Labrum. Variable, asymmetrical, bifurcate, subquadrate, or transverse. Epipharynx glabrous to densely covered with trichomes, forming bristle-trough or not, sometimes covered with sensilla.

Mandible. Variable in shape; subapical tooth, retinaculum and prostheca absent or present, forming lobe-like structure or not; mola-like structure usually absent, present only in a few groups (Oxyporinae, Solieriinae, Scydmaenus, Pseudopsis, and Diochus).

Maxilla. With cardo transverse. Stipes subdivided into basi- and mediostipes, the latter forming the base for both galea and lacinia. Lacinia fused with mediostipes. Galea inserted at mediostipes between lacinia and palpifer. Palpifer distinct, at base of maxillary palps, or also at base of galea (Megalopinus). Palpi consisting of 4 palpomeres.

Labium-hypopharynx. Prementum covered laterally with spine- or hair-like trichomes, glabrous medially or covered with sensilla. Distally with pair of ‘glossae’, ‘paraglossae’, and labial palpi consisting of 3 palpomeres. ‘Glossae’ usually reduced and integrated into prementum (Oxyporinae, Megalopsidiinae, Solieriinae, Steninae, Euaesthetinae, Scydmaeninae, Zalobius) and represented by sensilla trichodea or basiconica, or ‘glossae’ modified into medial lobes (Leptotyphlinae, Pseudopsis, Paederinae, and Staphylininae) that vary in form, usually covered with sensilla coeloconica. ‘Paraglossae’ unmodified antero-lateral lobes or modified into anterior digitiform lobes (Agnosthaetus and Austroesthetus) or modified into adhesive pads (Stenus). The base of the prementum is connected to the distal hypopharynx, with a transverse suture separating them.

Protarsus. Usually consisting of 5 tarsomeres (except in Leptotyphlinae, consisting of 3, and the Euaesthetinae genera Austroesthetus and Euaesthetus, consisting of 4), always with symmetrical pair of claws on pretarsus. Tarsal setae present on the ventral region can be divided into two major groups, i.e., unmodified hair-like (sometimes spine-like, a difference not taken into consideration in this context) or modified into a widened adhesive pad, covered by tenent setae. There is a great variety of tarsal setae with potential phylogenetic value; however, this is not within the scope of this study (for more information about tarsal seta types, see, for example, Stork 1980, Betz 2003, and Gnaspini et al. 2017). The overall width of each tarsomere is considered, and two main kinds are given: (1) slender, bearing spine- and/or hair-like setae, and (2) widened bilobed, bearing distally widened (tenent) setae, usually spatulate and/or discoid in shape.

Labrum (Fig. 1A, B). Asymmetrical, longer than wide, and strongly emarginate. Epipharynx flat, without furrow, covered by mesally directed hair- and scale-like trichomes; prominent dense medial tuft of posteriorly directed hair-like trichomes.

Figure 1. 

Labrum-epipharynx (ventral aspect), SEM micrographs: A Oxyporus stygicus, B Pseudoxyporus quinquemaculatus, C Megalopinus sanguinitriguttatus, D Solierius obscurus, E Dianous obliquenotatus, F Stenus puthzianus, G Austroesthetus passerculus, H Euaesthetus iripennis, I Fenderia chandleri, J Agnosthaetus cariniceps, K Cephennodes clavatus, L Veraphis sp., M Palaeostigus bifoveolatus, N Scydmaenus sp., O Stenichnus sp., P Eutyphlops sp., Q Homeotyphlus sp., R Pseudopsis subulata.

Figure 2. 

Labrum-epipharynx (ventral aspect), SEM micrographs: A Zalobius nancyae, B Astenus sp., C Lathrobium sp., D Medon vittatipennis, E Stilicoderus woodwardi, F Baryopsis sp., G Gnathymenus sp., H Homaeotarsus bicolor, I Paederus littoralis, J Pinophilus parcus, K Diochus schaumii, L Atrecus punctiventris, M Platyprosopus sp., N Erichsonius patella, O Philonthus politus, P Thyreocephalus albertisi.

Mandible (Fig. 3A, B). Somewhat broadened at base, blade-like shape, only slightly curved. Subapical tooth absent, incisor area smooth. Retinaculum absent. Small tooth-like projection (with unclear homology) present at outer rim of mandible. Prostheca present, not forming lobe-like projection, arising from elongate ledge-like inner margin, composed of brush-like fringe of fimbriate hair-like trichomes along medial margin. Mola present, forming a broad surface provided with grinding surface resembling a nipple-carpet (Pseudoxyporus) or an almost flat surface studded with faint ridges (Oxyporus); partially surrounded by trichomes of prostheca.

Figure 3. 

Left mandible (ventral aspect), SEM micrographs: A Oxyporus stygicus, B Pseudoxyporus quinquemaculatus, C Megalopinus sanguinitriguttatus, D Solierius obscurus (left and right mandibles), E Dianous obliquenotatus, F Stenus puthzianus, G Austroesthetus passerculus, H Euaesthetus iripennis, I Fenderia chandleri, J Agnosthaetus cariniceps, K Cephennodes clavatus, L Veraphis sp., M Palaeostigus bifoveolatus, N Scydmaenus sp., O Stenichnus sp., P Eutyphlops sp., mandibular teeth hidden by prostheca of right mandible, Q Eutyphlops sp. (prostheca damaged, indicated by arrow), mandibular teeth exposed, R Homeotyphlus sp. subapical tooth hidden because of the direction of view, S Homeotyphlus sp. (prostheca damaged, indicated by arrow), subapical tooth exposed. — Abbreviations: ml = mola, prst = prostheca, re = retinaculum, sat = subapical tooth.

Maxilla (Fig. 5A). Lacinia extending toward half the length of galea; from base nearly to apex densely covered with anteriorly directed short straight hair-like trichomes. In apical region, meso-posteriorly directed curved bristle-like trichomes present. Galea elongated, subequal to lacinia, reaching more than half length of second palpomere and bearing robust brush of posteriorly curved bristle-like trichomes. Apical palpomere longer than and similar in width to preceding one.

Labium-hypopharynx (Fig. 8A). Prementum strongly bilobed and dorsally, along lateral margins, with prominent dense fringe of mesally directed hair-like trichomes; without spine-like trichomes, densely covered medially with carpet of cone-shaped trichomes. Palps directed distad, palpomere 3 distinctly enlarged, flattened and strongly securiform. Ligula consisting of paired ‘glossae’ and ‘paraglossae’; ‘glossae’ represented by two pairs of sensilla trichodea; ‘paraglossae’ represented by well-developed antero-lateral lobes of prementum. Hypopharynx distally covered with arrays of mesally directed hair-like trichomes. Proximally, anteriad-directed hair-like trichomes, forming a medial tuft.

Protarsus (Fig. 10A, B): Consisting of 5 tarsomeres, all slender and subequal in width. Tarsomeres bearing spine-like setae laterally and medially glabrous in Oxyporus or hair-like setae completely covering tarsomeres 1–4 ventrally and 5 laterally in Pseudoxyporus.

Labrum (Fig. 1C). Longer than wide, strongly bifurcate, bearing row of long hair-like setae anteriorly and along inner margins of labral lobes. Lateral lobes bearing longer hair-like setae at tips. Epipharynx flat, without furrow, glabrous, except for margins of lateral lobes.

Mandible (Fig. 3C). Broad at base and acute at apex, strongly curved mediad near apex. Incisor area without subapical teeth. Retinaculum present with only weakly developed teeth-like protrusions. Prostheca absent. Mola absent. Distinct concave area covered with proprioreceptive setae near ventral mandibular condyle.

Maxilla (Fig. 5B). Lacinia reaching to apical part of galea, bearing few rows of hair- and bristle-like trichomes, basal half with straight trichomes antero-medially directed, apical half with curved trichomes medially directed, unarticulated spine present apically. Galea elongated, more than half of lacinia length, reaching base of third maxillary palpomere; bearing brush of antero-mesally directed hair- and bristle-like trichomes. Apical palpomere longer than and similar in width to preceding one.

