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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. 

Lampyrinae Lucidotini contains nearly a third of the world’s Lampyridae. The lack of revisions and the overlap of diagnostic features across taxonomic levels have hindered identification and, therefore, further studies of Lucidotini taxa. The use of terminalia and genital traits in Lucidotini phylogenies has been fundamental to inform and update the genus-level delimitations in this group. One important open question is whether the presence of increased-length abdominal segment VIII (in relation to segment VII) in the closely related genera Scissicauda and Haplocauda is synapomorphic or homoplastic. In recent collecting efforts combined with studies of specimens deposited in different scientific collections, we found specimens hypothesized as four new firefly species from the Amazon basin that share the unique characteristics of the male abdomen of Haplocauda species. To test the hypothesis that these new species are monophyletic and sisters to Haplocauda species, and that the augmented segment VIII is synapomorphic to Scissicauda + Haplocauda, we ran phylogenetic analyses building upon a pre-existing character matrix, including a wide sample of Lucidotini and neighboring branches. Our results support the placement of the four new species described here (H. lata sp. nov., H. amazonensis sp. nov., H. aculeata sp. nov. and H. antimary sp. nov.) in Haplocauda. Importantly, one species with regular-sized segment VIII sclerites was recovered as the earliest diverging lineages of Haplocauda, suggesting that segment VIII was augmented at least twice in the Lucidotini—the other one being within Scissicauda. We also report, for the first time, a sympatry between Haplocauda species. We revised the definition of Haplocauda and updated the distribution of H. mendesi and the existing key to species.