Out of Xanthopygus (Coleoptera: Staphylinidae): escape from polyphyly

Abstract Xanthopygus as currently defined is the largest genus in the subtribe Xanthopygina (Coleoptera: Staphylinidae: Staphylininae) with 40 described species. However, the genus is poorly defined, morphologically heterogeneous and previous studies have questioned whether it is a natural group. A morphological (51 characters) Bayesian phylogenetic analysis was performed to test whether Xanthopygus is a monophyletic group. The analysis indicated that Xanthopygus was polyphyletic, and therefore species were split into four different genera. Xanthopygusnigricornis Scheerpeltz was transferred to Oligotergus as Oligotergusnigricorniscomb. nov. and Xanthopygusskalitzkyi (Bernhauer) was transferred to Styngetus as Styngetusskalitzkyicomb. nov. A new genus, Photinopygusgen. nov. was erected to accommodate the majority of the species previously in Xanthopygus and Xanthopygus sensu novo is used in a new restricted sense to accommodate the remaining species. Diagnostic features are provided to distinguish species in the genera Photinopygus and Xanthopygus from each other and all other Xanthopygina genera.


Introduction
Recently, Chatzimanolis and Brunke (2019) produced a comprehensive phylogeny of the subtribe Xanthopygina using morphological and molecular data and established several lineages within Xanthopygina. Catapulting from that work was the description of several new genera of Xanthopygina (Chatzimanolis 2019;Chatzimanolis and Hightower 2019;Chatzimanolis and Brunke 2021) that were placed in a phylogenetic framework. And while description of new genera is always exciting, many problems still exist with genera that have received little taxonomic attention over the last 200 years. One of the most problematic areas within Xanthopygina is the Xanthopygus lineage (briefly summarized in the next paragraphs), a group that includes Elecatopselaphus Scheerpeltz, Phanolinus Sharp, Triacrus Nordmann, Xanthopygus Kraatz, Xenopygus Bernhauer, along with Gastrisus nitidus Bernhauer and Genus 1 (a potentially undescribed new genus).
Phanolinus is perhaps one of the most charismatic taxa within Xanthopygina, and even Staphylinidae, with bright metallic coloration covering the whole body. However, many species were described solely based on the differences in coloration and many of them are potential synonyms (Chatzimanolis unpublished data). Elecatopselaphus was recovered as the sister group of Phanolinus (Chatzimanolis and Brunke 2019) and whether or not it should be treated as a separate genus or Phanolinus is still a matter of investigation. Xenopygus was revised by Caron et al. (2016) and Chatzimanolis and Caron (2016) provided clarifications and additions, but it is doubtful that the two species groups currently recognized in Xenopygus form a monophyletic group (Chatzimanolis and Brunke 2019;and this paper). Gastrisus nitidus and Genus 1 may belong to the same (new) genus, but more data and analyses are needed to clarify their position. Triacrus was shown to be nested within Xanthopygus (Chatzimanolis & Brunke 2019) but without support.
Xanthopygus ( Fig. 1) as currently defined (referred to as Xanthopygus sensu Herman to include all taxa of Xanthopygus as presented in Herman's 2001 catalogue) is the most speciose genus in Xanthopygina with 40 valid species. The name Xanthopygus refers to the bright yellow or orange coloration of segments 7 and 8. Notes on the biology of adults and larvae are known for Xa. cognatus Sharp (Quezada et al. 1969) but the biology of the remaining species of the genus is largely unknown. Since there is no comprehensive taxonomic treatment of Xanthopygus, there are no good characters to define the genus, which has been typically diagnosed with a combination of the following: superior marginal line of pronotal hypomeron not continuing to anterior margin, postcoxal process present, and tergites 3-5 with arch-like carina (e.g., Hayashi 1997;Navarrete-Heredia et al. 2002;Navarrete-Heredia 2004;Rodríguez et al. 2012). Unfortunately, these three character states are not unique for Xanthopygus and have arisen multiple times within the subtribe (Chatzimanolis and Brunke 2019;and this paper), and as early as 2014; Chatzimanolis (2014) hypothesized that Xanthopygus is not monophyletic.