Labium-hypopharynx (Fig. 8B). Prementum dorsally along lateral margins with dense fringe of mesally directed hair-like trichomes; innermost row of these lateral fringes differentiated into spine-like trichomes, medial surface glabrous. Palpomere 3 slightly wider than palpomere 2. Ligula consisting of paired ‘glossae’ and ‘paraglossae’; ‘glossae’ represented by pairs of sensilla basiconica; ‘paraglossae’ represented by well-developed antero-lateral lobes of prementum. Hypopharynx dorsally along lateral margins with short mesally directed hair-like trichomes. Medially glabrous, but distally covered with loose series of proximally directed bristle-like trichomes.

Protarsus (Fig. 10C). Consisting of 5 tarsomeres, all slender and subequal in width. Tarsomeres loosely covered with spine- and hair-like setae, more densely so toward basal tarsomeres.

Labrum (Fig. 1D). Subquadrate with anterior margin not bilobed. Epipharynx flat without furrow, mostly glabrous, but with two pairs of obliquely anteriorly directed prominent bristle-like sensilla distally. With larger two pairs of sensilla at anterior rim. Mesally directed hair-like trichomes medially forming loose rows and small brush toward base. Rows of posteriad-directed hair-like trichomes proximally.

Mandible (Fig. 3D). Strongly broadened at base, acute at apex. Incisor area with subapical tooth. Retinaculum absent. Prostheca present, not forming lobe-like projection, short, bearing brush-like fringe of fimbriate hair-like trichomes. Mola present, forming smooth flat surface.

Maxilla (Fig. 5C). Lacinia reaching to apical part of galea; bearing rows of antero-medially directed bristle-like trichomes, restricted to apical half of lacinia, multi-branched unarticulated structure present apically. Galea length subequal to lacinia, reaching second palpomere; apex bearing rows of anteriorly directed bristle-like trichomes. Apical palpomere reduced and conical.

Labium-hypopharynx (Fig. 8C). Prementum slightly bilobed and dorsally along lateral margins with parallel continuous fringes of mesally directed comb-like trichomes; without spine-like trichomes, medially glabrous. Palpomere 3 distinctly thinner than palpomere 2. Ligula consisting of paired ‘glossae’ and ‘paraglossae’; ‘glossae’ represented by pair of sensilla trichodea; ‘paraglossae’ represented by antero-lateral lobes of prementum. Hypopharynx dorsally covered with mesally directed hair-like trichomes forming a medial tuft.

Protarsus (Fig. 10D). Consisting of 5 tarsomeres, all slender and subequal in width. Tarsomeres covered with hair-like setae.

Labrum (Fig. 1E, F). Strongly transverse, with anterior margin not bilobed. Epipharynx flat, without furrow, mainly glabrous with two thorn-like projections at proximal margin (Dianous). Distinct separate plate proximally covered with campaniform sensilla, with anteriad-directed hair-like trichomes (Dianous), or with only few lateral patches of hair-like trichomes (Stenus).

Mandible (Fig. 3E, F). Slender and of falciform shape. Incisor area with subapical tooth with serrate inner margin (Stenus) or not (Dianous). Retinaculum, prostheca and mola absent.

Maxilla (Fig. 5D). Lacinia reaching to nearly the apical part of galea; bearing a brush of curved bristle- and hair-like trichomes, ventral fringe anteriorly directed and dorsal fringe postero-medially directed (Dianous) or basal half with straight antero-medially directed trichomes and apical half of curved postero-medially directed trichomes (Stenus). Galea length subequal to lacinia, reaching first palpomere; bearing dense brush of curved bristle- and hair-like posteriorly directed trichomes. Apical palpomere vestigial and peg-like.

Labium-hypopharynx (Fig. 8D). In Dianous, prementum dorsally along lateral margins with dense fringe of meso-anteriorly directed hair-like trichomes, without lateral spine-like trichomes; medially covered with anteriorly directed spine-like setae. Palpomere 3 distinctly thinner than palpomere 2. In Stenus (the typical strikingly distinct elongated rod-like prementum is present, the distal part is however similar and comparable to the other investigated genera), prementum dorsally along lateral margins with rows of short bristle-like trichomes, becoming longer on the innermost margin, without lateral spine-like trichomes; single longitudinal row of spine-like setae is present medially. Ligula consisting of paired ‘glossae’ and ‘paraglossae’; ‘glossae’ represented by two pairs of sensilla trichodea; ‘paraglossae’ represented by antero-lateral lobes of prementum, more prominently so in Stenus, which bears ventrally specialized sticky pads covered with adhesive trichomes. Hypopharynx in Dianous dorsally along lateral margins with mesally directed hair-like trichomes. Medially provided with dense vestiture of anteriad-directed hair-like trichomes forming medial tuft and glabrous proximally or with only few scattered short hair-like trichomes. In Stenus, the hypopharynx sits on the base of the membranous arthrodial membrane of the elongated labium, forming a roof-shaped fold directed away from the mouth opening and covered by antero-mesally directed scale-like trichomes. Distally, it runs out into two folds that border the recess of the hypopharynx laterally and that are also occupied by anteriorly directed scale-like trichomes. SEM images of different species of Stenus labium and hypopharynx are presented in Betz (1996).

Protarsus (Fig. 10E, F). Consisting of 5 tarsomeres; tarsomeres (especially the antepenultimate and the penultimate ones), apart from the tarsi shown here, can vary from slender and weakly bilobed to wide and bilobed. Great species-specific diversity in number and shape of tarsal tenent setae has been recorded (Betz 2003; Betz et al. 2018).

Labrum (Fig. 1G–J). Strongly transverse, anterior margin denticulate with prominent teeth. Epipharynx flat, without furrow, glabrous. Medial membranous field bearing sensilla and / or gland openings present. Pair of concavely dented fields present proximally in Fenderia.

Mandible (Fig. 3G–J). Slender and moderately curved. Incisor area without subapical tooth. Retinaculum present, with (Euaesthetus) or without serrate inner margin (Austroesthetus, Agnosthaetus, Fenderia). Prostheca and mola absent.

Maxilla (Fig. 5E, F). Lacinia with corrugated ventral surface, almost completely divided from mediostipes; reaching half-length of galea; bearing brush of posteriorly curved bristle-like trichomes. Galea length subequal to lacinia, reaching second palpomere; apico-mesally covered by posteriorly directed curved bristle-like trichomes; distinct spine on lateral margin in Agnosthaetus. Apical palpomere vestigial and peg-like.

Labium-hypopharynx (Fig. 8E–H). Prementum deeply emarginate anteriorly (Austroesthetus, Agnosthaetus, and Fenderia) or not (Euaesthetus), and dorsally along lateral margins with dense fringe of mesally directed hair- and spine-like trichomes (Austroesthetus, Agnosthaetus, Euaesthetus) or with non-continuous fringe of mesally directed comb-like trichomes (Fenderia); medially glabrous. Palpomere 3 peg-like, distinctly thinner than palpomere 2. Ligula consisting of paired ‘glossae’ and ‘paraglossae’; ‘glossae’ represented by pair of sensilla trichodea, present medially (Euaesthetus and Fenderia) or shifted laterad toward inner margins of ‘paraglossal’ digitiform lobes (Austroesthetus and Agnosthaetus); ‘paraglossae’ represented by unmodified antero-lateral lobes of prementum (Euaesthetus and Fenderia) or sometimes modified into digitiform lobes (Austroesthetus and Agnosthaetus). Hypopharynx dorsally along lateral margins with mesally directed hair-like trichomes. Medially glabrous.

Protarsus (Fig. 10G–J). Consisting of 5 tarsomeres in Agnosthaetus and Fenderia, and 4 in Austroesthetus and Euaesthetus; all tarsomeres slender and subequal in width. Tarsomeres covered with spine- and hair-like setae in Euaesthetus and Agnosthaetus. In Austroesthetus and Fenderia, tarsomeres are glabrous medially and bear lateral rows of spine- and hair-like setae. In Fenderia, the first two tarsomeres are covered medially with distinct discoid tenent setae. Austroesthetus and Fenderia have distinctly elongated tarsomere 5 when compared with Euaesthetus and Agnosthaetus.

