The diagnostic characters given in this paper were established generally on the basis of current knowledge of the pupal stage in Staphylinini. The key covers 8 subtribes and 20 genera (highlighted in Table 1 by an asterisk) of pupae found in Europe. Most of the data and all the drawings have been taken from papers by the present authors, published between 1996–2014. Some information relating to Abemus and in part to: Creophilus, Ocypus, Philonthus, Platydracus, Quedius and Staphylinus, has been taken from papers by other authors (e.g., Szujecki 1965; Dajoz and Caussanel 1968; Orth et al. 1976; Boháč 1982, 1987; Schmidt 1994b; Moseley et al. 2006; Vorst and Heijerman 2015). The paper also includes new information on the pupa of Emus hirtus (L.) which was hitherto unknown, and photographs and notes supplementing existing descriptions of the pupae of nine genera, represented by species: Acylophorus wagenschieberi Kies., Creophilus maxillosus (L.), Gabrius appendiculatus Sharp, Ocypus fulvipennis (Er.), Quedius microps (Grav.), Rabigus tenuis (Fabr.) and Staphylinus erythropterus L. The photographs were taken with an Olympus DP72 digital camera mounted to an Olympus SZX16 compound microscope (Fig. 7) or with a VEGA3 TESCAN SEM (Figs 2a, 2b, 6b, 13, 14), and corrected using CorelDRAW Graphics Suite X6.

Material that was here examined for the first time includes one pupa of C. maxillosus (male) and one exuvium of E. hirtus, obtained from the collection of the Zoological Museum of the University of Copenhagen, Denmark (NHMD, the Natural History Museum of Denmark). The pupae of these four species, as well as others previously described by the authors, are deposited in the collection of the Department of Zoology, Marie Curie Skłodowska University, Lublin, Poland.

The terminology follows Staniec (1999a, b, 2002) and Pietrykowska-Tudruj and Staniec (2011). Measurements and their abbreviations are after Pietrykowska-Tudruj and Staniec (2012) and Pietrykowska-Tudruj et al. (2014b) marked on Figures 1, 2, 5, 18, 26, and 32 as follows: BL body length, BW body width, HW head width, HL head length, PW pronotum width, Abbreviations of the body parts as follows: A accessory, An antenna, As atrophied spiracle, El elytra, Fs functional spiracle, Fti fore tibia, H head, Hti hind tibia, K knee, Li labium, Lr labrum, Md mandible, Mp maxillary palp, Ms mesonotum, Mt metanotum, Mti mid tibia, P pronotum, Pr protuberance, Sap spiracular appendage, S spine, Sp setiform projection, St sternite, Tp terminal prolongation, Tr tergite, W wing, Vp ventral prolongation.

The phylogenetic analysis was carried out at the generic level. A data matrix was assembled in Nexus Data Editor for Windows v. 0.5.0 (Page 2001) that included 29 morphological characters of the well-known pupae from 22 species as the ingroup belonging to 20 genera of Staphylinini from 8 subtribes (Table 4). Each genus is represented by one species, except for Quedius (three species from the subgenera Distichalius, Raphirus, Microsaurus). As the pupae of the species from these three subgenera differ in certain morphological characters, they have been included in the data matrix. Some genera (Belonuchus, Cafius, Emus, Remus, Smilax, Triacrus, Hadropinus) have not been included because the available morphological data on their pupae are too fragmentary and superficial, and therefore deemed unreliable. The pupa of Hypnogyra angularis (Ganglbauer, 1895) from the tribe Xantholinini is added as an outgroup to the tribe Staphylinini (Staniec and Pietrykowska 2005a, Pietrykowska-Tudruj and Staniec 2006a, unpublished data). Inapplicable characters are assigned a gap value (‘−’) and treated as equivalent to missing data (‘?’). The matrix was analysed in TNT (Goloboff et al. 2008) under settings as follows: the ‘traditional search’ option for the parsimony analysis – 1000 replicates with tree bisection reconnection (TBR) branch swapping and saving 1000 trees per replicate, zero-length branches collapsed, all characters were treated as unordered and equally weighted.

Pupa obtect. Body clearly slender, almost cylindrical and weakly sclerotised (e.g., Neobisnius), or moderately elongate, slender and moderately sclerotised (e.g., Erichsonius, Gabrius, Heterothops, some species of Philonthus), or moderately stocky and well sclerotised (e.g., Astrapaeus, Quedius) to extremely stocky and strongly sclerotised (e.g., Atanygnathus). Colour: almost white or pale yellow shortly after pupation; from dark yellow to reddish brown a few days after pupation; usually almost black just prior to emergence of imago.

