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BackgroundLucidotini is a diverse tribe of lampyrine fireflies present throughout the New World, Europe, and Asia. Most of the over 30 genera have overlapping diagnoses, largely due to a lack of revisionary and phylogenetic studies. Widespread convergence in sensory morphology, traditionally used in genus-level diagnoses, further compounds the taxonomic issues surrounding the Lucidotini. Recent work has cast light on the value of terminalia and genitalic traits for Lucidotini taxonomy and called for a more thorough screening of morphological characters. Of special interest are basal outgrowths of the phallus (i.e., ventrobasal processes)—currently only known inAlychnusKirsch andPhotinusLaporte–that can be quite informative at the species level, but its variation within Lucidotini remains poorly studied. Most Lucidotini species remain only superficially described, while internal characters—including those of terminalia and genitalia—which could inform species identification and phylogenetic relatedness, remain unknown. Upon studying eight Lucidotini species superficially looking likePhotinusandPhotinoidesMcDermott—all of which bearing long ventrobasal processes–we raised the hypothesis that they belonged to a genus yet to be recognized. MethodsHere, we analyzed 97 morphological characters of 32 lampyrid species spanning 17 of 30 Lucidotini genera under Bayesian Inference. ResultsWe found evidence for the recognition and description ofSaguassugen. nov.to include seven new species (Saguassu acutumsp. nov.,Saguassu grossiisp. nov.,Saguassu manauarasp. nov.,Saguassu rebellumsp nov.,Saguassu rourasp. nov.,Saguassu serratumsp. nov.andSaguassu sinuosumsp. nov.), in addition toPhotinus dissidensOlivier ((transferred herein, thus generatingSaguassu dissidenscomb. nov.), for which we also designate a lectotype and two paralectotypes). This previously neglected lineage of Lucidotini spans four South American biomes: Amazon, Atlantic Rainforest, Cerrado, and Pampa. Interestingly,Saguassuspecies span a gradient of morphologies related to signaling: fromLampyris-style ventrally bulging eyes, tiny antennae and no lanterns; intermediate eyes and antennae, with complete lanterns as inPhotinus; to small eyes and long antennae and small lanterns as in manyLucidotaLaporte.Saguassugen. nov.was consistently found closely related to the three other Lucidotini taxa with ventrobasal processes (i.e.,Alychnus,Photinoides, andPhotinus). We provide an occurrence map of and a dichotomous key toSaguassuspecies, thoroughly compare this genus with co-occurring Lucidotini genera, and suggest steps towards a revision of the Lucidotini tribe. 

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.