Labrum (Fig. 1K–O). Subquadrate (Scydmaenus, Cephennodes, and Veraphis) or transverse (Stenichnus, Palaeostigus). Anterior margin not bilobed, with single pair of short and broad tooth-like projections in Palaeostigus. Epipharynx flat, without furrow, mostly covered by mesally directed hair-like (Palaeostigus), scale-like (Stenichnus), or both (Scydmaenus) trichomes, glabrous medially. Scydmaenus forming tuft medio-posteriad, and Scydmaenus and Veraphis forming pair of tufts apico-laterally. Proximal patch of hair-like trichomes anteriad-directed in Palaeostigus. In Cephennodes, the epipharynx has semi-membranous surface showing irregular folds.

Mandible (Fig. 3K–O). Broad at base, curved, and acute at apex (Cephennodes, Veraphis, Palaeostigus, Scydmaenus), or slender and falciform (Stenichnus). Incisor area with (Veraphis, Palaeostigus, and Scydmaenus) or without (Cephennodes and Stenichnus) subapical tooth. Retinaculum absent (Cephennodes, Veraphis, Scydmaenus, Stenichnus) or present (Palaeostigus). Stenichnus, however, have a serrated inner edge of the mandible. Prostheca well-developed as dense brush of hair-like trichomes covering nearly half of the ventral surface (Palaeostigus), limited to only medial inner margin (Scydmaenus), or close to base of inner margin (Veraphis); in Veraphis, prostheca is broader than usual and bears tooth-like structures. Prostheca is absent in Cephennodes and Stenichnus. Mola present in Scydmaenus, mesal edge flattened surface composed of small rows of bristle-like trichomes with ridge-studded surface just lateral to them.

Maxilla (Fig. 5G–K). Lacinia reaching basal (Palaeostigus) or meso-apical (Stenichnus, Cephennodes, Veraphis, Scydmaenus) part of galea; bearing curved bristle-like trichomes directed postero-medially and with unarticulated spine apically (Stenichnus, Cephennodes, Veraphis, Scydmaenus) or curved hair-like trichomes directed posteriorly without unarticulated spine (Palaeostigus). Galea length subequal to lacinia, reaching second palpomere; bearing mesally directed curved bristle- (Stenichnus, Cephennodes, Veraphis, Scydmaenus) or hair-like (Palaeostigus) trichomes. Apical palpomere subequal to preceding one (Palaeostigus) or much shorter (Stenichnus, Cephennodes, Veraphis, Scydmaenus).

Labium-hypopharynx (Fig. 8I–M). Prementum¹ dorsally along lateral margins with dense fringe of mesally directed hair- or spine-like trichomes. Palpomere 3 somewhat (Cephennodes, Veraphis, Stenichnus) to distinctly (Palaeostigus, Scydmaenus) thinner than palpomere 2. Ligula consisting of paired ‘glossae’ and ‘paraglossae’; ‘glossae’ represented by pair(s) of sensilla basiconica (Palaeostigus and Scydmaenus) or trichodea (Veraphis); ‘paraglossae’ represented by antero-lateral lobes of prementum. Hypopharynx dorsally along lateral margins with mesally directed hair-like trichomes, medially glabrous or with only one single pair of trichomes. More proximally, dense fringe of anteriad-directed hair-like trichomes (Palaeostigus and Veraphis). For the genera Cephennodes, Stenichnus, and Scydmaenus, in which the hypopharynx overlaps the prementum, the hypopharynx has a different appearance as follows. Hypopharynx dorsally along lateral margin with short thorn-like trichomes (Cephennodes and Scydmaenus) and, innermost to these, longitudinal striae, continuing medially (Stenichnus) or not (Cephennodes and Scydmaenus). More proximally with dense tuft of hair-like trichomes (Scydmaenus), or glabrous (Cephennodes and Stenichnus).

Protarsus (Fig. 10K–O). Consisting of 5 tarsomeres, all slender and subequal in width. Tarsomeres are ventrally covered with spine- and hair-like setae. Palaeostigus with pairs of distinct spines laterally on tarsomeres 1–3.

Labrum (Fig. 1P, Q). Transverse, anterior margin slightly denticulate. Epipharynx flat, without furrow, mainly glabrous. Anteriad-directed hair-like trichomes proximally, and mesally directed hair-like trichomes lateroproximally. With few campaniform sensilla medially.

Mandible (Fig. 3P–S). Slightly broadened at base, becoming acute toward apex, falciform. Incisor area with subapical tooth. Retinaculum absent (Homeotyphlus) or present (Eutyphlops). Prostheca present, forming prominent, robust, sclerotized lobe-like projection with brush-like fringe of fimbriate hair-like trichomes along medial margin. Mola absent.

Maxilla (Fig. 5L). Lacinia reaching to apical part of galea; bearing row of straight mesally directed bristle-like trichomes, unarticulated spine present apically. Galea length subequal to lacinia, reaching third palpomere (Eutyphlops) or about half of lacinia length, reaching only base of second palpomere (Homeotyphlus); bearing antero-medially directed slightly curved bristle-like trichomes. Apical palpomere distinctly shorter and thinner (peg-like) than preceding one.

Labium-hypopharynx (Fig. 8N). Prementum dorsally along lateral margins with parallel continuous fringes of mesally directed comb-like trichomes; without spine-like trichomes, medially glabrous. Palpomere 3 somewhat thinner than palpomere 2. Ligula consisting of paired ‘glossae’ and ‘paraglossae’; ‘glossae’ modified into spatulate meso-lateral lobes, projecting anteriad, bearing few sensilla coeloconica at tip; ‘paraglossae’ represented by antero-lateral lobes of prementum. Hypopharynx dorsally with arrays of mesally directed hair-like trichomes distally and anteriad-directed hair-like trichomes proximally.

Protarsus (Fig. 10P, Q): Consisting of 3 tarsomeres, all slender and subequal in width. Tarsomeres glabrous medially. Eutyphlops with spine-like setae and Homeotyphlus with spoon-like tenent setae with distinctly enlarged adhesive pads.

Labrum (Figs 1R, 2A). Subquadrate, not bilobed. Epipharynx without furrow, with sparsely distributed vestiture, with medio-lateral region covered by rows of mesally directed hair-like trichomes, forming bristle-trough. Campaniform sensilla present medially (Pseudopsis) or absent (Zalobius). Posteriorly with patches of hair- or scale-like anteriorly directed trichomes (Zalobius) or a tuft of hair-like trichomes (Pseudopsis). Short basiconic (Zalobius) or disc-like (Pseudopsis) sensilla distally.

Mandible (Fig. 4A, B). Broad, abruptly curved at incisor (Pseudopsis) or only slightly broad at base and falciform in shape (Zalobius). Incisor area with subapical tooth. Retinaculum absent (Zalobius) or present (Pseudopsis). Prostheca well-developed, without lobe-like projection but with only short extension distally; bearing row of branched hair-like trichomes along medial margin, extending anteriorly. Mola present in Pseudopsis, somewhat broad, covered by teeth- and bristle-like trichomes, thicker on edges and finer on inner region.

Figure 4. 

Left mandible (ventral aspect), SEM micrographs: A Pseudopsis subulata, B Zalobius nancyae, C Astenus sp., D Lathrobium sp., E Medon vittatipennis, F Stilicoderus woodwardi, G Baryopsis sp., H Gnathymenus sp., I Homaeotarsus bicolor, J Paederus littoralis, K Pinophilus parcus, L Diochus schaumii, M Atrecus punctiventris, N Platyprosopus sp., O Erichsonius patella, P Philonthus politus, Q Platydracus femoratus, R Thyreocephalus albertisi. — Abbreviations: dt = dorsal tooth, ml = mola, prst = prostheca, re = retinaculum, sat = subapical tooth.

Maxilla (Fig. 5M, N). Lacinia reaching to apical part of galea; bearing few rows of straight bristle- and hair-like trichomes directed mesally; unarticulated spine present apically. Galea about half of lacinia length, reaching only to base of second palpomere (Zalobius) or more than half length of lacinia, reaching third palpomere (Pseudopsis); bearing brush of curved hair- and bristle-like trichomes directed postero-medially. Apical palpomere slightly longer and thinner (Pseudopsis) than preceding one or distinctly longer than and same width as preceding one, tapering apically (Zalobius).

Figure 5. 