Xanthopygus was described by Kraatz (1857) and included the species that Erichson (1839;1840) listed in 'Staphylinus Fam. VII and Philonthus Erichs. pro parte' (Herman 2001). Species in Fam. VII included (in the order listed by Erichson) Staphylinus sapphirinus Er., St. calidus Er., St. hilaris Er., St. tepidus Er., St. iopterus Er., St. cyanelytrius Er., St. chrysopygus Er., St. pyraster Er., (a junior synonym of St. haemorrhoidalis Germar also listed by Erichson), St. chrysurus, and St. faustus Er. Kraatz (1857, p. 540) listed the species Philonthus xanthopygus (Nordmann), Ph. herilis Er., Ph. analis Er., Ph. bicolor (Laporte), and Ph. mirabilis Er. as those he intended to move from 'Philonthus Erichs. pro parte' to Xanthopygus. As stated by Herman (2001), all included species in a group must be cited by available names according to Article 67.2.1 of the ICZN (ICZN, 1999). Thus, the first included species in Xanthopygus were those cited by Gemminger and Harold (1868) who included in Xanthopygus all the species listed above and used X. abdominalis Gemminger and Harold as a replacement name (without justification) for X. Xanthopygus (Nordmann). Xanthopygus abdominalis has been treated as junior synonym of X. xanthopygus (Nordmann) by all subsequent authors. Sharp (1876) added several more species to Xanthopygus and was the first one to recognize that the genus (as proposed by Kraatz) was morphologically heterogeneous. Later, Sharp (1884) established the genus Lampropygus Sharp to include L. xanthopygus (Nordmann), L. cognatus (Sharp), L. analis (Er.) and L. bicolor (Er.). Unfortunately, the characters (ligula less emarginate, pronotum anterolaterally restricted) provided by Sharp (1884) to establish the concept of Lampropygus are present in Xanthopygus as well. In 1906, Bernhauer (1906 placed the last two species (analis and bicolor) in the genus Xenopygus Bernhauer. Based on his publications (e.g., Bernhauer 1905Bernhauer , 1906Bernhauer , 1917Bernhauer , 1927, Bernhauer agreed with Sharp on the concepts of Xanthopygus and Lampropygus as established by Sharp, although neither Sharp nor Bernhauer provided clear diagnostic characters for these genera. Bernhauer (1906) established the subgenus Heteropygus Bernhauer for two particularly large species, L. giganteus Bernhauer and L. oliveirae (Lynch) in Lampropygus. Lucas (1920) designated L. xanthopygus as the type species of Lampropygus. Blackwelder (1943) seemingly ignored the generic concepts that had been established by Sharp and Bernhauer for Lampropygus and Xanthopygus, and designated L. xanthopygus (Nordmann) as the type species of Xanthopygus, which resulted in Lampropygus becoming a junior synonym of Xanthopygus. This nomenclatural act established the concept of Xanthopygus as it stands today before the results of this paper. Perhaps to his credit, Blackwelder (1943;p.450 footnote) realized that he was giving a new meaning to Xanthopygus and suggested that new generic assignments would be needed in the future for some of the species in Xanthopygus.
While it is rather obvious from the taxonomic history above that Xanthopygus is not homogeneous, the goal of this paper is to use a phylogenetic framework to show that Xanthopygus sensu Herman can be confidently split into two or more taxa. Additionally, I seek to define diagnostic characters that can easily separate the various groups within Xanthopygus.

