Research Article |
Corresponding author: Margarita I. Yavorskaya ( margarita.yavorskaya@uni-tuebingen.de ) Academic editor: André Nel
© 2021 Rolf Georg Beutel, Xiao-Zhu Luo, Margarita I. Yavorskaya, Paweł Jałoszyński.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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The head anatomy of Pselaphus heisei (Pselaphitae) is described and documented. The structural features are evaluated in comparison with findings presented in earlier studies on the subfamily, with a special focus on correlations with predacious habits and the groundplan of Pselaphinae. We found the tentorium, labrum, maxillary palps, shape of head, and a system of dorsal pits and sulci highly variable within the subfamily, reflecting multiple transformations, including many homoplasious changes. The following major characters are identified as groundplan features of Pselaphinae: falciform mandibles; small mola; semiglobular neck; ventrolateral antennal articulation; steep clypeal region; setiform labial palpomere 3; tentorium with nearly vertical main branches and lacking laminatentoria; separation of tentorial bridge from tentorial arms; fusion of dorsal tentorial arms with the head capsule; large brain placed in the posterior third of the head; and a triple cluster of well-developed cephalic glands. The last feature supports a hypothesis that multiple and independent cases of adaptations to myrmecophilous habits observed in various lineages of Pselaphinae were possible by re-programming already existing glands to produce appeasement secretions. The cephalic muscle apparatus of P. heisei is similar to what is found in other staphylinoid groups, with some exceptions, whereas it is strongly modified in the myrmecophile Claviger testaceus. We propose that the unparalleled structural megadiversity in Pselaphinae is primarily linked with life in the upper soil layers combined with specialized carnivorous habits, with small and agile or mechanically protected arthropods as prey. Within the group, various specialized life habits have evolved, including myrmecophily, termitophily, and also life in deep soil or caves, each with unique morphological adaptations.
Glands, micro-CT, musculature, pselaphine beetles, 3D-reconstruction.
Pselaphinae, also known as short-winged mold beetles or ant beetles, are a group with small brownish adults, often with a cryptic lifestyle (e.g.
The external aspects of the structural megadiversity were treated in several substantial contributions, for instance
Pselaphitae: Pselaphus heisei; beetles collected from leaf litter in Turze Pole ad Brzozów, SE Poland, by Dariusz Twardy. Specimens were preserved in 75% ethanol.
Specimens were dehydrated with an ascending series of ethanol (70%–80%–90%–95%–100%), stained in iodine solution, transferred to acetone and then dried at the critical point (Emitech K850, Quorum Technologies Ltd., Ashford, UK). One of the dried specimens was scanned at the MPI for the Science of Human History (Jena, Germany) with a SkyScan 2211 X-ray nanotomograph (Bruker, Knotich, Belgium) with an image spatial resolution of 0.30 μm (isotropic voxel size) using the following parameters: 50 kV, 300 μA, 4600 ms exposure time, 0.16° rotation steps, frame averaging on (2), and using Filter (0.5 mm Ti). Projections were reconstructed by NRecon (Bruker, Knotich, Belgium) into TIFF files. Amira 6.1.1 (Thermo Fisher Scientific, Waltham, USA) and VG studio Max 2.0.5 (Volume Graphics, Heidelberg, Germany) were used for the three-dimensional reconstruction and volume rendering. The µ-CT-scan is stored in the collection of the Phyletisches Museum Jena (for access, please contact X.-Zh. Luo).
For microtome sectioning, one specimen of P. heisei was embedded in araldite CY 212® (Agar Scientific, Stansted/Essex, UK). Sections were cut at 1 µm intervals using a microtome HM 360 (Microm, Walldorf, Germany) equipped with a diamond knife, and stained with toluidine blue and pyronin G (Waldeck GmbH and Co.KG/Division Chroma, Münster, Germany). The sections are stored in the collection of the Phyletisches Museum.
Specimens were cleared in a warm 10% aqueous solution of KOH for 20–60 min, thoroughly washed in distilled water and dissected; isolated body parts were transferred from 75% to 99% ethanol for 15 min and air-dried, mounted on SEM stubs with carbon tabs and examined uncoated using a Helios Nanolab 450HP scanning electron microscope (FEI, Hillsboro, USA). Images were processed using CorelDraw Graphic Suite 2017; the following adjustments were made: overall brightness and contrast enhanced, and background manually replaced with black.
