Synthesis of Current Knowledge of the Morphology of the Larval Stages of Paederinae (Coleoptera; Staphylinidae), with a First Insight into the Mature Larva of Pseudomedon Mulsant & Rey, 1878, in the Light of a New Systematic Division

Simple Summary The subfamily Paederinae is an example of a, species-wise, highly diverse group about whose developmental biology a great deal remains to be discovered, especially the external structures of their developmental stages. Here, we give the first morphological description of the mature larva of the genus Pseudomedon with detailed illustrations of its structural features. We have carried out a comparative analysis of these traits at different taxonomic levels on the basis of the new tribal system established for Paederinae and the current knowledge concerning the larval morphology of these staphylinids. We also provide an identification key for known Paederinae larvae and review in detail the relevant literature and the state of knowledge of their morphology. Abstract This is the first morphological description of the mature larva (L2) of the genus Pseudomedon, belonging to the tribe Lathrobiini and subtribe Medonina. Detailed illustrations of its structural features are provided. Based on earlier published and new data, 10 and 18 diagnostic larval morphological characters for Paederinae and Pseudomedon, respectively, are proposed. In the light of the new tribal system established for the subfamily Paederinae and based on the current knowledge (including Pseudomedon) concerning the larval morphology of these staphylinids, a comparative analysis of the traits at different taxonomic levels was carried out: intertribal—between Lathrobiini and Paederini sensu nov., intersubtribal (Lathrobiini)—between Medonina and Lathrobiina, and intrasubtribal for Medonina. As a consequence, 12 intertribal, 2 intersubtribal and 3 intrasubtribal distinguishing features were selected. These features, appearing on the head, antennae and mouthparts of the larvae, confirm the validity of the recent proposals to alter the systematics of these staphylinids at higher taxonomic levels. Our proposed practical identification key to Paederinae larvae at the generic level is a synthesis of the current knowledge of Paederinae larvae, including new data. The work also gives a thorough review of the literature and the state of knowledge of the morphology of Paederinae larvae.


Introduction
Paederinae is one of the largest subfamilies of rove beetles (Staphylinidae and Coleoptera), containing around 7600 described extant species in 225 genera worldwide [1,2]. The majority of the studies relating to this group have mainly been taxonomic, in particular descriptions of new species or generic revisions, phylogenetic studies concentrating mainly on the morphological characters of adults, and to a lesser extent on larval stages, and very Table 1. Known larval morphology of Paederinae species. State of knowledge of morphology, descriptions: very good-descriptions extremely detailed; good-descriptions sufficiently detailed; fair-descriptions moderately informative; poor-descriptions insufficiently informative. State of knowledge of morphology, figures: very good-very well illustrated (the whole structure), usually with SEM photos; good-illustrations sufficient, but without SEM photos; fair-illustrations insufficient; poor-illustrations absent or sketches at best. State of knowledge of morphology, generally: very good-descriptions extremely detailed and well-illustrated, reliable for diagnostics and sufficient for phylogenetic analysis; good-descriptions sufficiently detailed with adequate (sometimes a little insufficient) illustrations, reliable for diagnostics but not fully so for phylogenetic analysis; fair-descriptions moderately informative, possibly with or without adequate illustrations, can be used for diagnostics but not for phylogenetic analysis; poor-descriptions sketchy, mostly without any illustrations or no description with inadequate illustrations, possibly ambiguous even for diagnostic purposes. Abbreviations: abdomen A, antenna An, epipharynx Ep, head H, hypopharynx Hp, labium Lb, labrum Lr, ligula Lg, labial palp Lp, leg L, mandible Md, maxilla Mx, maxillary palp Mp, nasale Na, pygopod Pg, thorax T, urogomphus Ug, all structures AS, diagnostic key characters only DKChO, general habitus GH, state of knowledge SK, selected structures SS, structures ST, total state of knowledge TSK. Symbols: * the table takes account of papers published since Bøving and Craighead [27], earlier ones, being solely of historical interest, were not analysed; ** after Paulian [19]?-difficult to assess because only a few key diagnostic characters were given.

