﻿A new species of Petalacmis firefly from Bolivia, with a key to species (Coleoptera, Lampyridae)

﻿Abstract Petalacmis Olivier, 1908 is a poorly known genus of firefly endemic to South America and is currently the only member of the subfamily Lampyrinae, tribe Lampyrini known to occur on the continent. Here, we describe a new species, Petalacmistriplehornisp. nov. from Bolivia and compare it to the two other described species in the genus. A key to Petalacmis species based on male traits, as well as illustrations of morphological features, are given in detail for the first time. We present unique, previously neglected traits of Petalacmis species and compare them to other Lampyrinae.


Introduction
Petalacmis E. Olivier, 1908 is an interesting and unique genus of fireflies (Coleoptera, Lampyridae) with distinctive antennal morphology: males have only nine antennomeres, the ninth very elongate and paddle-shaped. Petalacmis is poorly represented even in large collections worldwide (LS and MB pers. obs.), and even basic aspects of its morphology are lacking due to the rarity of specimens available for dissection. In fact, this genus is only known from male specimens, a widespread phenomenon in lampyrids Mermudes 2013, 2014;Ferreira et al. 2019Ferreira et al. , 2020Bocakova et al. 2022) and in elateroids as a whole (e.g., Bocak et al. 2016;Biffi et al. 2021). Therefore, detailed studies on the diversity of Petalacmis species are greatly needed in order to produce a more comprehensive understanding of the family Lampyridae, particularly in the tribe Lampyrini. Astonishingly, Petalacmis is the only known genus of its tribe known to occur in South America, where it remains more poorly known than its counterparts in both the Old World and North America.
Petalacmis was erected for its type species, Petalacmis praeclarus E. Olivier, 1908, with no subfamilial placement, by Olivier (1908. In a later work, Olivier (1910) placed the genus in the subfamily Photininae. In his 1959 work, Green moved Petalacmis to the superfamily Lampyrinae and the tribe Lampyrini. Green's placement of Petalacmis was followed in McDermott's subsequent taxonomic work on Lampyridae (McDermott 1964) and his 1966 catalog. Phylogenetic analyses consistently placed Petalacmis in the subfamily Lampyrinae, but its affinities remain unsteady. Phylogenetic analyses based on morphological data recovered Petalacmis close to the Neotropical Pleotomini Summers, 1875 (Jeng 2008), whereas molecular-based phylogenies found it closer to part of Lamprocerini Olivier, 1907(Martin et al. 2019. The most recent comprehensive classification places Petalacmis in Lampyrini (Martin et al. 2019).
Petalacmis currently consists of two species: P. praeclarus from Brazil, Bolivia, and Peru, and Petalacmis wittmeri Reichardt, 1963 known only from Brazil. A third species, Petalacmis triplehorni sp. nov. only known from Bolivia, is described here. We provide the first identification key to Petalacmis species based on male morphology and document the morphological features of this genus for the first time. A discussion of Petalacmis morphology and its comparison to other Lampyrinae is presented.

Materials and methods
Specimens were both studied and imaged under dissection microscope Leica M205 C. Digital images were obtained and stacked using the Leica Application Suite X. Specimens of P. praeclarus and P. triplehorni sp. nov. were measured under a Leica MZ16 microscope with a calibrated eyepiece graticule, and measurements were converted to millimeters (Table 1). A whole specimen, as well as the abdomen of a second were soaked in KOH 10% for 24 h before dissection to digest soft tissues. The classification scheme used in this study follows Martin et al. (2019), morphological terminology follows Silveira and Mermudes (2014), and wing venation nomenclature follows Lawrence et al. (2021) Diagnosis. The three species of Petalacmis are easily diagnosable by size (Table 1), along with the morphology of antennae, elytra, and pygidium (see key below). Petalacmis triplehorni sp. nov. can be identified by the following combination of characters: antennomeres V and VII slightly longer and wider than adjacent antennomeres ( Fig. 2H-I), elytron subparallel-sided (Fig. 3L), pygidium with sides divergent to basal third, then convergent apically with almost straight margins, posterior margin slightly bisinuose, lateral thirds subequal to or slightly longer than median third (Fig. 4A, C). Measurements are given in Table 1.
Description of male. Color pattern: overall brown, except for the dark brown pronotal disc, translucent parasagittal rounded spots on pronotal expansions, and translucent sterna VI-VIII (Fig. 1); pronotal expansions often light brown (Suppl. material 1: Fig. S1). Table 1. Comparative measurements (average, range between parentheses) between the three known species of Petalacmis. Measurements were taken from the material examined (see above), except for those of P. wittmeri, which were taken from Reichardt (1963).
Female and immature stages. Unknown. Etymology. This species is named for Dr Charles "Chuck" Triplehorn, Professor Emeritus of the Ohio State University, who collected the first specimens of Petalacmis praeclarus that one of us (MAB) first encountered as a graduate student while investigating the systematics of the family Lampyridae. Dr Triplehorn has been both a mentor and an inspiration to MAB. It is with great appreciation and respect for Dr Triplehorn that we name this species after him. (1). Antennomeres V and VII as wide as VI and VIII, apical antennomere nearly 2× longer than remaining antennomeres together (Fig. 5A); elytral outer expansion (also known as explanate margin) extending up to 2/3 of elytral length, yellowish (Fig. 3C, D) Antennomeres V and VII slightly longer and wider than neighbor antennomeres, apical antennomere as long as the remaining antennomeres together (Fig. 2H, I); elytral outer expansion feebly developed throughout, and of the same color as the rest of the elytron (brown) or slightly brighter (Figs 1A, B, 3L; Suppl. material 1: Fig. S1A); pygidium with sides almost straight, posterior margin slightly bisinuose, lateral thirds subequal to or slightly longer than median 1/3 ( Fig. 4A-C)