Left maxilla (ventral aspect), SEM micrographs: A Oxyporus stygicus, B Megalopinus sanguinitriguttatus, C Solierius obscurus, D Dianous obliquenotatus, E Euaesthetus iripennis, F Agnosthaetus cariniceps, G Cephennodes clavatus, H Veraphis sp., I Palaeostigus bifoveolatus, J Scydmaenus sp., K Stenichnus sp., L Eutyphlops sp., M Pseudopsis subulata, N Zalobius nancyae. — Abbreviations: bs = basistipes, ca = cardo, ga = galea, lac = lacinia, mp = maxillary palp, ms = mediostipes, pf = palpifer.

Labium-hypopharynx (Fig. 9A, B). Prementum dorsally along lateral margins with fringe of mesally directed hair-like trichomes; innermost row of lateral fringes differentiated into spine-like trichomes (Pseudopsis) or spine-like trichomes still present (but not on innermost row) and prementum medially covered with hair-like setae (Zalobius). Palpomere 3 somewhat thinner than palpomere 2. Ligula consisting of paired ‘glossae’ and ‘paraglossae’; ‘glossae’ spatulate, modified dorsally into medial lobes, projecting anteriad and covered apically with sensilla coeloconica in Pseudopsis. In Zalobius, the ‘glossae’ are integrated with the prementum plate and probably represented by hair-like setae, but we were not able to distinguish these clearly as this area of the prementum is densely covered by other hair-like setae; ‘paraglossae’ represented by antero-lateral lobes of prementum. Hypopharynx covered dorsally with anteriad-directed hair-like trichomes, forming small tuft more distally (Zalobius) or along lateral margins with mesally directed hair-like trichomes and medially mainly glabrous, forming a dense brush proximally (Pseudopsis).

Protarsus (Fig. 10R, S). Consisting of 5 tarsomeres, all slender and subequal in width. Tarsomeres in Pseudopsis with distinct rows of spine-like setae and in Zalobius densely covered with spine- and hair-like setae, somewhat longer than Pseudopsis.

Labrum (Fig. 2B–J). Transverse, not or only slightly bilobed; subquadrate and strongly bilobed in Lathrobium. Epipharynx extensively and densely covered by mesally directed hair- or scale-like trichomes combined with dense medial tuft of anteriorly directed hair-like trichomes (Lathrobium, Baryopsis, Homaeotarsus, and Pinophilus), or with some regions covered by mesally directed hair- or scale-like trichomes and glabrous furrow medially (Astenus, Medon, Stilicoderus, Gnathymenus, and Paederus); medially directed trichomes forming bristle-trough in all species, except Astenus and Medon. Longitudinal furrow is also present in Pinophilus, but mainly covered with anteriorly directed hair-like trichomes medially.

Mandible (Fig. 4C–K). Robust at base and falciform in shape at apical half (Lathrobium, Medon, Baryopsis, Gnathymenus, Homaeotarsus, Paederus, Pinophilus), or slender and strongly falciform (Astenus), or overall broad with incisor angled almost 90° in relation to base (Stilicoderus). Incisor area without subapical tooth. Retinaculum present, with one or two teeth; retinaculum in Astenus has one tooth with three cusps (tricuspid). Prostheca well-developed, forming prominent sclerotized lobe-like projection, which bears brush-like fringe of fimbriate hair-like trichomes along medial margin, sometimes extending anteriorly toward most proximal tooth of retinaculum. Prostheca absent in Astenus, Medon, and Stilicoderus. Mola absent.

Maxilla (Fig. 6A–I). Lacinia reaching to just basal part of galea; bearing robust brush of hair-like trichomes, basal region consisting of straight antero-mesally directed trichomes and apical region of curved trichomes (antero-)mesally directed. Galea about half length of lacinia, reaching first palpomere or only to base of second palpomere; bearing robust brush of curved hair-like trichomes curved mesally, trichomes sometimes with spatulate tip (Medon, Stilicoderus, Baryopsis, Homaeotarsus, Paederus). Apical palpomere subequal to longer (Pinophilus), or much shorter than preceding one (Paederini) or vestigial and peg-like (Lathrobiini).

Figure 6. 

Left maxilla (ventral aspect), SEM micrographs: A Astenus sp., B Lathrobium sp., C Medon vittatipennis, D Stilicoderus woodwardi, E Baryopsis sp., F Gnathymenus sp., G Homaeotarsus bicolor, H Paederus littoralis, I Pinophilus parcus, J Diochus schaumii, K Atrecus punctiventris. — Abbreviations: bs = basistipes, ca = cardo, ga = galea, lac = lacinia, mp = maxillary palp, ms = mediostipes, pf = palpifer.

Labium-hypopharynx (Fig. 9C–J). Prementum dorsally along lateral margins with prominent and dense fringe of mesally directed hair-like trichomes; innermost row of lateral fringes differentiated into spine-like trichomes, medially with sensilla coeloconica (Lathrobium, Baryopsis, Homaeotarsus), with spine-like erect setae (Stilicoderus) or glabrous (Medon, Gnathymenus, Paederus, Pinophilus); pair of prominent peg-like protuberances are visible in Paederus. Palpomere 3 about as wide to half as wide as, or distinctly (Medon, Stilicoderus) thinner than palpomere 2. Ligula consisting of paired ‘glossae’ and ‘paraglossae’; ‘glossae’ dorsally modified into anterior lobes, sometimes bulbous, covered with sensilla coeloconica; ‘paraglossae’ represented by antero-lateral lobes of prementum. Hypopharynx dorsally mainly with dense homogeneous array of anteriad-directed hair-like trichomes (Baryopsis, Gnathymenus). Alternatively, along lateral margins with mesally directed hair-like trichomes and medially with dense vestiture of hair-like trichomes converging to form medial tuft (Stilicoderus, Homaeotarsus, Paederus, Pinophilus). Lathrobium with such vestiture, but with distinct glabrous furrow medially. In Lathrobiini, the hypopharynx is narrowed distally and suddenly enlarged proximally where most of hypopharynx sclerite is present (the connection between these regions is fragile and susceptible to tearing during dissection).

Protarsus (Figs 10T–X, 11A–E). Consisting of 5 tarsomeres, tarsomeres covered by tenent setae are widened and bilobed. Tarsomeres 1–4 of Lathrobium, Baryopsis, Gnathymenus, Paederus, and Pinophilus ventrally densely covered by distally widened spatulate (tenent) setae. Pinophilus is much more densely covered by distally widened spatulate (tenent) setae than the other genera. In Astenus, the only tarsomere that bears tenent setae is 4, whereas 1–3 are covered by rows of unmodified prominent spine-like setae. In Medon, Stilicoderus, and Homaeotarsus, tarsomeres 1–5 are mainly covered by spine- and hair-like setae, and all tarsomeres are subequal in width.

Labrum (Fig. 2K–P). Transverse, but subquadrate in Diochus and Thyreocephalus, and in Platyprosopus rhomboidal, longer than wide including hidden part beneath clypeus, or, for only externally exposed part, gradually widening toward exposed base; anterior margin not or only slightly bilobed. Epipharynx flat, without furrow (except Platyprosopus), covered by mesally directed hair-like trichomes, forming dense medial tuft of anteriad-directed trichomes medially (Erichsonius, Philonthus, Platydracus, Quedius). Platyprosopus distinctly longitudinally furrowed, with ramified trichomes distally and anteriad-directed cone-shaped trichomes medially. In Diochus and Thyreocephalus, epipharynx is mainly glabrous, Thyreocephalus with transverse row of anteriorly directed hair-like trichomes, and grooved peg-like sensilla medially, distal to transverse row; and Diochus with paired patches of anteriad-directed hair-like trichomes proximally and row of bowl-like (campaniform?) sensilla proximally and row of prominent peg-like sensilla just distal to that. Atrecus with loose row of mesally directed hair-like trichomes laterally, row of anteriad-directed thorn-like setae distally, and few rows of prominent peg-like sensilla meso-proximally; medially directed trichomes forming bristle-trough in all species, except Diochus, Atrecus and Thyreocephalus.

