hypothesis for the evolution of these antennae is presented (myrmecophily), and morphological changes associated with myrmecophilous Heteroptera are surveyed.