Head directed ventrally towards thorax, without any setiform projection or spines, rarely with a few protuberances. Labrum usually V-shaped, exceptionally U-shaped, with short, longitudinal groove running from its anterior margin. Mandibles elongate, usually pointing posteriorly, falcate or almost straight. Maxillae usually moderately long. Antennae curved, rest on knees of fore and mid legs; apex usually protruding beyond knee of mid tibia. Scutiform pronotum widest at the base, usually about as wide as long with 6–32 setiform projections, or a pair of micro spines or 8–26 protuberances, sometimes with no structures. Mesonotum separated from pronotum by a furrow, distinctly wider than long. Metanotum narrower than mesonotum with deeply bisinuate anterior margin. Elytra shortened. Wings protruding to ventral side. Apex of wings protruding at most beyond posterior margin of I (morphologically III), clearly visible abdominal segment. Tibiae and tarsi directed obliquely towards body middle. All tibiae, or only some of them with pointed protuberances. Hind tarsi at most reaching midpoint of V (morphologically VII), clearly visible abdominal segment.

Abdomen with 9 somewhat flattened tergites and 7 convex sternites visible. Abdominal tergite I wider than others and about twice as long as tergite II. Abdominal shape of three kinds: arcuate, with parallel sides or funnel-shaped. Sides of abdomen with: spines on segments II–VIII or II–VII, or setiform projections on segments III–VIII or VII–VIII. Rarely abdomen without any lateral cuticular projections. Last segment usually strongly protruding into two terminal, elongated prolongations, sometimes weakly protruding into two triangular prolongations, exceptionally without prolongations. Terminal sternite with well-marked sexual dimorphism. Gonotheca in female double, in male single. In female pupae, terminal sternite often with a pair of prolongations. Abdominal tergites I–IV with tuberculate, functional spiracles, the first pair usually situated more laterally, most often larger and protruding farther than the others; tergites V–VIII with externally visible, but apparently atrophied spiracles.

The following crucial characters distinguish the pupae of the tribe Staphylinini from the tribe Xantholinini within the subfamily Staphylininae for which the pupae are known: abdominal segments divided laterally into ventral and dorsal sclerites (not grown into uniform rings); body with setiform projections, spines or protuberances, apart from the genus Astrapaeus which has no cuticular processes (Staniec and Pietrykowska 2005a, Pietrykowska-Tudruj and Staniec 2006a). The combination of characters distinguishing the pupae of Staphylinini within the family Staphylinidae, i.e., the subfamilies Aleocharinae, Omaliinae, Oxyporinae, Oxytelinae, Paederinae, Steninae and Tachyporinae for which the pupae are known, includes: i. exarate pupa; ii. no projections whatsoever on head; iii. short labium; iv. lack of short setae on dorsal and/or ventral part of abdominal sclerites; v. lack of setae on hind margin of prothorax.

Based on current knowledge of the pupal morphology of Staphylinini species, eight morphological types were distinguished: Acylophorus (genus: Acylophorus), Astrapaeus (species: Astrapaeus ulmi (Rossi)), Atanygnathus (species: Atanygnathus terminalis (Er.)), Erichsonius (genus: Erichsonius), Heterothops (genus: Heterothops), Philonthus (genera: Bisnius, Gabrius, Hesperus, Neobisnius, Philonthus and Rabigus), Quedius (genera: Quedius and Quedionuchus) and Staphylinus (genera: Abemus, Creophilus, Emus, Ocypus, Ontholestes, Staphylinus and Tasgius). These types take into consideration pupae from 20 genera, most of which have been described by the present authors. The diagnosis of the types is presented in Table 2.

Key to subtribes of Staphylinini

Subtribe Staphylinina

1. Setiform projections or spines on the body (excluding segment IX): 0. absent, 1. present.

2. Protuberances on head: 0. absent, 1. present.

3. Number on protuberances on head: 0. 2, 1. 7.

4. Labrum, shape in outline: 0. U-shaped, 1. V-shaped.

5. Mandibles, shape of apices: 0. rounded, 1. pointed.

6. Maxillary palps, length: 0. protruding beyond apices of mid legs, 1. not protruding beyond apices of mid legs.

7. Antennae length: 0. at most reaching apices of mid tibiae, 1. protruding beyond apices of mid tibiae.

8. Protuberances on pronotum: 0. absent, 1. present.

9. Number of protuberances on pronotum: 0. about 10, 1. about 25.

10. Setiform projections on pronotum: 0. absent, 1. present.

11. Number of setiform projections on pronotum: 0. four, 1. more than four.

12. Length of setiform projections on pronotum: 0. shorter than half pronotum length, 1. at least half as long as pronotum.