Discussion
Distribution of Petalacmis spp.
To date, all Petalacmis species are known from lowland localities across South America east of the Andes. Petalacmis praeclarus was described from "Brazil". It has since been collected in Bolivia and Peru (Olivier 1908;Reichardt 1963;McDermott 1964McDermott , 1966Lawrence et al. 2001) and the Atlantic Rainforest in Brazil (see Material examined above). Such a widespread distribution is uncommon in Neotropical fireflies, and the existence of overlooked or cryptic species should be considered in future comprehensive taxonomic reviews. In 1963, Reichardt described the second species, P. wittmeri which was collected Ananindeua, in Pará state, Brazil. Petalacmis triplehorni sp. nov. was collected near Buena Vista, Santa Cruz, Bolivia. Due to its small size and nocturnal habit, the genus is likely to have been overlooked, particularly in South America, where taxonomic expertise in fireflies was largely lacking until fairly recently.
Thoughts on the mating system of Petalacmis species Due to the presence of large eyes and photic organs in the male, one might expect that both the male and female of these species are luminous and use luminous signals for pair-formation, as seen in several firefly subfamilies (Branham and Wenzel 2003;Branham 2010;Stanger-Hall et al. 2018). The fact that no females are currently known for any of the three species in this genus may suggest that females are sedentary, perhaps brachypterous or even apterous, as seen in other lampyrids (e.g., Faust 2017;Stanger-Hall et al. 2018). Alternatively, these species may be so uncommonly encountered that no females have been collected.

