Based on morphological characters, Guyanymphula cayennensisHeppner, 2015, new synonym, the type species, is synonymized with Cryptocosma perlalisLederer, 1863, the type species. As a result, GuyanymphulaHeppner, 2015, new synonym, and Cryptocosma Lederer (1863) are subjective generic synonyms because they are not based on the same type specimen, and as the oldest name, Cryptocosma Lederer has nomenclatural priority. Cryptocosma is transferred from the Acentropinae to the Glaphyriinae with the new tribe, Guyanymphulini Heppner, created for this genus. Lectotypes are designated for the type species, Cryptocosma perlalis Lederer, and Cryptocosma pigrissima Dyar (1914), revised status, which is removed from synonymy with C. perlalis. We provide illustrations of type specimens and a new diagnosis for the genus. We discovered new country records for C. perlalis from Colombia, Costa Rica, Ecuador, Guatemala, Guyana, Peru, and for C. pigrissima from Venezuela. The morphology and distribution of reduced adult mouthparts in the Glaphyriinae and the cataclystiform pattern, or metallic spots, along the hindwing outer margin in crambid subfamilies is discussed.