Material Examined
Larval stages of P. obsoletus were obtained by rearing 4 adult specimens ( Figure 1A-C). The beetles were collected by sifting wet leaf litter in the ecotone at the water's edge of Lake Łukcze ( Figure 1D) near the village of Rogóźno (51 • 23 29.32 N, 22 • 57 24.07 E; Łęczna-Włodawa Lake District, SE Poland) on 18 March 2019. Rearing took place from 20 March to 1 June 2019 at room temperature (20 ± 3 • C). Adults and larvae were kept separately in plastic containers (18 × 18 × 7 cm in height), filled with humid soil. Larvae and adults were fed with mites and Collembola of various species. The material for the morphological examination and measurements is listed in Table 2.

Study Techniques
Measurements (mm) of the first (L1) and second (L2-mature) larval instars were made with cellSens Dimension v1.9 software using an Olympus BX63 compound microscope. Photographs showing the overall habitus of the larvae (L2) and the male adult used for rearing were taken with an Olympus DP72 digital camera mounted on an Olympus SZX16 compound microscope.
To prepare the microscope slides for the morphological analyses, the preserved larvae were soaked in 10% KOH for several days, rinsed in distilled water, then immersed in lactic acid. Photographs showing the various details of the external structure of the larvae and male aedeagus were taken with an Olympus DP21 digital camera mounted on an Olympus BX63 compound microscope or with a VEGA3 TESCAN SEM and subsequently corrected using CorelDRAW Graphics Suite X6. The style and terminology of the morphological description are according to Staniec et al. [6,7].
The voucher specimens are deposited in the collection of the Department of Zoology and Nature Protection, Institute of Biological Sciences, Marie Curie-Sklodowska University, Lublin. The material used for the morphological examination and measurements (15 larvae-8 L1 and 7 L2) is listed in Table 2.

Subfamily Diagnosis of the Mature Larvae (L2) of Paederinae
The combination of the characteristics that enable the mature larvae of Paederinae to be distinguished from the known larvae of other Staphylinidae are as follows (3, 6, 7, 10-13; 19, the present study): (1) body narrow, elongate, usually dorso-ventrally flattened, weakly sclerotized, small or medium-sized (usually 3-10 mm); (2) antennae four-articled, usually elongate and slim, articles III and IV clubbed, distinctly winding in the anterior portion; (3) antennal article III with three long setae and three solenidia-two long, one short; (4) each side of head with six oval or circular stemmata in clusters; exceptionally, head without stemmata; (5) stipes of maxilla, head and pronotum sides each with trichobothrium; (6) macro setae setose, micro setae (if they occur) match-shaped; (7) legs elongate and slim, sides of femur and tarsus almost parallel with long setae, setose; (8) article III of maxillary palp strongly tapering to top, distinctly longer than article II; (9) ligula elongate, tapering apically, pointed with tuft of microtrichia; and (10) urogomphi elongate, article I distinctly longer than segment X. Only 4 and 5 among the above characters are diagnostic solely of Paederinae; the others have been described in other, known staphylinid larvae.

Generic Diagnosis of the Mature Larva (L2) of Pseudomedon
The diagnostic characters of the genus Pseudomedon are defined on the basis of the morphological data presented for the first time in this paper. The combination of the characters (indicated by arrows on the illustrations) that distinguish the mature larvae of Pseudomedon from the previously known larvae of the closely related Paederinae (6, 7, 11-13, 19, 30, 31) are the following: (1) head narrowest at around half-length; (2) head 2.5× wider than neck; (3) apotome reaching tentorial pits; (4) antennal article III longer than article II; (5) antennal article IV at most only slightly (1.2×) longer than sensory appendage of article III; (6) nasale without median tooth; (7) epipharynx with 22-25 triangular cuticular processes in anterior row; 8) mandible serrate with 15-17 teeth; (9) mala curved; (10) mala 3× as long as wide; (11) mala as long as article I of maxillary palp; (12) seta 2 of article II of maxillary palp tiny, located at about half-length of article; (13) microtrichia of hypopharynx form a vertical, median band; (14) article I of labial palp longer than ligula, at least twice as long as article II; (15) ligula at least 3× as long as wide at base; (16) urogomphi 4.3-4.4× longer (without apical seta) than pygopod; (17) article I of urogomphi about 2.5× as long as article II; and (18) article I of urogomphi with 1 club-shaped (coded: L3) seta.