The nomenclature of v.
The distinctly prognathous head (Figs
SEM images, head of Pselaphus heisei. (A) dorsal view; (B) ventral view; (C) lateral view. Abbreviations: ce, compound eye; fcl, frontoclypeal lobe; fhs, flattened hyaline setae; fp, frontal pouch; fr, frons; ge, gena; gs, gular sutures; lp, labial palp; lr, labrum; md, mandible; mf, median furrow; mp1–4, maxillary palpomere 1–4; mt, mentum; nr, neck region; occ, occipital constriction; pe, pedicellus; sc, scapus; smt, submentum; vt, vertex.
Ocelli are absent. The frontal region is strongly differentiated; widely spaced long and curved setae are inserted on different areas, except for the smooth and glabrous median furrows. The posterior frontal part between the compound eyes is medially divided by a deep furrow, ca. 20 µm wide anteriorly, narrowing posteriorly, and obliterating at the level of the posterior ocular margins; the posterior frontal portion is demarcated from the frontal roof of the rostrum by a deep semicircular emargination, which contains the large openings (diameter ca. 30 µm) of deep frontal pouches (fp; Figs
The ventral side of the neck region (nr; Figs
SEM images, head of P. heisei, (A) frontal view; (B–D) ventral view, flattened hyaline setae removed. Abbreviations: bs, basistipes; ca, cardo; ce, compound eye; ga, galea; gp, glandular pore; gs, gular suture (vestigial); lp, labial palp; lr, labrum; md, mandible; mt, mentum; nr, neck region; ppf, palpifer; ptp, posterior tentorial pit; sc, scapus; smt, submentum.
The main part of the tentorium (t; Figs
The labrum (lr; Figs
Musculature (Fig.
The antennae are inserted on the ventral side of the rounded frontoclypeal supraantennal lobes (fcl; Figs
SEM images, antenna (A), maxillary palp (B), apical region of palpomere 4 (C), and spatulate projection on apical region of palpomere 4 (D) of P. heisei. Abbreviations: fcl, frontoclypeal lobe; fl1–9, flagellomeres 1–9; mp2–4, maxillary palpomeres 2–4; pe, pedicellus; sap, sensory appendage; sc, scapus.
Musculature (Figs
The slightly asymmetrical, broadly falciform mandibles are largely concealed below the labrum in their resting position, but well-developed and prominent when extended. They are mostly flat but a large, conspicuous protuberance (mpb; Fig.
SEM images, mouthparts of P. heisei. (A) mandibles, dorsal view; (B) right mandible, mentum and part of maxilla, ventral view; (C–D) labrum-epipharynx, ventral view. Abbreviations: at, apical tooth; bs, basistipes, ca, cardo; clc, curved longitudinal concavity; lml, lamella; lp, labial palp; lr, labrum; mpb, mandibular protuberance; mt, mentum; pll, plate-like lobe; ppf, palpifer; sat, subapical tooth; smdj, secondary mandibular joint.
Musculature (Figs
The maxillary groove is very shallow; a smooth peristomal concavity is present laterad the basal half of the maxilla, delimited by a distinct, rounded ridge. The cardo (ca; Fig.
3D reconstruction, head of P. heisei. (A) head, dorsal view; (B) nervous and digestive system, head capsule rendered transparent, dorsal view; (C) head, ventral view; (D) nervous and digestive system, head capsule rendered transparent, ventral view; (E) head, lateral view; (F) nervous and digestive system, head capsule rendered transparent, sagittal view; (G) head with right maxillary palp, ventral view; (I) head, posterior view. Abbreviations: br, brain; bs, basistipes; ca, cardo; ce, compound eye; cfr, clypeofrontal ridge; cl, clypeus; fcl, frontoclypeal lobe; fg, frontal ganglion; fp, frontal pouch; fr, frons; ge, gena; lp, labial palp; lr, labrum; md, mandible; mf, median furrow; mp, maxillary palp; mt, mentum; nr, neck region; oln, olfactory nerve; opl, optic lobe; p, pharynx; pll, plate-like lobe; ppf, palpifer; sc, scapus; smt, submentum; soe, suboesophageal ganglion; t, tentorium; tb, tentorial bridge; vt, vertex.