Mandible (Fig. 4L–R). Somewhat broadened at base, acute at apex, falciform (Erichsonius, Philonthus, Quedius, Platydracus, Thyreocephalus,); Diochus and Platyprosopus with broader base, only slightly falciform and with blunt incisor; Atrecus with broad base but not falciform shape and more robust than other genera studied; incisor blunt. Incisor area with (Atrecus and Platyprosopus) or without (Diochus, Erichsonius, Quedius, Philonthus, Platydracus, Thyreocephalus) subapical tooth. Retinaculum absent (Diochus, Atrecus, Platyprosopus) or present (Erichsonius, Philonthus, Quedius, Platydracus, Thyreocephalus), usually partially covered by more posterior prostheca; Diochus and Thyreocephalus bearing one dorsal mesal tooth. Prostheca well-developed, forming prominent sclerotized lobe-like projection bearing brush-like fringe of fimbriate hair-like trichomes along medial margin, sometimes extending anteriorly toward most proximal tooth of retinaculum. Mola present only in Diochus, forming broader region covered with bristle-like trichomes.

Maxilla (Figs 6J, K, 7A–F). Lacinia reaching to just base or half-length of galea; bearing brush-like structure with bristle-like trichomes on basal region directed straight anteriorly and on apical region curved and directed postero-medially (Diochus), or hair-like trichomes, basal region consisting of straight anteriorly directed (Atrecus, Platyprosopus, Erichsonius, Thyreocephalus), or mesally directed (Philonthus, Quedius, Platydracus) directed trichomes and apical region with curved mesally directed trichomes; brush especially dense and long in Platydracus and Platyprosopus and significantly shorter in Diochus. Galea about half lacinia length, reaching first or second palpomere; apex bearing brush of curved bristle- (Diochus) or hair-like trichomes (Atrecus, Platyprosopus, Philonthus, Quedius, Platydracus, Thyreocephalus) or curved hair- and bristle-like trichomes (Erichsonius); brush especially dense and long in Platydracus and Platyprosopus. Apical palpomere subequal to or slightly longer than preceding one, or vestigial (Diochus).

Figure 7. 

Left maxilla (ventral aspect), SEM micrographs: A Platyprosopus sp., B Erichsonius patella, C Philonthus politus, D Quedius capucinus, E Platydracus femoratus, F Thyreocephalus albertisi. — Abbreviations: bs = basistipes, ca = cardo, ga = galea, lac = lacinia, mp = maxillary palp, ms = mediostipes, pf = palpifer.

Figure 8. 

Labium-hypopharynx (dorsal aspect), SEM micrographs: A Oxyporus stygicus, B Megalopinus sanguinitriguttatus, C Solierius obscurus, D Dianous obliquenotatus, E Austroesthetus passerculus, F Euaesthetus iripennis, G Fenderia chandleri, H Agnosthaetus cariniceps, I Cephennodes clavatus, J Veraphis sp., K Palaeostigus bifoveolatus, L Scydmaenus sp., M Stenichnus sp., N Eutyphlops sp. — Abbreviations: gl = ‘glossa’, hph = hypopharynx, hph scl = hypopharynx sclerite, lp = labial palp, pgl = ‘paraglossa’, prm = prementum.

Labium-hypopharynx (Fig. 9K–Q). Prementum dorsally along lateral margins with dense fringe of mesally directed hair-like trichomes; innermost row differentiated into spine-like trichomes, medially covered with sensilla coeloconica (Atrecus, Platyprosopus, Quedius, Thyreocephalus), with hair-like setae (Philonthus and Platydracus) or glabrous (Diochus and Erichsonius). Palpomere 3 about as wide to half as wide as palpomere 2. Ligula consisting of paired ‘glossae’ and ‘paraglossae’; ‘glossae’ dorsally modified into anterior lobes (Platyprosopus and Staphylinini) covered with sensilla coeloconica, sometimes bulbous, or modified into single medial projection covered with sensilla coeloconica (Diochus, Atrecus, and Thyreocephalus); ‘paraglossae’ represented by antero-lateral lobes of prementum. Hypopharynx dorsally along lateral margins with mesally directed hair-like trichomes. Medially provided with dense vestiture of hair-like trichomes forming medial tuft (Erichsonius, Philonthus, Platydracus), medially glabrous or with only few scattered short hair-like trichomes (Diochus, Atrecus, Platyprosopus, Quedius, Thyreocephalus).

Figure 9. 

Labium-hypopharynx (dorsal aspect), SEM micrographs: A Pseudopsis subulata, B Zalobius nancyae, C Lathrobium sp., D Medon vittatipennis, E Stilicoderus woodwardi, F Baryopsis sp., G Gnathymenus sp., H Homaeotarsus bicolor, I Paederus littoralis, J Pinophilus parcus, K Diochus schaumii, L Atrecus punctiventris, M Platyprosopus sp., N Philonthus politus, O Quedius capucinus, P Platydracus femoratus, Q Thyreocephalus albertisi. — Abbreviations: gl = ‘glossa’, hph = hypopharynx, lp = labial palp, pgl = ‘paraglossa’, prm = prementum.

Protarsus (Fig. 11F–M). Consisting of 5 tarsomeres, tarsomeres covered by tenent setae are widened and bilobed (except in Thyreocephalus). Tarsomeres 1–4 of all investigated Staphylininae are covered by distally widened (tenent) setae of various shapes (depending on species). Although Thyreocephalus bears tenent setae like the other genera, they have a different arrangement, forming a pair of setal patches on each tarsomere.

Figure 10. 

Left protarsi (ventral aspect), SEM micrographs: A Oxyporus stygicus, B Pseudoxyporus quinquemaculatus, C Megalopinus sanguinitriguttatus, D Solierius obscurus, E Dianous obliquenotatus, F Stenus puthzianus, G Austroesthetus passerculus, H Euaesthetus iripennis, I Fenderia chandleri, J Agnosthaetus cariniceps, K Cephennodes clavatus, L Veraphis sp., M Palaeostigus bifoveolatus, N Scydmaenus sp., O Stenichnus sp., P Eutyphlops sp., Q Homeotyphlus sp., R Pseudopsis subulata, S Zalobius nancyae, T Astenus sp., U Lathrobium sp., V Medon vittatipennis, X Stilicoderus woodwardi.

Figure 11. 

Left protarsi (ventral aspect), SEM micrographs: A Baryopsis sp., B Gnathymenus sp., C Homaeotarsus bicolor, D Paederus littoralis, E Pinophilus parcus, F Diochus schaumii, G Atrecus punctiventris, H Platyprosopus sp., I Erichsonius patella, J Philonthus politus, K Quedius capucinus, L Platydracus femoratus, M Thyreocephalus albertisi.

In Staphylinoidea, the groundplan condition is the form that also occurs most often within the Staphylinine group, with a transverse labrum (Blackwelder 1936; Naomi 1988a). Our analysis regarding the anterior margin of the labrum and the medial surface of the epipharynx has yielded the groundplan condition of these characters for Staphylinine as ambiguous (Fig. 12). On the other hand, our analysis has shown that the groundplan state of the epipharyngeal surface in Staphylinoidea is the same as found in Betz et al. (2003), i.e., bearing a bristle-trough bordered by hairs or spines involved in concentrating and directing the food stream in the median line. Half of the Staphylinine species that we studied possess the character state of the epipharyngeal surface with a bristle-trough. This fact supports the view that bristle-troughs in staphylinoids are functionally connected not only to a microphagous feeding style but might also have an important role in the context of predation in some members of the Staphylinine group. The rows of trichomes forming the bristle-trough have also been described by other authors in Ptiliidae (Yavorskaya et al. 2017) and Leiodidae (e.g., Wheeler and Miller 2005; Antunes-Carvalho et al. 2017), but also in non-staphylinoid polyphagans (e.g., Anton and Beutel 2004, 2012), and in Myxophaga (e.g., Anton and Beutel 2006; Yavorskaya et al. 2018b).

