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Keywords:

  • Aleocharinae;
  • Coleoptera;
  • comparative morphology;
  • head anatomy;
  • hypopharynx;
  • labium;
  • microtomography;
  • prementum;
  • sclerite;
  • spore-feeding;
  • Staphylinidae;
  • synchrotron;
  • tentorium

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

We elucidate the configuration of the tentorium and the sclerites of the hypopharynx–prementum complex in selected spore- (pollen-) and non-spore-feeding Aleocharinae (Staphylinidae) by presenting the first comparative 3D reconstructions of these structures for 19 staphylinoid beetle species (six outgroups, 13 Aleocharinae). General organization of the tentorium follows the groundplan previously proposed for adult Staphylinidae, although some taxa have reduced or lost the dorsal (all Aleocharinae studied, Agathidium mandibulare [Leiodidae]) or anterior (Omalium rivulare [Omaliinae], Anotylus sculpturatus [Oxytelinae]) tentorial arms. All species investigated have premental and hypopharyngeal sclerites that are partly homologizable across taxa. We clarified that Musculus praementopalpalis externus originates from the premental sclerite, resolving its unclear origin reported in our previous publications. Eight of 13 investigated Aleocharinae species are spore/pollen feeders, six obligatorily. Three of these six (Eumicrota, Gyrophaena fasciata, G. gentilis) have grinding pseudomolae and a fully developed hypopharyngeal suspensorium with posterior bridge and anterior elongations; the remaining three (Oxypoda, Pagla, Polylobus) lack pseudomolae and suspensorial bridge, but have the suspensorium elongated anteriorly. The dorsolateral side of the hypopharyngeal sclerite interacts with the pseudomola. Obligate sporophagy/pollinivory apparently arose at least three times in Aleocharinae, not always involving the pseudomola–hypopharynx grinding mechanism.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

Predation is thought to be the basal and probably most common feeding type within the beetle family Staphylinidae (Hansen 1997), but other feeding habits also occur widely in staphylinid beetles, including saprophagy, phytophagy, and mycophagy (Thayer 2005). A specialized and uncommon feeding mode is sporophagy, which we define here as feeding—usually obligately—on numerous discrete small particles such as fungal spores or pollen (see Betz et al. 2003; Weide et al. 2010). Betz et al. (2003) found that specialization from general microphagy (e.g., saprophagy, mycophagy in general) to sporophagy within major lineages of staphylinoids is not necessarily strongly reflected in mouthpart morphology. Nevertheless, they found that in some lineages the organs used in food intake and processing have undergone fine structural modifications, probably convergently, in evolving specialized mycophagy such as spore feeding. Similar arguments have been used by, for example, Leschen (1993) and Hansen (1997).

Weide and Betz (2009) and Weide et al. (2010) investigated Staphylinidae: Aleocharinae in more detail with regard to the head musculature and overall structure of the hypopharynx–prementum complex. Three types of hypopharynx–prementum complexes are recognized based on the position of the prementum relative to the hypopharynx and mentum. The configuration of this region seems to be highly correlated with the fine structures of the mouthparts. A wide preoral cavity occurs in species that possess a distinct molar region, for instance. Taxa having a pseudomola (i.e., a secondary grinding surface that evolved de novo on the ventral side of the mandible; cf. Betz et al. 2003) show distinct modifications of this region, in particular the highly derived prementum sandwiched between the ventral mentum and the dorsal hypopharyngeal region.

The present study is an extension of those studies to investigate the internal skeletal features of the head capsule and hypopharynx–prementum complex. We focused on comparative investigation and analysis of 3D reconstructions of the tentorium and sclerites of the hypopharynx–prementum complexes within a group of closely related staphylinid taxa.

The inner head skeleton (tentorium) of Hexapoda was described in detail by Snodgrass (1993). Although the groundplan of the coleopteran tentorium is known (Beutel 1997), it has not been interpreted as a whole in a phylogenetic context for this group. In this study, the tentoria of ‘higher Aleocharinae’ (Ashe 1993, 2005), a clade that includes all known aleocharine mycophages, are investigated comparatively. To test whether the inner head skeleton of spore feeders differs from that of non-spore feeders, these structures are portrayed three-dimensionally for Staphylinidae for the first time in the literature, and a matrix of tentorial characters is analyzed. Inference of evolutionary changes is made possible by considering not only obligate sporophages, but also facultative spore feeders and non-mycophagous Aleocharinae and outgroups.