3D reconstruction, head of P. heisei, tentorium. (A) dorsal view; (B) ventral view, flattened hyalinous setae removed, (A–B) upper half with intransparent cuticle, lower half with transparent cuticle; (C) dorsal view, uppermost part of frontal pouch cut off, cuticle intransparent on left side, transparent on right side; (D) sagittal view, upper half with cuticle intransparent, lower half with transparent cuticle. Abbreviations: dat, dorsal attachment of tentorium; fp, frontal pouch; ptp, posterior tentorial pouch; t, tentorium; vat, ventral attachment of the tentorium.
Musculature (Figs
3D reconstruction, head of P. heisei. (A–B) antennal muscles; (C–D) mandibular muscles; (E–F) maxillary muscles; (G) labral-epipharyngeal, pharyngeal and labio-hypopharyngeal muscles; (A), (C) and (E) dorsal view, (B), (D) and (F) ventral view, (G) sagittal view. Abbreviations: M1 – M. tentorioscapalis anterior (0an1); M2 – M. tentorioscapalis posterior (0an2); M4 – M. tentorioscapalis medialis (0an4); M5 – M. scapopedicellaris lateralis (0an6); M6 – M. scapopedicellaris medialis (0an7); M7 – M. labroepipharyngalis (0lb5); M9 – M. frontoepipharyngalis (0lb2); M11 – M. craniomandibularis internus (0md1); M12 – M. craniomandibularis externus (0md3); M15 – M. craniocardinalis externus (0mx1); M17a, M17b - M. tentoriocardinalis (0mx3); M18 – M. tentoriostipitalis (0mx4/0mx5); M19 – M. craniolacinialis (0mx2); M21 – M. stipitogalealis (0mx7); M22 – M. stipitopalpalis externus (0mx8); M23 – M. stipitopalpalis internus (0mx10); M26 – M. palpopalpalis tertius (0mx14); M27 – M. palpopalpalis quartus (0mx15); M28 – M. submentopraementalis (0la8); M29 – M. tentoriopraementalis (0la5); M30 – M. tentoriopraementalis superior (0la6); M41 – M. frontohypopharyngalis (0hy1); M43 – M. clypeopalatalis (0ci1); M44 – M. clypeobuccalis (0bu1); M45 – M. frontobuccalis anterior (0bu2); M46 – M. frontobuccalis posterior (0bu3); M48 – M. tentoriobuccalis anterior (0bu5); M50 – M. tentoriobuccalis posterior (0bu6); MmIII – Mm. compressores epipharyngis; md, mandible; t, tentorium.
The submentum (smt; Figs
Musculature (Fig.
The anteriormost epipharynx, i.e. the ventral side of the labrum (Fig.
Histological sections of P. heisei. (A) labral region; (B–C) middle frontal region; (D) posterior frontal region (see inserted figure with dotted lines indicating position and orientation of sections). Abbreviations: br, brain; ce, compound eye; fp, frontal pouch; lhg, labiohypopharyngeal gland; lr, labrum; lrg, labral gland; mg, mandibular gland; mp, maxillary palp; oes, oesophagus; pph, prepharynx; t, tentorium.
Musculature (Fig.
The closed prepharyngeal tube (pph; Fig.
Musculature (Fig.
The brain (br; Figs
Three well-developed glandular clusters are present in the head, unpaired labral and labiohypopharyngeal glands, and paired mandibular glands. The relatively small labral glands (lrg; Fig.
A rather diffuse tissue is present in the apical antennomere. However, an unambiguous interpretation as gland is not possible with the material at hand.
Even though the present contribution on the unspecialized predacious Pselaphus heisei adds information on external and internal head structures, the available morphological data for Pselaphinae are still too limited for a formal character evaluation. Internal soft parts of crucial taxa are completely unknown, notably of Protopselaphus Newton and Thayer, 1995, the sister taxon of Pselaphinae, and also of Faronitae, probably the sister group of all remaining pselaphine supertribes (
It was pointed out by
That predaceous habits belong to the groundplan of Pselaphinae is clearly indicated by one feature found in P. heisei and many other species (e.g.