The rotary mill behavior as described for Philonthus by Evans (1964) is closely associated with the labrum-epipharynx complex. The shape of the labrum might give hints as to the performance of this behavior among the staphylinines. Evans (1964) provided an illustration and described how a food item passes through the medial notch of the labrum during the rotary mill process. Leschen and Newton (2003) suggested that the mill might serve as a filter for larger indigestible particles for fluid feeders. The feature of additional filtering would therefore only be possible in groups that have such a notch present in the labrum. Such filtering has been observed in Megalopsidiinae (Leschen and Newton 2003) and some Staphylininae (Philonthus, Evans 1964). The Oxyporinae possess a notch in the labrum, although the rotary mill behavior has never been observed in this group, despite studies in which their feeding behavior has been closely observed (Newton 1984; Leschen and Allen 1988; Lipkow 1997). The rotary mill behavior has also been reported in Steninae (Betz 1999); however, their labrum shape (Fig. 1E, F) is different from that of Megalopsidiinae and Staphylininae in that the medial notch is missing. The absence of the medial notch prevents the movement of pushing the food dorsally as in Megalopinus (as shown in Leschen and Newton 2003) and Philonthus (as shown in Evans 1964). Therefore, a special additional filtering feature of the rotary mill is unlikely to be present in the Steninae.

The orientation of the trichomes present on the surface of the epipharynx can be associated with the function of directing solid food particles, either by preventing the ingestion of solid food when they are anteriorly directed (Evans 1964) or by directing food particles toward the pharynx when they are posteriorly directed (Yavorskaya et al. 2017; Antunes-Carvalho et al. 2017), the latter case being the groundplan condition in Staphylinoidea (Betz et al. 2003). As shown by our results (Figs 1, 2), the orientation or even presence of the trichomes varies quite substantially among members of the Staphylinine group. For example, in Oxyporinae, the epipharynx trichomes are directed posteriorly (Fig. 1A, B), contradicting its hypothesized function of preventing the ingestion of solid food. Even though the diet of these beetles consists of fluid via extra-oral digestion (Newton 1984; Leschen and Allen 1988; Thayer 2016), when the trichome orientation is considered, the function of the epipharynx apparently deviates from the other members of the group regarding at least the way in which the food is directed to the mouth opening. The high variability found in this character leads to the hypothesis of the Staphylinine group members showing a great diversity of feeding styles, the direction of the trichomes of the epipharyngeal (together with the hypopharyngeal) tuft being the main indication for determining whether larger food particles are directed toward (i.e., trichomes posteriorly directed) or away from (i.e., trichomes anteriorly directed) the mouth opening. Megalopsidiinae, Steninae, Euaesthetinae, some Scydmaeninae, Leptotyphlinae, and some Staphylininae have an epipharynx that is completely or mainly devoid of trichomes, so such a function of directing food items seems to be absent in these species.

The potential groundplan condition of the mandibles in the staphylinine group according to our character mapping analysis based on maximum parsimony (Fig. 12) is the following: subapical tooth absent; retinaculum absent; prostheca present, not forming a lobe-like projection; mola present. Subapical tooth and retinaculum are terms usually used to refer to mandibular teeth present in Coleoptera larvae (Böving and Craighead 1931; Lawrence et al. 2011). In adult beetles, they were consistently used and clearly differentiated by Betz et al. (2003) in their study on Staphylinoidea and are similarly interpreted and used herein. Nevertheless, the term retinaculum has also been used by Ball et al. (2011) in Carabidae but is not homologous to our usage. Luo et al. (2021), in their anatomical study of a pselaphine (Staphylinidae) species, state that it is difficult to interpret the phylogenetic significance of series of subapical mandibular teeth. Many authors in their descriptions refer only to “mandibular teeth” while disregarding their positions (as being on the incisor area of the mandible or not), thereby making it difficult to homologize their designation with our current interpretation (sensu Betz et al. 2003). Notwithstanding, Naomi (1988a) proposed that the likely groundplan condition for the mandible of staphylinoids is without any teeth, i.e., the absence of both subapical teeth and retinaculum. On the other hand, Betz et al. (2003) found reti­nacula widespread among adult sporophagous staphylinoids. Two other groundplan aspects of the assumed microphagous primitive staphylinoid are the well-developed prostheca and mola (Naomi 1988a; Betz et al. 2003; Beutel and Yavorskaya 2019). The Staphylinine group typically lacks the mola, with few exceptions (Thayer 2016). Our data show the presence of a mola in the Oxyporinae, Solieriinae, Scydmaeninae (Scydmaenus), Pseudopsinae (Pseudopsis), and Staphylininae (Diochus). According to our tree, the only group to possess a “true” plesiomorphic mola is the Oxyporinae, the other groups having a secondarily evolved (pseudo)mola. This hypothesis is however difficult to establish, as the relationships of these groups remain doubtful (Hansen 1997; Grebennikov and Newton 2009; McKenna et al. 2015). Since the absence of a mola is usually associated with predators (Hansen 1997; Beutel and Yavorskaya 2019; Krenn 2019) (see discussion further below), the secondary evolution of a (pseudo)mola in these beetles might indicate feeding strategies that differ from the specialized extra-oral digestion feeding type that is markedly present in the Staphylinine group.

Steninae have been the subject of several ecomorphological studies and investigations into feeding behavior (e.g., Jenkins 1960; Betz 1996, 1998, 1999, 2002, 2003; Kölsch and Betz 1998), whereas the Euaesthetinae have largely been neglected in this respect. The only records known to us are from Remillet (1969) who fed diplurans to a specimen of Octavius massatensis Coiffait, 1959 and from Orousset (1988) who reported the presence of a specimen of oribatid mite attached to the mouthparts of a dead specimen of Tyrannomastax Orousset, 1988. We can, however, infer, based on their morphology, that the Euaesthetinae are predators that feed on small arthropods, because their mandibles are slender and falcate (Clarke 2018), similar to those of Steninae, which are optimized for hunting agile prey. The retinaculum in the Euaesthetinae might function similarly to the subapical tooth present in the Steninae for holding prey after capture, but more closely to the body. The size of the lacinia and galea are notably shorter in Euaesthetinae when compared with its sister group Steninae. This might also be an adaptation toward handling food items that are more proximally located, as is the retinaculum.

The prostheca can be interpreted as a tool for gathering and transporting food particles toward the mouth, as stated by Betz et al. (2003), although the taxa that they studied were microphagous spore feeders. In contrast, the taxa examined herein are predators that feed on fluids, and therefore, the prostheca might have evolved a different function. Evans (1964) suggests that the prostheca (referred to as “penicillus”) of Philonthus assists with both the fluid intake and extrusion of digestive fluids. Stocker et al. (2022) have also proposed that the presence of the prostheca in staphylinines allows a “mixed-feeding strategy”, and that its lack represents a specialized predatory morphology. Veraphis possesses a distinct type of prostheca (Fig. 3L) that might be used as a pseudomola because of its morphology and position.

Only six species of the ingroup studied here possess a developed molar region: the two oxyporine species, which are interpreted as having retained its plesiomorphic form as stated previously, and four other species (Solierius, Scydmaenus, Pseudopsis, and Diochus) that evolved it secondarily. Therefore, the feeding conditions that can be linked to the secondary evolution of a pseudomola are worthy of discussion. Solierius is a rare group, and nothing is known about its feeding behavior (Thayer et al. 2012; Thayer 2016). Scydmaeninae feeding habits have been reviewed in Jałoszyński (2018). According to Jałoszyński (2012a, 2012b), two species of Scydmaenus have been shown to prefer Collembola or soft-bodied Acari and display no interest in specimens of Oribatida under controlled laboratory conditions. Additionally, that author has reported the scavenging of dead arthropods and cannibalistic behavior exhibited by these species. Jałoszyński (pers. comm.) has however pointed out the difficulties in observing precisely the way that such small beetles use the specific parts of their mandible. Therefore, we currently lack information concerning the employment of the pseudomola of Scydmaenus or the pseudomola-like prostheca of Veraphis during the feeding process. Regarding the feeding habits of Pseudopsis, Klimaszewski et al. (2013) found that the gut of these beetles contains traces of yeast, but no arthropod cuticle. The authors have nevertheless concomitantly determined the presence of eight unidentified DNA strands indicating that an extra-oral digestion feeding strategy is a plausible explanation for the missing cuticle particles in the examined gut. Little is known about the biology of Diochus.