The internal sclerites, that is, in our study defined as the internal hypopharyngeal and premental sclerites, have been described for various hexapod species (Matsuda 1965; Snodgrass 1993). As these structures are often associated with the salivarium, which is reduced in the majority of Coleoptera (Beutel 2003), homologizing these internal sclerites cannot rely on connections to the salivarium and can be done only on the basis of the positional and structural criteria of Remane (1952). Descriptions of these sclerites have typically not been provided in past studies (e.g., Pradhan 1938; Honomichl 1975; Ashe 2005). This is mainly a result of conventional preparation techniques that make it easy to overlook the structures, which are usually very fragile and closely connected to the labium (i.e., partly fused to the lateral walls). Using synchrotron X-ray microtomography (SR-μCT), which renders complete tomography datasets with good spatial resolution (e.g., Betz et al. 2007; Weide and Betz 2008, 2009; Weide et al. 2010), overcomes this difficulty.

We here elucidate the hypothesis elaborated in Betz et al. (2003) and Weide et al. (2010) that, in sporophagous representatives of the aleocharine tribes Homalotini (subtribes Gyrophaenina and Homalotina), Placusini, and Oxypodini, the hypopharynx has moved closer to the ventral mandibular surface, where it can function as an abutment for the ventral trituration surface of the mandibles, and the bowl-like depression of the hypopharyngeal surface functions as a mortar concentrating the spores, where they are triturated between the hypopharynx and the mandibles (Betz et al. 2003; Lipkow and Betz 2005). To do this, we rendered the internal sclerites three-dimensionally for the first time and interpreted sclerotized elements not previously recognized as part of this system. Using these new resources, we comparatively investigated characters of the internal sclerites and coded them for tree-mapping analyses of character evolution.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

Selection of taxa

We selected 17 staphylinid species (Table 1) from the omaliine, tachyporine (including the Aleocharinae), and oxyteline groups of Staphylinidae (subfamily groups of Lawrence and Newton 1982). The species examined comprise both assumed obligatory and facultative spore- (or pollen-) feeders as well as non-spore (or non-pollen) feeders. For details on the diet of the taxa examined, see Weide and Betz (2009), Weide et al. (2010: esp. table 4), Newton (1998), and Zwick (1981).

Table 1. Species included in this study. Non-European species were from the alcohol collection of the Field Museum of Natural History (Chicago, USA)
Family: subfamilyGenusTribeSpecies [collecting site]Feeding strategy
Agyrtidae: Necrophilus  Necrophilus subterraneus (Dahl, 1807) [Rhoen area, Germany]Non-spore (non-pollen) feeders
Leiodidae: Agathidium  Agathidium mandibulare (Sturm, 1807) [Tübingen, Germany]Obligate spore feeders
Staphylinidae:
Omaliine group
Omaliinae Lesteva AnthophaginiLesteva longoelytrata (Goeze, 1777) [Tübingen, Germany]Non-spore (non-pollen) feeders
Omalium OmaliiniOmalium rivulare (Paykull, 1789) [Tübingen, Germany]Non-spore (non-pollen) feeders
Oxyteline group
Oxytelinae Anotylus OxyteliniAnotylus sculpturatus (Gravenhorst, 1806) [Tübingen, Germany]Non-spore (non-pollen) feeders
Tachyporine group
Tachyporinae Tachyporus TachyporiniTachyporus chrysomelinus (Linnaeus, 1758) [Tübingen, Germany]Non-spore (non-pollen) feeders
Aleocharinae Aleochara AleochariniAleochara haematoptera (Kraatz, 1858) [Genf, Switzerland; Wallern, Austria]Non-spore (non-pollen) feeders
Autalia AutaliiniAutalia impressa (Olivier, 1795) [Tübingen, Germany]Non-spore (non-pollen) feeders
Oligota HypocyphtiniOligota parva (Kraatz, 1862) [Tübingen, Germany]Non-spore (non-pollen) feeders
Homalota HomalotiniHomalota sp [Veracruz, Mexico]Facultative spore feeders
Eumicrota  Eumicrota sp. [Mississippi, USA]Obligate spore feeders
Gyrophaena  Gyrophaena fasciata (Marsham, 1802) [Tübingen, Germany]Obligate spore feeders
 Gyrophaena gentilis Erichson, 1839 [Kiel, Germany]Obligate spore feeders
Stictalia  Stictalia sp. [Washington, USA]Facultative spore feeders
Pagla PagliniPagla sp. [Malleco Prov., Chile]Obligate pollen feeders
Atheta AthetiniAtheta laticollis (Stephens, 1832) [Tübingen, Germany]Non-spore (non-pollen) feeders
Pontomalota  Pontomalota opaca (J.L. Leconte, 1863) [Oregon, USA]Non-spore (non-pollen) feeders
Oxypoda OxypodiniOxypoda alternans (Gravenhorst, 1802) [Tübingen, Germany]Obligate spore feeders
Polylobus  Polylobus sp. [Talca Prov., Chile]Obligate pollen feeders