Larval Characters at the Tribal Level in Paederini and Lathrobiini Sensu Novo
The higher classification of the subfamily Paederinae has quite a long history, which stretches back to the first half of the 19th century. The changes in the classification sys-tems of this group of staphylinids was described in detail by Frania [3], Schomann and Solodovnikov [2] and in abbreviated form byŻyła et al. [1]. The Paederinae taxonomy currently recognises four tribes-Pinophilini, Cylindroxystini, Paederini and Lathrobiiniinstead of the two previously recognised ones-Pinophilini and Paederini. In this new tribal system, established on the basis of molecular data but supported by adult morphological data, the former tribe Paederini, apart from Cylindroxystini stat. resurr., was split into Paederini sensu novo and Lathrobiini sensu novo [2,34].
We assembled larval morphological data sampled from sufficiently detailed, current descriptions of species or genera of Paederinae and utilised them to assess their usefulness for confirming the validity of new tribal arrangements for two tribes, i.e., Lathrobiini and Paederini sensu novo. We did not analyse the other two tribes, i.e., the Neotropical Cylindroxystini and Pinophilini, as we are unfamiliar with their preimaginal stages. The characters common to Lathrobiini sensu novo presented in Table 3 are based mainly on those given by Staniec et al. [7] and the present study for the larvae of the subtribe Lathrobiina (Lathrobium, Pseudomedon and Tetartopaeus). In addition, certain data from Frania [3] have been taken into consideration for Stilicina (Rugilus and Eustulicus) and Medonina (Medon) and in the case of Paederini sensu novo, data given by Staniec et al. [6,7] and limited data from Kasule [12] and Pototskaya [11] for the subtribe Paederina (Paederus and Paederidus). This analysis has shown that the characters of Paederinae larvae perfectly match and wholly justify the new tribal arrangement of Lathrobiini and Paederini. Of course, it is well to bear in mind that the present state of knowledge of this group of rove beetles is still generally very unsatisfactory. Among the subtribes Paederina, Cryptobiina, Dolicaonina and Dicaxina in Paederini sensu novo, the larvae of only the first mentioned have been used for tribal diagnoses, the limited and confusing diagnoses of the second one given by Pototskaya [11] are poorly informative, while nothing is known about the other two. Likewise, within Lathrobiina, only the sets of characters established for Lathrobiina, Medonina and Stilicina could be extrapolated, because the other four-Scopaeina, Astenina, Stilicopsina and Echiasterina-are unknown.
Frania [3], in the study of the morphology of Medon in the subtribe Medonina and of some genera in other subtribes, i.e., Stilicina, Scopaeina, Stilicopsina and Astenina (all members of Lathrobiini sensu novo), notes that the larvae share a number of derived characters, which indicates that these subtribes are closely related. That author considers the following three characters to be the most important ones: (i) swollen, small setae (Fl2) on the nasale, (ii) branched microtrichia on the ligula and (iii) cuticular processes on the anterior margin of the buccal cavity arranged in rows. By contrast, Frania [3] considers that the larvae of the genera in the subtribes Paederina (now Paederini sensu novo) have the primitive condition for these three characters. However, the morphology of the larva of another member of the Medonina subtribe (Pseudomedon) analysed in the present paper does not uphold the hypothesis that the above-mentioned characters are common to this subtribe. Our observations show that the larvae of Pseudomedon do not possess a seta (F12) of a different structure on the nasale; it is of the simple type, and likewise, the microtrichia on the ligula also have a simple, not bifurcate, structure. Only the third of the three characters given by Frania [3] is valid for