As the shape of the mandible might not fit precisely into a categorical description, we performed a 2D geometric morphometric analysis based on (semi-)landmarks. The value of PC 1 in this analysis (Figs 13, 14) refers mainly to the extent to which the mandibles are sickle-shaped and their apices are pointed. The value of PC 2 (Figs 13, 15) refers to their overall curvature, ranging from straight toward strongly curved. Whether the investigated subfamilies have diverged with regard to their mandibular morphology and their connected feeding ecology (as indicated by their partly exclusive occupation of certain regions of the morphospace in Fig. 13) needs to be substantiated in future studies by examining more representatives per subfamily. The shape corresponding to lower PC 1 might be functionally associated with: (1) non-predatory staphylinoids (e.g., Tachinus (Klimaszewski et al. 2013) and Anotylus (Thayer 2016; Okuzono and Tokuda 2022)), or (2) more generalist feeders (including predators) possibly using their mandibles for a wide variety of food items and using strong force against certain food types, for example, Platydracus beetles have a relatively robust mandible (Figs 4Q, 13) used to feed on various kinds of prey, ranging from soft and lightly sclerotized dipteran maggots to hard and heavily sclerotized oribatid mites (ELS unpublished observations), or (3) more specialized predators of heavily sclerotized organisms such as some Scydmaeninae that are specialized for feeding on sclerotized mites (Jałoszyński 2018). The more robust shapes corresponding to lower PC 1 also correspond to high inlever/outlever ratios (Fig. 16) because of the increased length of the inlever and shortened outlever length (see, for example, the hypothetical model in Fig. 13 at -0.225 along the x-axis), which translate into higher force output but lower velocities to close the mandibles (see Table S3). The slender falcate and curved shapes corresponding to higher PC 1 values are characterized by low inlever/outlever ratios because of the narrow base (small inlever length) and long length (large outlever length) allowing faster closing movements and can be considered as adaptations by predators specialized on elusive prey that are not heavily sclerotized (Betz et al. 2018; Stocker et al. 2022). Astenus (Paederinae) is a good representative of such a specialized predatory mandible shape, and indeed, members of this genus feed on entomobryid springtails (Collembola) (Assing 2015). Stocker et al. (2022) suggested that the paederine genus Rugilus, which is closely related to Astenus (Żyła et al. 2021) and which possesses falcate mandibles, is also adapted toward feeding on elusive springtails. Other representatives of the Staphylinine group that fall in the regions of the morphospace of falcate mandibles (e.g., Pinophilus or Medon) might also represent predators that are capable of preying upon especially vigilant prey. In the ancestral character reconstruction of PC 1 (Fig. 14), the Staphylinine group exhibits lower PC values, suggesting a tendency towards mandibles with a predatory shape when compared to the outgroup.

The functional consequences and feeding types that are related to the shape changes associated with PC 2 (straight versus curved mandibles) remain to be clarified. Future comparative studies on the feeding ecology of rove beetles should help to improve our understanding of relationships between morphology and ecology.

Mahalanobis distances between the subfamilies (and outgroup species) with resulting p values of the CVA to quantify the separations between the subfamilies (and the outgroup species) are shown in Table 2.

For a definition of the groundplan aspects of the maxilla in Staphylinoidea, Naomi (1988b) considered the major parts of the maxilla, i.e., cardo, stipes, palpifer, lacinia, and galea. Blackwelder (1936) stated that the cardo, stipes, galea and lacinia are subject to much variation and Hansen (1997) suggested that the lacinia was probably not a particularly informative character at higher taxonomic levels because of its high variability. Betz et al. (2003), however, focused on the functional parts of the maxilla, i.e., the lacinia and galea, and showed that the groundplan condition in Staphylinoidea was most likely a microphagous maxilla with brush-, rake-, or comb-like structures for sweeping in diffuse food material. Yavorskaya et al. (2017) also paid close attention to some structures of the maxilla regarding the fimbriate galea and the curved rows of trichomes. Such features were interpreted by them to be present in the ancestor of Staphyliniformia and related to a microphagous feeding style. Our data confirm that the lacinia and galea are diverse among staphylinines, and therefore, we follow Hansen’s (1997) interpretation that these characters are uninformative at higher taxonomic levels and ignore them in our character mapping analysis. The high variability reflects the strong selective demands on these structures, as they probably serve as important tools in the context of feeding (Betz et al. 2003). Hansen (1997) nonetheless considered maxillary palpomere 4 to be informative for higher taxonomic levels, and we also have included it in our character mapping analysis. In addition, Betz et al. (2003) pointed out that the palpifer may serve as a mechanical support for the galea, which can also be seen in Megalopinus (Fig. 5B).

For the character mapping analysis, we investigated the apical unarticulated spine of the lacinia and maxillary palpomere 4. The apical unarticulated spine of the lacinia, even though it was rendered as ambiguous for the groundplan of the Staphylinine group, was identified as a groundplan feature of the hypothetical ancestor between the split of Oxyporinae + the rest of the staphylinines (Fig. 12). Maxillary palpomere 4, on the other hand, was unambiguously rendered as “well developed, fully sclerotized, similar in width to palpomere 3”, as the groundplan state for the Staphylinine group. Hansen (1997) interpreted this state as being basal to most of the Staphyliniformia families and subfamilies. A spine on the lateral margin of the galea was present in Agnosthaetus, but we did not score it on our character mapping analysis since it is an autapomorphic character for this genus and therefore not informative for our purposes. Nonetheless, this character was used by Clarke and Grebennikov (2009) as they had analyzed another genus that also possessed this feature.

No suggestions have been made in the aforementioned literature that the maxillae are used for fluid uptake. However, Wilhelmi and Krenn (2012) reported that the maxillary palps, together with the galea and lacinia, might be adapted in some groups of beetles to feed on nectar. In Scarabaeidae, the maxillae have been described as being moistened with nectar, enabling the adherence of pollen grains (Karolyi et al. 2009). The maxillary brushes of the highly specialized leiodid Platypsyllus castoris Ritsema, 1869 were considered a tool for transferring fluid to the mouth by capillary forces (Yavorskaya et al. 2023). The feeding behavior of Philonthus was closely studied by Evans (1964), whose findings confirmed the occurrence of extra-oral digestion involving a fluid feeding technique. In addition to these behavioral observations, an analysis of the gut has revealed the presence of finely divided solid content (Evans 1964). Such information (together with the maxilla’s dense brushes present on both lacinia and galea) sustain a hypothesis that in Philonthus and closely related groups the dense brushes aid the ingestion of a fluid diet, probably via capillarity, as described in Platy­psyllus. The combination of dense brushes on the maxillae together with an epipharynx with anteriorly directed trichomes might help to determine whether a beetle is an exclusive fluid feeder. Interestingly, Diochus is the only member of the Staphylininae that we have studied that possesses a developed molar region and is the only one to have a lacinia bearing significantly short trichomes. This suggests that Diochus does not feed exclusively on fluids.

The most phylogenetically informative characters of the labium-hypopharynx complex probably concern the labial palpomeres. In this study, we have focused on the apical palpomere, which serves as a diagnostic feature of some groups, such as the Oxyporinae in which it is strongly expanded, whereas the Solieriinae and the clade Steninae + Euaesthetinae have a distinctly reduced type. According to our results, the most likely hypothesis is that the staphylinine ancestor would have had a labial palpomere 3 “about as wide to half as wide as penultimate palpomere”. Naomi (1988b) also considered the labial palpomere 3 when establishing the groundplan conditions of Staphylinoidea, considering it as “about as long and a little thinner than the 2^nd”, which does not agree with our description model but might overlap our definition. Hansen (1997), however, considered this to be an informative character only at lower taxonomic levels.