13. Spines on pronotum: 0. absent, 1. present.

14. Protuberances on mesonotum: 0. absent, 1. present.

15. Pairs of tibiae with protuberances: 0. only mid, 1. mid and fore, 2. mid and hind, 3. all pairs.

16. Position of hind tarsi in relation to abdominal segments: 0. not adhering to abdomen, 1. adhering to abdomen.

17. Wing length: 0. at most reaching hind margin of segment III (I visible), 1. protruding beyond hind margin of segment III (I visible).

18. Structure of abdominal segments: 0. tergites and sternites fused into uniform rings, 1. tergites and sternites separate.

19. Abdomen shape: 0. gradually tapering from first to last segment, 1. tapering to last segment only in hind part.

20. Cuticular processes on sides of abdominal segment VII: 0. absent, 1. present.

21. Type of cuticular processes on sides of abdominal segment VII: 0. setiform projections, 1. spines.

22. Length of projections as spines on abdominal segment VII in relation to the width of that segment: 0. tiny, 1. long.

23. Cuticular processes on sides of abdominal segment VIII: 0. absent, 1. present.

24. Segment IX, terminal prolongations: 0. absent, 1. present.

25. Terminal prolongations, apical accessories: 0. absent, 1. present.

26. Apical accessories, shape: 0. straight, 1. curved.

27. Apical accessories, apex: 0. pointed, 1. rounded.

28. First pairs of spiracles, position: 0. in the same longitudinal line as others, 1. protruding laterally much more than others.

29. Appendages at functional spiracles of abdominal segments II–IV: 0. absent, 1. present.

The parsimony analysis retrieved 100 most parsimonious trees. The 50% majority rules consensus tree showed the following: i) separation of Astrapaeus from a clade of all other Staphylinini; ii) a well-supported clade of Erichsonius+Heterothops+Atanygnathus+ Acylophorus; iii) a well-supported clade of Staphylinini propria represented here by the subtribes Philonthina and Staphylinina (Fig. 35).

The occurrence of obtect pupae in the subfamily Staphylininae (including the tribe Staphylinini) is exceptional compared to other rove beetles and the majority of Coleoptera. The obtect pupa type with a compact body and usually heavily sclerotised cuticle appears to be far more resistant to negative impacts like attack from predators, parasitoids or mechanical damage. Therefore, an obtect pupa is probably a defensive adaptation. Presence of a tough cuticle in a pupa reduces the need for fully-grown larvae to construct a protective pupal cocoon, such as has been reported in a few Staphylinini species (G. splendidulus (Grav.), O. murinus (L.), R. tenuis) (Staniec 2004b; Pietrykowska-Tudruj and Staniec 2007; Staniec and Pietrykowska-Tudruj 2008c). Pupal cocoons are often encountered in representatives of the Aleocharinae, which have free, delicate pupae with weakly sclerotised cuticle (Frank and Thomas 1984; Staniec et al. 2010; Zagaja et al. 2014).

Another form of defensive adaptation is cuticular processes, which occur on various parts of the body (pronotum, abdominal segments) in almost all known pupae of the Staphylinini. They can take several forms: flexible, often arcuate setiform projections (e.g., Gabrius, Philonthus, Staphylinus), stiff spines (e.g., Quedius, Erichsonius, Heterothops) and, more rarely, convex protuberances (e.g., Acylophorus). These structures, besides having a defensive function (Hinton 1955), appear to minimise direct contact between the pupal body surface and the surrounding substrate (e.g., soil, leaf litter, plant remains, decaying wood), which probably allows the spiracles to function more efficiently. The number, length and shape of setiform projections or spines are also usually associated with the degree of cuticular sclerotisation and pupal body size. Their presence is particularly important for pupae with a weakly sclerotised cuticle, less resistant to damage. For this reason, such pupae usually bear numerous long, flexible, arcuate (especially on the pronotum) setiform projections (e.g., G. splendidulus, N. villosulus (Steph.) and most Philonthus species). In contrast, pupae with highly sclerotised cuticle bear far fewer such structures on the pronotum (e.g., Quedius sp. and A. terminalis), and in a few cases, the entire body surface is devoid of them (e.g., A. wagenschieberi and A. ulmi).

The number of setiform projections on the pronotum is a variable character within a species, the range of which is usually small (e.g., R. tenuis – 10–12, P. nigrita (Grav.) – 9–13), but occasionally much larger (e.g., P. rectangulus – 13–21, P. politus (L.) – 15–22). On small pupae there are usually fewer setiform projections on the pronotum (e.g., G. splendidulus – 7–9, N. villosulus – 7–8) than on much larger ones (S. erythropterus – 19–25, P. decorus (Grav.) – 22–24), a fairly obvious observation.