Morphology and systematics of Petalacmis
The affinities of Petalacmis have been investigated in two comprehensive phylogenies of Lampyridae. Based on morphological data, Jeng (2008) found Neotropical Pletomini Summers, 1875 (i.e., Calyptocephalus Gray, 1832, Phaenolis Gorham, 1880, Ophoelis Olivier, 1911, andRoleta McDermott, 1962) sister to Petalacmis. On the other hand, the molecular-based phylogeny by Martin et al (2019) found Petalacmis sister to Lucio Laporte, 1833 and Lamprocera Laporte, 1833, both in the Lamprocerini Olivier, 1907. Neither phylogenetic hypothesis found evidence of exclusive shared ancestry with the Lampyrini, where they are currently placed (Martin et al. 2019). However, the taxa which were more closely associated with Petalacmis by Jeng (2008) were not included in Martin et al. (2019); hence, the sister-lineage of Petalacmis remains unclear. As our study was the first to thoroughly survey the anatomy of Petalacmis, we provide some comparisons to inform the ongoing debate on the phylogenetic affinities of this genus. We assume that our description of P. triplehorni sp. nov. includes traits that are likely to be shared with other species in the genus, or even traits common to all of them. In addition to the very distinctive "petal-like" antennal morphology, we observed previously obscured traits of Petalacmis that differ significantly from those of other lampyrine genera. For example, Petalacmis is unique among lampyrine genera in having (i) an intumescent and laterally keeled frons ( Fig. 2A, D), (ii) mandibles short apically rounded (i.e., obtuse, not pointed) frons ( Fig. 2A, C-D), and (iii) a dorsal plate of the phallus much shorter than the phallobase and strongly bent dorsally (in ventral view), with sides parallel and straight in apical view, deeply grooved (in apical/ posterior view), and apically truncate (Fig. 4H-J). Other genera of Lampyrini often have frons that are not intumescent (e.g., flat or depressed between antennal sockets), mandibles apically very acute (i.e., needle-like), and phalli at least slightly longer than phallobase, sinuose or almost straight (but never strongly bent dorsally), with sides sinuose, and apex variably acute, often with arrow-shaped apices (e.g., Green 1959;Geisthardt 1986;Kazantsev 2010;Constantin 2014). Interestingly, the intumescent frons seen in Petalacmis is known in at least some Pleotomini (e.g., Roleta; Jeng et al. 2006), and in the amydetine taxa Magnoculus and Memoan (Silveira and Mermudes 2013;Campello-Gonçalves et al. in press).
Concerning aedeagal morphology, lampyrine taxa very often have parameres with an elongate, membranous apex-a trait also commonly found across Lamprocerini and Cratomorphini (e.g., Campos et al. 2018;Silveira et al. 2019). The same membranous apex is found on the aedeagus of P. triplehorni sp. nov., but in a rudimentary form (Fig. 4). The typical inner lobe of parameres (as seen for example in Lampyris (e.g., Geisthardt 1986;Kazantsev 2010;Constantin 2014) is also present in P. triplehorni sp. nov., although distinctly thinner and less sclerotized.
Recently, it has become more common that taxonomic studies report detailed exoand endoskeletal traits of the thorax, like the shape of meso and metatergal ridges, as well as those of the endosternites. Currently, no such information is available for any taxa of Lampyrini, which hampers any comparison. Nevertheless, we observed some interesting traits in Petalacmis triplehorni sp. nov. and compare it to known lampyrine taxa. For instance, the mesoscutellum of P. triplehorni sp. nov. is so reduced that it is almost triangular, despite the median pointed projection at the posterior margin (Fig. 3F). To our knowledge, no such rudimentary shape has been observed before in any other lampyrid. Moreover, no other Lampyrini is known to possess an extremely reduced and posteriorly pointed mesoscutellum. However, we observed a rather pointed mesoscutellum in the lamprocerine genus Tenaspis (e.g., T. angularis, T. sinuosa; LFLS pers. obs.).
The alinotum of P. triplehorni sp. nov. is clearly distinct, with a scutum-prescutal ridge that extends to less than half the length of the metanotum (Fig. 3G), otherwise reaching or almost so the posterior margin in most other lampyrids, where known (e.g., Silveira et al. 2019). Such a short scutum-prescutal ridge is only known in the distantly related Amydetinae (Silveira and Mermudes 2014;Campello-Gonçalves et al. in press). The function of that modified alinotum in Lampyridae remains unknown, but it likely reflects changes in the flight muscles that attach to these ridges, and possibly associated with changes in flight pattern. The metascutelum of P. triplehorni sp. nov. has rather oblique anterior ridges (about 45° to the lateral ridges), otherwise almost transverse as found in most other lampyrids (e.g., Silveira et al. 2019). It is also noteworthy that P. triplehorni sp. nov. has a metaendosternum with anterior margin wide and emarginate (a trait so far only observed in Cratomorphini; Campos et al. 2018).
Petalacmis shares with Lampyrini, Pletomonini, and Lamprocerini, the ventral position of abdominal spiracles, as well as the reduced mandibles. However, this genus lacks key traits of all these three tribes. For example: both Lucio and Lamprocera, the taxa placed close to Petalacmis in Martin et al.'s (2019) phylogenetic analysis, have biflabellated antennae, elytra broadly expanded laterally and bent ventrally, and a transverse pygidium. In contrast, Petalacmis has a petal-shaped antenna, elytra feebly expanded and almost flat in lateral view (not bent ventrally), and a pygidium much longer than wide. Petalacmis also lacks the typical biflabellate antennae, and the bilobate sternum VIII, of the Neotropical Pleotomini. Yet, Petalacmis and the Neotropical Pleotomini share an intumescent frons, and an elongate pygidium. Lampyrini taxa usually have a transverse pygidium (elongate in Petalacmis), and their parameres have a well-developed apical membranous projection (e.g., Geisthardt 1986;Kazantsev 2010;Constantin 2014), which is at best rudimentary in Petalacmis. Therefore, the unique combination of characters seen in Petalacmis makes its taxonomic placement within the family Lampyridae challenging. A well-substantiated tribal placement of Petalacmis in Lampyrinae would benefit from its inclusion in future phylogenetic analyses with expanded taxon sampling across the Lampyridae.
Taken together, these observations highlight the potential value of traits typically neglected in lampyrid taxonomy, and invite future anatomical studies concerning the Lampyrinae, particularly the Lampyrini. We hope that our study fosters a future comprehensive review of this interesting firefly genus.