The role of the antennae in prey detection and capture in different pselaphine species was described by
The German common name “Palpenkäfer” (palp beetles) refers to another complex autapomorphy of Pselaphinae, the greatly modified (and extremely diverse) maxillary palps. This appendage is usually characterized by a short and often triangular palpomere 3, a large, terminal club-shaped palpomere 4, and a peg-like sensorial apical ‘pseudosegment’ (e.g.
The long range of the palps is certainly important for the small predacious species of Pselaphinae. Palpomere 4 of Pselaphus displays an unusually dense array of various sensilla and hair-like structures, including curved and apically spatulate cuticular projections, presumably with glandular openings (Figs
The documentation of the labrum of Pselaphinae is sparse, as this structure is usually partly concealed (e.g.
A typical pselaphine feature is a very steep clypeus, strongly declining from the anterior region of the frontal rostrum. This condition is present in P. heisei (Fig.
The cephalic digestive tract of P. heisei is similar to what is found in other beetles in its general configuration (e.g.
An intriguing and characteristic but puzzling character system of Pselaphinae is the presence of furrows, foveae, non-foveal pits, and other cephalic (and also thoracic and abdominal) surface structures, and also various modes of ‘deformation’ of the head capsule (e.g.
The presence of a very distinct longitudinal median frontal furrow is arguably a derived groundplan feature of Pselaphinae. This structure is absent in Protopselaphinae (
A conspicuous feature observed in P. heisei (Fig.
A potential synapomorphy of Protopselaphus and Pselaphinae is the V- or U-shaped tentorium, with nearly vertical main branches, each comprising the posterior and dorsal arm, and lacking laminatentoria completely. This is in clear contrast to the presumably ancestral condition found in other staphylinoid beetles (
An unusual feature of P. heisei, possibly an autapomorphy of the genus, is a dense vestiture of broadened and flattened hyaline setae on the anterior gular region. The arrangement of these structures is so dense that they form a continuous mass covering a large ventral area of the head. Mechanical removal of the setae revealed small pores, presumably glandular openings, at each setal insertion. Similar setae densely cover the ventral precoxal region of the prothorax, a large anterior area of the mesoventrite, and the first exposed abdominal sternite. The massive, bulging submental region of the head seen in anterior view (Figs
A very unusual, apparently derived antennal feature observed in P. heisei and other pselaphine genera (e.g.
In contrast to the maxillae and especially the maxillary palps, the prementum of pselaphine beetles is inconspicuous, more or less retracted, and not visible externally at all in P. heisei (Fig.
Compound eyes are present and functional in the groundplan of Pselaphinae (e.g.
The very large relative size of the brain and its placement in the posterior third of the head is likely an autapomorphy of Pselaphinae, or a possible synapomorphy shared with Protopselaphus. The increase in size and posterior shift are obviously a result of small body size, an effect of miniaturization as observed in many beetles with very small body size (e.g.
The cephalic musculature of P. heisei (Figs
Musculature of Pselaphinae, some other Staphyliniformia and Elateriformia (Clambidae) (partly based on
Family | Subfamily | Genus | No. of muscle | ||||||||||||||||||||||||||
1 | 2 | 4 | 7 | 9 | 11 | 12 | 15 | 17 | 18 | 19 | 28 | 29 | 30 | 41 | 42 | 43 | 44 | 45 | 46 | 48 | 50 | 51 | 52 | 67 | 68 | 69 | |||
Ptiliidae | Ptiliinae | Mikado | + | + | + | + | + | + | + | + | + | + | + | + | - | + | + | - | - | + | + | + | + | - | - | + | + | + | - |
Ptiliidae | Ptenidium | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | - | + | + | + | + | + | - | - | + | + | + | - | |
Leiodidae | Cholevinae | Catops | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | - | + | + | + |
Hydraenidae | Ochtebiinae | Ochtebius | + | + | + | + | + | + | + | + | + | + | + | ? | + | + | + | + | + | ? | + | + | ? | - | - | + | ? | ? | ? |
Staphylinidae | Aleocharinae | Aleochara | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | ? | + | + | + | + | - | + | ? | ? | ? | ? | ? |
Autalia | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | ? | + | + | + | + | - | + | ? | ? | ? | ? | ? | ||
Oligota | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | ? | + | + | + | + | - | + | ? | ? | ? | ? | ? | ||