The pupae of some species from very wet habitats (Atanygnathus terminalis, Acylophorus wagenschieberi) exhibit special adaptations to their environment in the structure and localisation of spiracles (Staniec 2005a, b). They pupate among unsubmerged peat mosses (Sphagnum). However, water levels frequently vary in the peat-bogs they inhabit. The first pair of spiracles is thus especially large and protrudes strongly from the body outline, which facilitates gas exchange even at high levels of moisture. In the case of A. terminalis, the next three active spiracles additionally bear peculiar phylliform lobes. These probably serve to accumulate a supply of air in case the peat mosses are suddenly flooded or act as an additional respiratory surface (Staniec 2005b).

In the natural environment, larvae and adults of Staphylinini mostly live in the same microhabitats; pupation usually takes place there, too (Table 3). Only in the case of the above-mentioned species from wet microhabitats subject to flooding were pupae found in distinctly drier locations than those inhabited by mobile adults or larvae. These were usually unsubmerged layers of peat mosses, directly adjacent to higher-lying areas of bogs, 2–3 m from small bodies of standing water (Staniec 2005a, b). In the field, pupae of Staphylinini were found from spring to autumn (IV–X), although the pupae of most Quedius species were found in spring (IV–V). Among Quedius, only species confined to special microhabitats, such as tree hollows, crevices under tree bark or the vicinity of hornets’ nests, pupated during the summer (VII) or autumn (X). These phenological observations are in broad agreement with laboratory breeding data. In the laboratory, however, specimens from the same species pupated earlier (mainly V–VI) than in nature (Table 3), probably because of the more stable and warmer conditions (T = 20–24 °C) there, where the pupal stage lasted from 7 to 13 days, depending on the species. Larger species, e.g., A. ulmi and S. erythropterus, took distinctly longer to pupate than smaller ones, e.g., R. tenuis, P. nigrita (Table 3) (Staniec and Pietrykowska-Tudruj 2008a, c; Pietrykowska-Tudruj and Staniec 2012; Pietrykowska-Tudruj et al. 2014b).

Two major morphological groups were clearly distinguishable among the pupae of Staphylinini: ‘Staphylinini propria’, represented here by the subtribes Philonthina and Staphylinina only and forming a well-supported clade in our analysis, and ‘non-Staphylinini propria’, represented by the genera Acylophorus, Astrapaeus, Atanygnathus, Erichsonius, Heterothops and Quedius, whose systematic affiliation at the subtribal level has in recent years been the topic of much debate and has undergone far-reaching changes (e.g., Solodovnikov and Schomann 2009; Chatzimanolis et al. 2010; Brunke et al. 2016).

The critical characters distinguishing the two groups are: i) the presence of setiform projections on the pronotum in all species of the Staphylinini propria group, but their absence in non-Staphylinini propria; ii) the type of cuticular process on abdominal segment VII – setiform projections in the former clade, but spines in the latter (except for Acylophorus, which has no cuticular processes). Among the non-Staphylinini propria, a clade was recovered with species whose pupae bear protuberances on the body (the genera: Acylophorus, Atanygnathus, Heterothops and Erichsonius). These protuberances may be situated on the head, pronotum and mesonotum (Acylophorus, Atanygnathus), only on the head and pronotum (Heterothops), or only on the pronotum (Erichsonius). Pupae of Quedius do not possess any protuberances.

Since subtribal classification within the non-Staphylinini propria has undergone substantial changes in recent years. We discuss below the phylogenetic potential of the external pupal structures of some taxa in the light of such taxonomic revolutions.

There are practically no cuticular structures on the pupa of A. ulmi. The cuticular surface is devoid of any visible processes or protuberances (not including those on the legs of all pupae of the Staphylinini and the tiny accessories on terminal prolongations), which makes this species unique among the known pupae of the Staphylinini. Phylogenetic research based on adult and larval morphology, including fossil taxa, suggests that the monotypic genus Astrapaeus is not related to the subtribe Quediina (its traditional placement) but is a member of a rather isolated and basal lineage within Staphylinini (Solodovnikov and Schomann 2009; Solodovnikov 2012; Brunke and Solodovnikov 2013; Solodovnikov et al. 2013; Pietrykowska-Tudruj et al. 2014b). Based on molecular and morphological evidence, Astrapaeus is now included in subtribe Cyrtoquediina Brunke et al. 2016. Within Cyrtoquediina, a subtribe comprising species with mostly isolated distributions in the Neotropical, Oriental or Palaearctic regions (e.g., Bolitogyrus, Cyrtoquedius, Parisanopus, Sedolinus), only Astrapaeus occurs in and is restricted to Europe. The pupal characters of Astrapaeus support the isolated position of A. ulmi (and potentially other Cyrtoquediina) within the tribe, and outside the Quediina sensu Brunke et al. 2016. However, given the lack of data on the pupae of other members of the subtribe Cyrtoquediina, it is it is difficult to tell which morphological features of Astrapaeus are representative of the subtribe versus just genus level.

