Here, we propose Uanauna gen. nov. (Lampyridae: Photinini), a new monotypic genus of fireflies endemic to the Brazilian Atlantic Forest. Uanauna angaporan sp. nov. occurs in Submontane and Montane forest habitats at the Serra do Mar mountain range, in the Southern Fluminense region (Rio de Janeiro State, Southeastern Brazil). The new species was recorded both on the mainland, at the municipality of Itaguaí and Rio de Janeiro, and at the continental island of Ilha Grande, in Angra dos Reis, at the Rio de Janeiro State. An occurrence map and illustrations of diagnostic features are given for the new taxa, as well as a key to Lucidotina genera in Brazil.

Photinini is the largest tribe of Lampyrinae fireflies, with over 30 genera and nearly 750 species, and includes taxa where adults may or may not have light organs. A focus on taxa with bioluminescent adults left the taxonomy of dark fireflies in poor condition compared to taxa with lit adults. A recent phylogenetic study based on molecular data supported the transfer of Scissicauda McDermott, 1964—traditionally placed in Amydetinae due to the flabellate antenna of its type species—to Photinini at a branch sided with Pyropyga Motschulsky, 1852 and Pyractonema Solier, 1849. Such placement had never been recovered before in morphology-based studies, and the consequences for the interpretation of character evolution in this lineage are yet to be addressed. Moreover, Scissicauda was previously thought to be endemic to the Atlantic Forest. Here, through phylogenetic analyses of 38 taxa and 108 morphological characters, we (i) expand on the concept of Scissicauda to include S. antennata sp. nov., S. asymmetrica sp. nov., S. aurata sp. nov., S. biflabellata sp. nov., S. gomesi sp. nov., S. jamari sp. nov., S. neyi sp. nov., S. truncata sp. nov., and (ii) transfer Lucidota malleri Pic, 1935 and, therefore, S. malleri comb. nov. We also recover Pyropyga + Pyractonema as sister to Scissicauda + Haplocauda, providing the first morphological evidence for their sisterhood. Noteworthy, the new species S. biflabellata sp. nov. and S. asymmetrica sp. nov. stand out as the first known Photinini species with biflabellate antennae. In addition, our study reports the first Scissicauda species in the Amazon (S. antennata sp. nov., S. aurata sp. nov., S. gomesi sp. nov., S. jamari sp. nov., and S. truncata sp. nov.) and Cerrado biome (S. neyi sp. nov.). Our study highlights the value of continued sampling and phylogenetic analyses of South American fireflies towards a revised classification and deeper understanding of this lineage.