Oxypoda | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | ? | + | + | + | + | - | + | ? | ? | ? | ? | ? | ||
Eumicrota | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | ? | + | + | + | + | - | + | ? | ? | ? | ? | ? | ||
Gyrophaena | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | ? | + | + | + | + | - | + | ? | ? | ? | ? | ? | ||
Homalota | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | ? | + | + | + | + | - | + | ? | ? | ? | ? | ? | ||
Pselaphinae | Claviger | + | + | + | + | - | + | + | + | + | + | +? | + | - | + | + | - | + | + | + | + | + | - | - | - | + | + | + | |
Bergrothia | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | - | + | - | + | + | + | - | - | - | + | + | + | ||
Pselaphus | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | - | + | + | + | + | + | + | - | - | + | + | + | ||
Hydrophilidae | Helophorus | + | + | + | + | + | + | + | + | + | + | + | + | + | + | + | - | + | - | + | + | - | + | + | + | + | + | + | |
Clambidae | Clambus | + | + | + | + | + | + | + | + | + | + | + | + | + | - | + | - | + | + | + | + | - | + | - | + | + | + | + |
The extrinsic and intrinsic antennal muscles are also well-developed in Bergrothia saulcyi (
In contrast to other staphyliniform beetles (e.g.
An intriguing character system documented in P. heisei, Claviger (
The presently available morphological information of Pselaphinae is not even remotely sufficient for a formal character analysis. However, an overwhelming morphological diversity of head structures is obvious, by far surpassing what is found in related groups of staphylinid beetles (e.g.
The study of cephalic structures of the free-living and predacious P. heisei made it possible to compare presumably unspecialized conditions with features previously described for the extreme myrmecophile Claviger testaceus and the blind Bergrothia saulcyi, the latter presumably adopted to periodic shifts into deep soil layers. These species belong in three different supertribes of Pselaphinae, and although such a small sample out of over 10,000 known species is insufficient to draw general conclusions, we identify possible groundplan features of the subfamily: falciform mandibles with several sharp subapical teeth and a vestigial mola but lacking a mesal microtrichial brush (secondarily developed in specialized, liquid-feeding Clavigeritae); a semiglobular neck; a ventrolateral articulation of the antennae below supraantennal frontal lobes; a steep clypeal region; a setiform labial palpomere 3; V- or U-shaped tentorium, with nearly vertical main branches, each comprising the posterior and dorsal arm, and lacking laminatentoria; separation of the tentorial bridge from the tentorial arms; a firm fusion of dorsal tentorial arms with the head capsule, with fusion sites usually visible externally as dorsal tentorial pits; functional compound eyes with a limited number of ommatidia; a very large brain placed in the posterior third of the head; a constriction separating the anterior protocerebral part from the posterior portion (linked with the development of the occipital constriction); a triple cluster of well-developed cephalic glands. A strongly modified, largely vertical labrum is a possible synapomorphy of Pselaphitae and Clavigeritae. The shape of the head capsule, especially of its preocular region, the dorsal system of foveae and sulci, the shape and orientation of the labrum, and especially the maxillary palps were all found very variable within only three examined pselaphine species, documenting an enormously high morphological diversity, likely including frequent homoplasious transformations. To explore the genetic background of the structural megadiversity in Pselaphinae will be an intriguing target of future investigations, screening genomes or transcriptomes of pselaphines and other staphylinid beetles to detect changes enabling accelerated evolution of morphological structures.
Our work was partly funded by project CGL2013-48950-C2 (AEI/FEDER, UE). We are indebted to Dariusz Twardy (Brzozów, Poland) who collected specimens used in our study; Anna Siudzińska (PORT Polish Center for Technology Development, Wrocław) is acknowledged for taking the SEM images; and Peter Hlaváč (Prague, Czech Rep.) kindly provided rare literature and valuable data on the taxonomy of Pselaphinae.
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Explanation note: Figure S1. Comparison of pharyngeal, labral-epipharygeal and labial-hypopharygeal muscles of Claviger testaceus, Begrothia saulcyi, Pselaphus heisei, in sagittal view.