Taxon sampling
The focus of this paper was to determine whether the species currently in Xanthopygus sensu Herman form a monophyletic group. Thus, the analysis conducted focused on this goal rather than attempting to decipher the exact placement of all the different Xanthopygus species groups within Xanthopygina. For the ingroup, I included 21 species from Xanthopygus, comprising all the different species groups in that genus (Chatzimanolis unpublished data). Xanthopygus borealis Hatch was listed as a valid species of Xanthopygus by Herman (2001) but that species is a junior synonym of Tympanophorus puncticollis (Erichson). I also included 14 species as outgroup taxa, which included representatives of all genera belonging in the Xanthopygus lineage except Elecatopselaphus. From the Xanthopygus lineage I included the following taxa: Gastrisus nitidus, an undescribed taxon referred to as Genus 1 (Chatzimanolis and Brunke 2019), Phanolinus colombinus Bernhauer, Triacrus dilatus Nordmann, and four species of Xenopygus, representing both species groups within Xenopygus. In addition to the taxa of the Xanthopygus lineage, I included species from Gastrisus Sharp, Oligotergus Bierig and Styngetus Sharp since the overall habitus of these taxa is sometimes confused with that of Xanthopygus, and Philothalpus Kraatz (as distant outgroup). I examined the type specimens of all ingroup taxa included in the analyses, except for Xa. cyanelytrius (Perty), Xa. oliveirae Lynch and Xa. pexus (Motschulsky) that are considered lost. Specimens were examined from the following collections: the American Museum of Natural History (AMNH), the Natural History Museum of London (BMNH); the Canadian National Collection of Insects, Arachnids and Nematodes (CNC), the Field Museum (FMNH); the Naturhistorisches Museum Wien (NMW); the Natural History Museum of Denmark (NHMD), the Senckenberg Deutsches Entomologisches Institut (SDEI), the Snow Entomological Collection, Biodiversity Institute, University of Kansas (SEMC), the University of Tennessee at Chattanooga Insect Collection (UTCI), and the Museum für Naturkunde der Humboldt-Universität (ZMHB). A DarwinCore format file with the voucher numbers for all the material examined can be found as Suppl. material 1. Because not all specimens had catalogue numbers, I added a new label to every specimen examined to serve as the voucher number; these labels had the following format: 'Xanthopygus phylogeny voucher SC-123'. In addition to the specimens listed in the Supp. File 1, I have access to virtually all specimens of Xanthopygus sensu Herman since I have borrowed materials from museums around the world for the revisions, and I had the ability to check a wide range of specimens for characters that are difficult to observe.

Phylogenetic analysis
Bayesian analysis were conducted in MrBayes v.3.2.7 (Ronquist et al. 2012) running on the CIPRES Science Gateway v3.3 (https://www.phylo.org). Convergence was assessed by examining the Potential Scale Reduction Factor (PSRF) and Average Standard Deviation of Split Frequency values (ASDSF) in the MrBayes output. The matrix (Suppl. material 2) was treated as a single partition and the analyses were performed using the Mkv model with gamma distribution and correction for ascertainment bias, with two runs of four chains each, default temperature (temp = 0.1) and 10,000,000 generations. I used the 'trace all characters' analysis in Mesquite to map all character states on the tree and the results of this analysis are presented as Suppl. material 3. A maximum parsimony analysis was not performed since Bayesian analysis outperforms parsimony for analysis of discrete morphological data (e.g., Wright and Hillis 2014;O'Reilly et al. 2016).

Phylogenetic analysis
The Bayesian analysis (Fig. 5) of the morphological matrix converged after 10 million generations with ASDSF = 0.001 and all PSRF values = 1.000. The analysis strongly supported the monophyly of the Xanthopygina (PP = 1) but most of the backbone clades were either weakly supported or not supported. Species from Xanthopygus sensu Herman appeared in four different parts of the phylogenetic tree (see below for details), and based on these results, Xa. skalitzkyi is transferred to Styngetus as Styngetus skalitzkyi comb. nov., Xa. nigricornis is transferred to Oligotergus as Oligotergus nigricornis comb. nov., a large group of Xanthopygus species are transferred to a new genus, named here Photinopygus gen. nov. (see Table 2 for details on the taxonomy) and the remaining taxa are left in Xanthopygus sensu nov.