Medonina, but it is not, as assumed, in opposition to Paederini. However, it is difficult to give a firm, unequivocal response to Frania's [3] conclusions, because the author did not identify the species of the analysed larvae, remaining on the generic level. It may be that characters (i) and (ii), analysed above, are intra-or intergenerically variable. Moreover, the specimens examined by Frania [3] were first-instar larvae: although these are similar to the second (final) instars, they are not identical to them. That is why transferring the morphological characters of larvae from the known genera of Paederinae worked out on the basis of model L2 to members of L1 from the Frania [3] paper is unreliable and may lead to erroneous inferences. Table 3. Larval characters (in order of power) common to Lathrobiini sensu novo and Paederini sensu novo. Abbreviations: Acp-row of cuticular processes along anterior margin of buccal cavity, Ar-article, At-antennal, Lg-ligula, Lp-labial palp, Ma-mala, Mp-maxillary palp, Na-nasale, Pmt-paramedian tooth, Rw-ring of wrinkled cuticula, Sa-sensory appendage, Sts-stipes; * at least 2× as narrow as Ar I of Mp, ** at most only slightly narrower than Ar I of Mp.        Table 3. Larval characters (in order of power) common to Lathrobiini sensu novo and Paederini sensu novo. Abbreviations: Acp-row of cuticular processes along anterior margin of buccal cavity, Ar-article, At-antennal, Lg-ligula, Lp-labial palp, Ma-mala, Mp-maxillary palp, Na-nasale, Pmt-paramedian tooth, Rw-ring of wrinkled cuticula, Sa-sensory appendage, Sts-stipes; * at least 2× as narrow as Ar I of Mp, ** at most only slightly narrower than Ar I of Mp.  Table 3. Larval characters (in order of power) common to Lathrobiini sensu novo and Paederini sensu novo. Abbreviations: Acp-row of cuticular processes along anterior margin of buccal cavity, Ar-article, At-antennal, Lg-ligula, Lp-labial palp, Ma-mala, Mp-maxillary palp, Na-nasale, Pmt-paramedian tooth, Rw-ring of wrinkled cuticula, Sa-sensory appendage, Sts-stipes; * at least 2× as narrow as Ar I of Mp, ** at most only slightly narrower than Ar I of Mp.  Table 3. Larval characters (in order of power) common to Lathrobiini sensu novo and Paederini sensu novo. Abbreviations: Acp-row of cuticular processes along anterior margin of buccal cavity, Ar-article, At-antennal, Lg-ligula, Lp-labial palp, Ma-mala, Mp-maxillary palp, Na-nasale, Pmt-paramedian tooth, Rw-ring of wrinkled cuticula, Sa-sensory appendage, Sts-stipes; * at least 2× as narrow as Ar I of Mp, ** at most only slightly narrower than Ar I of Mp.  Table 3. Larval characters (in order of power) common to Lathrobiini sensu novo and Paederini sensu novo. Abbreviations: Acp-row of cuticular processes along anterior margin of buccal cavity, Ar-article, At-antennal, Lg-ligula, Lp-labial palp, Ma-mala, Mp-maxillary palp, Na-nasale, Pmt-paramedian tooth, Rw-ring of wrinkled cuticula, Sa-sensory appendage, Sts-stipes; * at least 2× as narrow as Ar I of Mp, ** at most only slightly narrower than Ar I of Mp.  Table 3. Larval characters (in order of power) common to Lathrobiini sensu novo and Paederini sensu novo. Abbreviations: Acp-row of cuticular processes along anterior margin of buccal cavity, Ar-article, At-antennal, Lg-ligula, Lp-labial palp, Ma-mala, Mp-maxillary palp, Na-nasale, Pmt-paramedian tooth, Rw-ring of wrinkled cuticula, Sa-sensory appendage, Sts-stipes; * at least 2× as narrow as Ar I of Mp, ** at most only slightly narrower than Ar I of Mp.  Table 3. Larval characters (in order of power) common to Lathrobiini sensu novo and Paederini sensu novo. Abbreviations: Acp-row of cuticular processes along anterior margin of buccal cavity, Ar-article, At-antennal, Lg-ligula, Lp-labial palp, Ma-mala, Mp-maxillary palp, Na-nasale, Pmt-paramedian tooth, Rw-ring of wrinkled cuticula, Sa-sensory appendage, Sts-stipes; * at least 2× as narrow as Ar I of Mp, ** at most only slightly narrower than Ar I of Mp.

