Nine new species of Thrypticus (Diptera, Dolichopodidae) are described from South America: T. truncatus, T. sagittatus, T. yanayacu, T. circularis, T. chanophallus, T. romus, T. azuricola, T. formosensis, and T. taragui. All nine species were reared from the petioles and stems of aquatic and semiaquatic Pontederiaceae, five of them from water hyacinth, Eichhornia crassipes. These species appear to comprise the monophyletic truncatus Group characterized by a short and rather weakly sclerotised oviscapt, probably adapted to egg-laying in the soft petioles of Pontederiaceae. By contrast, most other Thrypticus species have a longer and more strongly sclerotised oviscapt for oviposition in hard-stemmed Poaceae, Cyperaceae, and Juncaceae. T. truncatus and T. sagittatus have potential as biological control agents for Eichhornia crassipes, which is a serious invasive weed in the Old World tropics. Notes are presented on the life history of these two species, based on field and rearing chamber observations.