In a tree rooted by Philothalpus, all other taxa were placed in four different clades in a polytomy. The first clade contained Phanolinus colombinus, and the second clade Table 1. List of Xanthopygus species sensu Herman and their current name based on this paper. Bold type font on the first column indicates taxa included in the phylogenetic analysis. Taxa not included in this analysis but transferred to Photinopygus have all the diagnostic features of Photinopygus. Similarly, taxa that remained in Xanthopygus but were not included in the analysis have all the diagnostic features of Xanthopygus sensu novo.
: The species was listed as nomen dubium by Herman (2001), and was originally described as distributed in Austria, which is peculiar given that no Xanthopygina are known from the Palearctic. I have contacted the Curator of Coleoptera in the Natural History Museum of Graz, Austria, where the Grimmer collection is housed and no taxa matching this name exist in the collection (Hausl-Hofstätter personal communication). It is unlikely that any specimens exist that can be attached to this name. is composed of the sister groups Gastrisus sp. and Gastrisus mimetes (PP = 1). The third clade was unsupported (called here the Xanthopygus clade); it contained several species (Gastrisus nitidus, Triacrus dilatus, Genus 1 and the fours species of Xenopygus) and a large portion of the Xanthopygus species. The species of Xanthopygus in this clade formed a monophyletic group that was strongly supported (PP = 0.92) and will be treated as the Xanthopygus sensu nov. (for details see below on the Taxonomy section). Taxa included here were the ones placed in the genus Lampropygus by early taxonomists. Xanthopygus giganteus was the sister group of Xa. oliveirae (PP = 0.99) and together were the sister group of Xa. major but without support. This clade was placed in a polytomy with Xa. xanthopygus, Xa. cognatus, Xa. pexus and Xa. max. For a list of characters that support Xanthopygus sensu nov. see the Taxonomy section below and Table 2.
The remaining taxa in the fourth clade all belonged in Xanthopygus sensu Herman and were strongly supported as a monophyletic group (PP = 0.99). Xanthopygus punctatus was recovered as the sister group of Xa. flohri but without support (PP = 0.74) and together as the sister group of Xa. sapphirinus (PP = 0.65). That clade was placed in a polytomy with Xa. mirabilis, Xa. cyanelytrius, Xa. puncticollis, Xa. calidus, and a strongly supported clade (PP = 0.90) of Xa. chapareanus + Xa. faustus (PP = 0.95) as the sister group of Xa. rufipennis + Xa. dimidiatus (PP = 0.93). All these taxa previously in Xanthopygus are transferred to a new genus, Photinopygus gen. nov. and the apomorphies supporting this new genus are given below in the Taxonomy section and in Table 2.

Species included. The genus includes 20 species listed in Newton
Diagnosis. The genus is not revised so the following characters (in combination) should be considered only as a partial list: left mandible with single tooth; antennomere 1 less than twice as long as antennomere 2; eyes large; pronotum with dense micropunctures (not in all species).
Remarks. The type species was not available for the phylogenetic analysis. A formal revision of the genus is forthcoming (Chatzimanolis in preparation) where all species belonging to this genus will be treated and illustrated. Fig. 1F Type species. Philonthus viduus Erichson, fixed by subsequent designation by Blackwelder (1952) (Herman 2001). (2021) and Styngetus skalitzkyi comb. nov. based on the results of the phylogenetic analysis presented in this paper. For a complete taxonomic history of the genus see Herman (2001).

Species included. The genus includes 16 species listed in Newton
Diagnosis. The genus is not revised so the following characters (in combination) should be considered only as a partial list: left mandible with bicuspid tooth; protarsi without ventral pale macrosetae (not present in all taxa); metafemur with upper posterior margin crenulate; sternites 3-5 with arch-like carina.