Comparison of the Larval Characters of Lathrobiini Sensu novo at the Inter-and Intrasubtribal Level
The detailed description of the morphological structures of the Pseudomedon obsoletus L2 larvae given in this paper is the first thorough elaboration of Medonina at both the generic and subtribal levels. Table 4 presents an inter-and intrasubtribal morphological comparison of the tribe Lathrobiini sensu novo based on our data and those gleaned from the literature relating to the four genera (3 to a limited extent, 7, 10]. The following inferences can be drawn from the data contained in this table. Among other known Lathrobiini sensu novo, the principal characters unique to the subtribe Medonina are: (i) the absence of micro setae, (ii) the small (the smallest in this tribe) number of macro setae on the surface of the epicranium and iii) the smooth surface of the epipharynx, with no microstructure. At the subtribal level, the labrum and maxilla are very useful structures, exhibiting intertribal similarities and differences. The distinct differences between Medonina-M (Pseudomedon) and Lathrobiina-L (Lathrobium plus Tetartopaeus) are: (i) the median tooth on the labrumabsent in M but present in L-and (ii) the mala relative to article I of the maxillary palp-no shorter in M and shorter in L. These characters link Medonina (Pseudomedon) with Stilicina (Rugilus), but these subtribes are distinguished by the apotome, which is a distinctly separate structure in the former but absent in the latter. Table 4. Comparative characters of the larva of Pseudomedon and some previously described larvae of Paederinae. Abbreviations: Acp-row of cuticular processes along anterior margin of buccal cavity; Ap-apotome; Ar-article; Asg-antennal segment; Bm-bunch of microtrichia at the mandible base; Hr-height ratio; Lg-ligula; Lr-length ratio; Lwr-length-to-width ratio; Mt-median tooth; Ncp-number of cuticular process; NmaS-number of macro setae; NmiS-number of micro setae; Npgs-number of peg setae; Ns-number of setae; Nt.Am.-number of well-developed teeth, anterior margin; Mss-match-shaped setae; Pg-pygopod (abdominal segment X); Pmnt-prementum; Pmtparamedian tooth; Rw-ring of wrinkled cuticula; Sa-sensory appendage; Sap-apical seta; Stssclerotised transversal strip; Tp-tentorial pits; Tsn-terminal sensilla.

Key to Genera of the Known Larvae of Paederinae
The identification key for the Paederinae larvae given below, encompassing eight genera, was constructed on the basis of the species inhabiting the Palearctic region. The key includes the previously undescribed larvae of Pseudomedon, the earlier well-known Paedridus, Paederus, Lathrobium and Tetartopeus [6,7], and Medon, Ochtephilum and Rugilus, known from incomplete descriptions [3,10,12]. Unfortunately, the morphologies of a large number of paederine larvae are still too poorly recognised to include them in this key (see Table 1). Here, we attempt to differentiate the taxa using mostly simple and user-friendly characters, the analysis of which does not require any prior preparatory effort.

Conclusions
The diagnostic characters of the Paederinae larvae at different taxonomic levels, given in this paper, are in accordance with the current state of knowledge and based on the limited larval material of the European fauna. Our intention is to expand these studies of the developmental stages of this rove beetle subfamily. Establishing new and certain (i.e., constant, valid) morphological data of these larvae at the species level will not only enable or facilitate them to be distinguished but also allow them to be included in phylo-genetic studies, which to date have been conducted solely on the basis of imaginal and molecular data.