Within Erichsonius, a genus including more than 160 species distributed over almost all the world, the pupal stage is known for just three: the Nearctic E. alumnus Frank and E. pusio (Horn) (Schmidt 1996) and the Palearctic E. cinerascens (Grav.) (Pietrykowska-Tudruj and Staniec 2006c). Until the end of the 20^th century, this genus was placed the subtribe Philonthina. But many recent phylogenetic analyses of adults utilising morphological and molecular data have indicated that the original placement of Erichsonius was incorrect (e.g., Brunke and Solodovnikov 2013; Chani-Posse 2013; Brunke et al. 2016). Initially, the genus was withdrawn from Philonthina and allocated to the Anisolinina grade within Staphylinini propria. However, the latest analyses show that Erichsonius is monophyletic and forms a separate subtribe Erichsoniina (Brunke et al. 2016, Chani-Posse et al. 2018).

The pupa of Erichsonius possesses a series of characters clearly distinguishing it from species classified among Staphylinini propria. They are: i) a lack of setiform projections on the pronotum; ii) the presence of protuberances on the pronotum; iii) cuticular processes on the abdominal segments in the form of spines. At the same time, these characters are shared with species of four genera of non-Staphylinini propria, i.e., Atanygnathus and Heterothops (all characters), Acylophorus (characters i and ii) and Quedius (characters i and iii). The results of our analyses suggest Erichsonius is distinguished from all other non-Staphylinini propria with known pupae by the number of protuberances on the pronotum. Pupae of Erichsonius have few protuberances (10 at most), whereas they are more numerous (more than 10) on the pupae of other taxa. Since the pupal stage is unknown in many other genera of Staphylinini and Erichsonius species, it is hard to assess the extent to which the number of protuberances is consistent within and unique to the genus. Given the present state of knowledge of pupae, we can regard it as unique to Erichsonius, and therefore evidence in favour of the recently erected subtribe Erichsoniina (Brunke et al. 2016).

Within Heterothops, a globally distributed genus with 149 described species, the pupal stage is known only in H. praevius (Herman 2001; Pietrykowska-Tudruj and Staniec 2006c). This poorly defined genus was moved from the conventional subtribe Quediina and initially included in the large lineage Tanygnathinina sensu Solodovnikov and Schomann 2009; later it was placed in the subtribe Amblyopinina, containing fauna mainly from the Neotropical and Australian regions (Solodovnikov and Schomann 2009; Assing and Schülke 2012; Solodovnikov 2012).

Our analyses have demonstrated that the pupa of Heterothops has many characters in common with Atanygnathus. They are: processes on the head and pronotum, spines on abdominal segments II–VII, broad elytra, short hind leg tibiae (not reaching the lateral margin of the body), protuberances on the mid and hind legs, and long antennae. There are not many characters (not present in Staphylinini propria) shared between Heterothops (H) and Quedius (Q) (spines on abdominal segments and protuberances on the mid and hind tibiae), whereas there are many differences: head size (in proportion to the rest of the body) (small – H, large – Q), protuberances on the head and pronotum (present – H, absent – Q), antenna length (long – H, short – Q), width of elytra (wide – H, narrow – Q). In the light of current knowledge of Staphylinini pupae, one can state unequivocally that the morphology of Heterothops pupae supports the separation of this genus from the subtribe Quediina. There are several recent studies based on adult characters, or in combination with DNA that have confirmed its placement within the subtribe Amblyopinina (e.g., Brunke et al. 2016; Chani-Posse et al. 2018; Brunke et al. 2019). Among the 17 genera forming this group, only the pupa of Heterothops, the single taxon in this group which occurs beyond the southern hemisphere, is known (Brunke et al. 2016).

The present study has shown that the external structures of Staphylinini pupae could be a useful, alternative source of evidence for resolving the relationships of some higher taxa within the tribe. However, much more descriptive work is needed – mainly expanding the data matrix to include new species/genera and compiling new morphological data. Unfortunately, the pupae of many species of phylogenetic interest will probably remain unknown owing to the great difficulties with their collection and identification.