Diagnosis. This genus can be distinguished from all other genera in Xanthopygina based on the combination of the following characteristics: head shape rectangular; posterior margin of head slightly extended posteriad on each side of the neck (apomorphy; except in Ph. corcovadoensis and Ph. mirabilis); antennomeres 1-5 elongate; labial palpomere 3 not securiform; medium size eyes; superior marginal line of pronotal hypomeron not continuing to anterior margin; postcoxal process present; pronotum with sparse micropunctures but no transverse lines visible at 70× magnification (apomorphy); mesoscutellum without dense micropunctures (apomorphy); mesoventral process narrow and rounded (apomorphy); metatarsi with setose dorsal surface (apomorphy); tergite 3 (at minimum, some species 3-4 or 3-5) with arch-like carina; and sternite 7 in males with emargination at posterior margin. For a list of characters that distinguish Photinopygus from Xanthopygus, see Table 2.
Etymology. The name is a combination of the Greek words φωτεινός (shining, bright) and πυγή (rump), and refers to the bright coloration of abdominal segments 7 and 8. The name is masculine.
Remarks. A formal revision of the genus is forthcoming (Chatzimanolis in preparation) where all species belonging to this genus will be treated and illustrated. Even though some of the species transferred to Photinopygus were not included in the phylogenetic analysis, they can be confidently placed in this genus since they have all the diagnostic features of Photinopygus (see Tables 1 and 2 for details).
Diagnosis. This genus can be distinguished from all other genera in Xanthopygina based on the combination of the following characteristics: head shape rectangular; head wider than pronotum (apomorphy; however, head size can be variable among specimens of the same species but wider than pronotum); antennomeres 7-10 transverse; left mandible with two teeth separated by deep emargination (apomorphy);  (Bernhauer), arrow points to the anterior transverse lines D abdominal tergites 3-8 of Xanthopygus cognatus Sharp, arrow points to arch-like carina on tergite 3 E lateral view of the aedeagus of Xanthopygus faustus (Erichson), arrow points to the serrated apical carina F lateral view of the aedeagus of Xanthopygus dimidiatus Bernhauer, arrow points to the hook-like carina. Not to scale. labial palpomere 3 not securiform; small size eyes (apomorphy); superior marginal line of pronotal hypomeron not continuing to anterior margin; superior and inferior marginal line of hypomeron produce anterolateral angles not parallel to one other (apomorphy); postcoxal process present; elytra coloration black (except with blue metallic overtones in Xa. xanthopygus); dorsal 1/3 of metepisterstum without punctures (apomorphy; state also present in Ph. mirabilis); with more than four spines on the posterior surface of metacoxae (apomorphy; less than four in Xa. xanthopygus); tergites 3-5 with arch-like carina; and sternite 7 in males with emargination at posterior margin. For a list of characters that distinguish Xanthopygus from Photinopygus, see Table 2.
Remarks. A formal revision of the genus is forthcoming (Chatzimanolis in preparation) where all species belonging to this genus will be treated and illustrated.

Discussion
The phylogenetic analysis presented here strongly rejected the hypothesis that Xanthopygus sensu Herman is a monophyletic group. As was previously defined, Xanthopygus included species that belonged in four distinct (and, as far as it is known, they are not sister to each other) clades, the genera Oligotergus, Photinopygus, Styngetus and Xanthopygus. The classification changes implemented in this paper resolve this issue by defining Xanthopygus in a new sense that includes some species that were described in Lampropygus (a synonym of Xanthopygus), although of the four species originally included in Lampropygus (Sharp 1884) two are now placed in Xenopygus (Xe. analis and Xe. bicolor, both included in the analysis here). However, Lampropygus was never clearly defined and included species (e.g., L. skalitzkyi) that clearly did not belong in that genus. Most of the species that belonged in Xanthopygus sensu Herman are placed in the new genus Photinopygus. Both Xanthopygus sensu novo and Photinopygus as presented in this paper are well-defined with clear diagnostic features that would hopefully prevent future misplaced species in these genera.