Another character that has some phylogenetic potential is the ‘glossa’, which has not previously been explored in such a context. Our interpretation suggests that it is a good character for separating the clade Paederinae + Staphylininae from the other groups. These subfamilies share mainly a ‘glossa’ that is dorsally modified into anterior lobes, which are sometimes bulbous and covered with sensilla coeloconica. Naomi (1988b) states that most staphylinoids have ‘glossae’ and ‘paraglossae’ fused (ligula) in various degrees, characterizing the ligula as synglossa or hologlossa according to its degree of fusion. Betz et al. (2003) refrained from using the terms ‘glossa’ or ‘paraglossa’ and instead referred to ligula, e.g., by saying: “Ligula represented by antero-lateral lobes of prementum plus anteriorly directed setae in different numbers and arrangements at distal margin of prementum.” However, in the present work on the Staphylinine group, we have not followed this terminology any longer, because, at the distal margin of the prementum, we can often clearly distinguish between lateral lobe (i.e., the ‘paraglossae’) and a median assemblage of paired sensilla or differentiated lobes (i.e., the ‘glossae’) as separate structures. However, since many authors consider the ‘glossa’, together with the ‘paraglossa’, to be lacking or completely reduced in Coleoptera (e.g., Crowson 1981; Beutel and Lawrence 2016), we have put these terms in quotation marks throughout the text.

A notable feature in the genus Stenus is the ‘paraglossae’ on which the sticky pads of its highly specialized adhesion-capture apparatus are located (Betz 1996; Betz et al. 2018). Few studies have been carried out on the function of this region in other staphylinids. Indeed, the morphological definition of the ‘paraglossae’ as a distinct structure is not clear in Coleoptera, it being usually considered as completely reduced or fused with the ‘glossae’, forming a ligula (Crowson 1981; Beutel and Lawrence 2016). Clarke and Grebennikov (2009) discussed the potential presence of sticky pads also being present in an undescribed genus originally suspected of being a member of the Euaesthetinae (Leschen and Newton 2003; Betz and Kölsch 2004). However, following a phylogenetic analysis, the genus has ended up being considered a member of the Steninae, as the potential sister group of Stenus. In the same study, Clarke and Grebennikov (2009) pointed out the difficulties of the term ‘paraglossa’ and ‘glossa’ being treated as distinct and cited studies reinforcing the idea that they are probably fused with the ‘glossa’ as a ligula. Nonetheless, as explained in the last paragraph we refer to both ‘glossa’ and ‘paraglossa’ between quotation marks as there are no accepted terms at the moment for these distinct structures. We score our character matrix with three distinct states for the ‘paraglossa’: the highly specialized type present in Stenus, the type with inconspicuous antero-lateral lobes, and the type in which the lobes are represented by prominent anterior digitiform lobes. Most members of the Staphylinine group have the potential groundplan condition of inconspicuous antero-lateral lobes (e.g., Figs 8B, 9O), whereas in Euaesthetinae, the genera Austroesthetus and Agnosthaetus possess the third type represented as anterior digitiform lobes (Fig. 8E, H), whose specific function (e.g., during prey-capture) needs to be elucidated.

The dorsal part of the prementum has been poorly documented in morphological studies. We have explored it here, as we consider this region to be closely involved in the food intake process as this is the side that the food item will first pass through while being directed to the mouth opening via the hypopharynx. A glabrous prementum surface is the trend that most repeats within the Staphylinine group, but it varies considerably in the presence of setae or sensilla, or both. We have characterized the lateral margin with regard to the types of trichomes present: hair-like, conspicuous spine-like, or comb-like. These types have been used in our character mapping analysis (Fig. 12), however for the establishment of the groundplan feature of the Staphylinine group, it was rendered ambiguous.

Betz et al. (2003) designated the groundplan of the staphylinoid hypopharynx as having a tuft of posteriorly directed trichomes working together with the epipharynx to direct food toward the pharynx. In the investigated staphylinines, hair-like trichomes are usually present on its surface, but not always forming a tuft. Great variation has been found in this area within the Staphylinine group. A comparison of Quedius (Fig. 9O) with Platydracus (Fig. 9P) illustrates its high variability. Even though these two groups are both members of the same tribe, namely the Staphylinini, Quedius is medially glabrous, whereas Platydracus is provided with a dense vestiture, forming a tuft. We have not included this character in our trait mapping analysis because of its being a highly variable (apparently evolutionarily labile) character and therefore uninformative for higher taxonomic level analysis.

The function of the labium-hypopharynx has been extensively studied in the extreme case of Stenus (e.g., Betz 1996, 1998; Kölsch and Betz 1998; Koerner et al. 2012), which has an elongated labium type with the addition of adhesive pads at the tip, adapted for sticking to elusive prey such as collembolans. Another case of a highly modified labium is found in the scydmaenine Cephennodes, with labial discs adapted for suction (Jałoszyński and Beutel (2012). Similarly to the state in some other species within Scydmaeninae in our study, this region was entirely covered by the hypopharynx, which prevented us from more closely describing these structures. The functional aspect of this region in other Staphylinidae has been poorly studied. Weide et al. (2010) explored its potential protrusibility and retractability in species of Aleocharinae and the way in which the hypopharynx serves as a receptacle for receiving spores, which are subsequently ground by the mola or pseudomola. To infer the degree of protrusibility and retractability, they used microCT data. We cannot infer this motility with such precision from our data, but there are examples of at least potential movements of the prementum-hypopharynx complex in Scydmaeninae, as some of the specimens that we studied had their premental dorsal surface covered by the hypopharynx (see Fig. 8L for both covered/uncovered views in the same species). Interestingly, Scydmaenus is one of the few members of the Staphylinine group studied herein to possess a mola that could potentially work together with the retractability of the prementum. In Ptiliidae and Leiodidae (Staphylinoidea), the longitudinal arrangement of abundant trichomes on the dorsal surface of the prementum and the hypopharynx has been interpreted to work in combination with the epipharyngeal trichomes, gathering and directing the food flow in the midline (Betz et al. 2003; Antunes-Carvalho et al. 2017; Yavorskaya et al. 2017).

Functional studies of the ‘glossa’ and ‘paraglossa’ in beetles are also lacking, as they are usually reduced and sometimes even considered lost in many groups of Coleoptera (Crowson 1981; Beutel and Lawrence 2016). In Coleoptera, the ‘glossa’ and ‘paraglossa’ are usually fused forming a ligula, which has been reported to be involved, for example, in fluid (nectar) uptake in Scarabaeidae (Karolyi et al. 2016) and Lucanidae (Krenn et al. 2002). Evans (1964) described the function of the labium-hypopharynx complex in Philonthus. He mostly referred to the hairs present on the hypopharyngeal surface, suggesting that they help prevent large pieces of food from being directed to the mouth. He also mentioned the prominent lateral spine-like trichomes on the dorsal region of the prementum, assuming that their function is to hold the food item between the mandibles and maxillae while it is being processed.

The groundplan tarsal formula for Staphylinoidea is 5-5-5 (Naomi 1989). Several deviations from this can be found within the family, but, as we have investigated only the protarsi, we will not explore this character in depth, only listing the subfamilies that show some variation on the protarsus regarding the setal type. In the species of Euaesthetinae that we studied, the tarsomeres vary, ranging from five to four. Both species of Leptotyphlinae analyzed possess three tarsomeres. In Leptotyphlinae, this reduction is associated with an endogean lifestyle that leads to several reductions, such as body miniaturization, eye reduction, and pigmentation loss (Andújar and Grebennikov 2021). This character has been studied in depth in Steninae (Betz 2003) in which the multiple (currently 14 recognized) types of tenent setae might also have potential for within-genus phylogenetic analyses. However, the presence or absence of spatulate and/or discoid tenent setae is not an informative character for higher level taxonomic analysis as this can be variable among the species of a same genus (see Betz 2003 as an example for the genus Stenus). Therefore, we did not include this character in our character mapping analysis. Another character system of phylogenetic value for Staphylinoidea that we have not explored, is the pretarsus and distal margin of the terminal tarsomere (Antunes-Carvalho and Gnaspini 2016).

While the setal type is not significant at higher taxonomic levels for a character mapping analysis, it is a valuable character that should be considered when the functional morphology of some species is studied. Some Staphylinine group species have been reported to use the first pair of legs to capture prey (Betz and Mumm 2001; Barthold and Betz 2020; Stocker et al. 2022), and the presence of tenent setae might be a cue for a more recurrent usage of this prey capturing and handling method. Another important function associated with the tenent setae in the Staphylinine group is the ability of the beetles to climb plants, as observed in the genus Stenus (Betz 2003). In addition, some tenent setae show sexual dimorphism and are probably used for mating (Gnaspini et al. 2017). Therefore, the presence of tenent setae in both sexes might be indicative of active usage in prey capturing behavior.