The subgenus Tarnetrum consists of several large Sympetrum species of the Nearctic and Neotropical regions. Larval structure indicates that four species – Sympetrum corruptum, S. illotum, S. madidum and S. nigrocreatum — belong to this group. The large size of the larvae of these species, their reduced lateral spines and the lack of abdominal dorsal hooks separate them from the larvae of other Sympetrum species. The differences among the larvae of the Tametrum group have not been analyzed adequately. In this paper, these differences are diagnosed, and a key separating the species is presented. In addition, variation within a single species, S. illotum, is recorded. Larvae from the northern part of the range (northwestern United States) are compared with specimens from the south (southern Mexico and Costa Rica). Measurements of several diagnostic characters (metafemur, lateral abdominal spine length, etc.) are significantly larger in the northern specimens. The larva of S. nigrocreatum is described from specimens collected in Costa Rica.