Styngetus skalitzkyi and Oligotergus nigricornis were clearly placed in Xanthopygus sensu Herman by mistake by Bernhauer (1906) and Scheerpeltz (1969), respectively. In both of these species, the superior marginal line of the hypomeron continues to the anterior end, which should have been a clear indication that the placement in Xanthopygus sensu Herman was erroneous. Granted, both of these species are atypical for either Styngetus or Oligotergus and these genera are still in dire need of revision since they contain multiple species of uncertain affinities (Chatzimanolis, unpublished data), not to mention the lack of clearly defined diagnostic features. Most species of Styngetus have a much narrower head than Styngetus skalitzkyi and some species of Styngetus have narrow protarsi (not seen in Styngetus skalitzkyi). However, the crenulate upper posterior margin of the metafemur is present in all species of Styngetus examined by me (and Styngetus skalitzkyi) and seems to be a good diagnostic character for the genus, pending its further review and phylogenetic analysis. In any case, Styngetus is probably more homogeneous than Oligotergus as currently defined. Oligotergus seems to include at least two distinct species groups, roughly split into species with dense small uniform punctures on the pronotum and species with larger, less dense punctures on pronotum. Oligotergus nigricornis belongs in the second group and whether these two species groups both should be included in Oligotergus is matter of further investigation.
One of the major issues with Xanthopygus sensu Herman was that the characters used to define the genus (superior marginal line of pronotal hypomeron not continuing to anterior margin, postcoxal process present, and tergites 3-5 with arch-like carina) are not unique to Xanthopygus and the genus was not easily recognizable. Even if somebody were to argue that the phylogeny presented here is not properly resolved, meaning that perhaps Xanthopygus sensu novo and Photinopygus might be sister groups and therefore do not have to be in separate genera, the reality is that Xanthopygus sensu Herman was impossible to diagnose with just the characters presented above. Perhaps more impor- tantly, Photinopygus and Xanthopygus sensu novo do not share any apomorphies that could be used to diagnose Xanthopygus sensu Herman 2001. The split of Xanthopygus sensu Herman into Xanthopygus sensu novo and Photinopygus makes both of these genera easily recognizable and identifiable, given the characters presented in Table 2.
One of the characters used to define Xanthopygus sensu Herman was the superior marginal line of pronotal hypomeron not continuing to anterior margin. Until recently, it was not clear how widespread this character state is among Xanthopygina. Triacrus dilatus has the same character state and this feature along with the position of Triacrus on the phylogeny of Xanthopygina (Chatzimanolis and Brunke 2019) led these authors to hypothesize that perhaps Triacrus belonged in Xanthopygus (but see below for characters that exclude Triacrus from Xanthopygus). However, the particular state of the superior marginal line of pronotal hypomeron seen in Xanthopygus and Triacrus is more common than previously thought and certain species of Gastrisus, Genus 1, and even some species of Plociopterus Kraatz exhibit this particular character state. It is likely that this character state is much more widespread in Xanthopygina than previously documented and has evolved multiple times in several different lineages. Additionally, given the parallel evolution, this character state alone should not be used as a justification for a hypothetical close relationship between Xanthopygus sensu novo and Photinopygus.
A caveat in this paper is that the backbone relationships presented are unsupported. This is certainly not uncommon in morphology-only analyses using Bayesian statistics, and previous morphology-only Bayesian analyses of Staphylinini had low support values (e.g., Brunke 2019, 2021). Usually combining morphological with molecular data alleviates nodes with low posterior probability values. But one problem with the addition of molecular data in papers that target relationships among species within genera rather than among higher taxonomic groups is the scarcity of DNA-grade material. For example, several of the species of Xanthopygus sensu novo are only known from the type and/or few additional specimens. While modern techniques have enabled the use of museum specimens in molecular analyses of Staphylininae (e.g., Brunke et al. 2021), using type materials for DNA analyses is still a sensitive subject with museum curators. Also, such techniques are expensive and thus may not be feasible for smaller standalone projects like this paper. Even though molecular data would have improved the resolution of the relationships presented here, the goal of this paper was to eliminate an obvious non-monophyletic group, Xanthopygus sensu Herman. For this purpose, the morphology-only analysis presented here was adequate and clearly indicated that Xanthopygus sensu Herman was polyphyletic. Discovering the exact phylogenetic placement of every species is a pending future task. The phylogenetic analysis presented in this paper differs to the one presented by Brunke (2019, 2021) regarding the placement of Photinopygus among Xanthopygina lineages. The analysis presented in this paper indicated Photinopygus belonging in a different lineage of Xanthopygina (the Plociopterus lineage) than Xanthopygus (that belongs in the Xanthopygus lineage) but that result was unsupported (in terms of posterior probabilities). However, further analyses are needed to test how closely related Photinopygus and Xanthopygus may be.
The phylogenetic position of Xenopygus within the Xanthopygus lineage of genera remains unresolved. Chatzimanolis and Caron (2016) proposed two species groups within Xenopygus (punctatus, which includes Xe. punctatus and Xe. excellens, and analis, which includes Xe. analis and Xe. bicolor) and cautioned that these species groups may need to be placed in different genera in the future. In Chatzimanolis and Brunke (2019) the two species of Xenopygus (Xe. excellens and Xe. analis) included did not form a monophyletic group. In this paper, I added two more species (Xe. punctatus and Xe. bicolor) in the analysis, hoping that the addition of these taxa may help clarify their phylogenetic position. The analysis in this paper failed to find support for a monophyletic Xenopygus and it is unclear if morphological data alone can resolve this puzzle. In any case, it seems unlikely (and unsupported by the current data) that the analis species group of Xenopygus and Xanthopygus sensu novo are closely related as had been hypothesized early on by Sharp (1884) by his placement of these taxa in Lampropygus.
Likewise, the position of Triacrus remains unresolved. In Chatzimanolis and Brunke (2019) Tr. dilatus was placed as the sister group of Xa. chapareanus (now Photinopygus chapareanus) and in Chatzimanolis and Brunke (2021) in a polytomy with Xa. xanthopygus and Xa. chapareanus. In this paper, Tr. dilatus is in a polytomy with Genus 1, Xenopygus and the clade that leads to Xanthopygus sensu novo. While the exact position of Tr. dilatus is unclear, it is likely that this species is not closely related to Photinopygus, and current data does not support its placement within Xanthopygus sensu novo. Triacrus can easily be excluded from Photinopygus or Xanthopygus sensu novo based on the shape of antennomere 5 (transverse), the shape of teeth on left mandible (one large tooth and one bicuspid tooth) and the lack of postcoxal process.
It is perhaps unfortunate that most of the species that used to belong in Xanthopygus sensu Herman required a new name and were transferred to Photinopygus. However, this action corrected existing taxonomic problems and was necessary. Unfortunately, changing the meaning of an existing genus name is not uncommon; for example, Chatzimanolis and Ashe (2005) completely changed the meaning of Philothalpus, and multiple other times a genus name has been drastically redefined in Xanthopygina (e.g., Dysanellus Bernhauer: Chatzimanolis 2012; Trigonopselaphus Gemminger and Harold: Chatzimanolis 2015b; Torobus Herman: Chatzimanolis 2018). It is very likely that further changes in the name usage might be necessary in Xanthopygina as revisionary work progresses, especially in poorly defined genera such as Gastrisus or Oligotergus.

Conclusions
The Bayesian phylogenetic analysis performed in this paper showed that Xanthopygus sensu Herman is polyphyletic. To solve this problem, one species was transferred to Oligotergus, another to Styngetus, a new genus Photinopygus was erected for many taxa previously in Xanthopygus and Xanthopygus sensu novo was restricted to the remaining species. The new diagnostic characters provided in this paper can be easily used to define Photinopygus or Xanthopygus. Even though this paper helped to untangle the relationships within Xanthopygus sensu Herman, the exact relationships of the genera within the Xanthopygus lineage are still uncertain and would probably require comprehensive molecular phylogenetic analyses to decipher.