For comparative purposes, we chose Necrophilus subterraneus (Agyrtidae) and Agathidium mandibulare (Leiodidae) as additional outgroup representatives (Table 1). These are the same taxa studied by Weide et al. (2010). For simplicity, we refer to all taxa in the text using just generic names, unless it is necessary to distinguish species.

To consider possible intraspecific variations, we studied two up to six specimens per taxon and reconstructed the entire head of all species.

Use of abbreviations

The abbreviated names of characters used in the text and figures are summarized in Table 2. The abbreviated names of head muscles are summarized in Table 5.

Table 2. List of abbreviations used in text and figures
AbbreviationMeaningUse, if limited
aLateral column-like extension of suspensoriumFigs 7-11
antAntennaFig. 1 only
ATAAnterior tentorial armsFigs 2-6, 12, 12
bProximal end of suspensoriumFigs 7-11
CConnection between PTAFig. 5 only
cAnterior elongation of suspensoriumFigs 7-11
CCCharacter combination 
d1/d2Unpaired anterior/posterior bridge-like structures of suspensoriumFigs 7-11
DTADorsal tentorial armsFigs 2, 3, 12
f, g, h, iUnnamed premental sclerites of non-aleocharines discussed in textFigs 7, 8
FSFeeding styleFig. 12 only
gaGaleaFig. 1 only
HCHead capsuleFigs 2-6
lbLabrumFig. 1 only
lcLaciniaFig. 1 only
liLigulaFig. 1 only
LTLaminatentoriumFigs 2\x966, 12
mdMandibleFig. 1 only
m, n, o, p, r, tUnnamed premental sclerites of Aleocharinae discussed in textFigs 8-11
MVLMedial vertical lamellaFigs 2-6
mxbMaxillary baseFig. 1 only
plbLabial palpusFig. 1 only
pmxMaxillary palpusFig. 1 only
psmPseudomolae 
PTAPosterior tentorial armsFigs 2-6
PTWPosterior tentorial wallsFigs 2-6
SCInternal sclerite condition of hypopharynx–prementum complexFig. 12 only
Scl1Premental sclerite (anterior)Figs 2-6
Scl2Hypopharyngeal sclerite (posterior)Figs 2-6
Scl1+2Fused condition of Scl1 and Scl2Figs 2, 3
Susp cAnterior elongations of suspensoriumFig. 12 only
Susp dSuspensorial bridge(s)Fig. 12 only
TBTentorial bridgeFigs 2-6, 12, 12
TentTentoriumFigs 2-6
ULTUnfused laminatentoriaFigs 2, 3

Sample preparation

Specimens were fixed in 70% ethanol or FAE (formaldehyde: acetic acid: 100% ethanol; 3 : 1 : 6). The head and prothorax together were separated from the mesothorax and stepwise dehydrated in ethanol and critical point–dried (Balzers, CPD 020).

Synchrotron X-ray microtomography and light microscopy

The data were produced at the beamline ID 19 at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, using Synchrotron X-ray micro-tomography (SR-μCT) at an energy of 20.5 keV.

The 3D voxel datasets were reconstructed from the 2D radiographs using the filtered back-projection algorithm (Cloetens et al. 1997, 2002) developed for absorption contrast tomography. Additional details on the applied SR-μCT methods are provided in the supplemental material (see also Betz et al. 2007 and Weide and Betz 2008; see Data S1).

To improve and complete the data, additional sagittal and transverse histological sections of Necrophilus, Agathidium, Lesteva, Omalium, Oligota, Eumicrota, Pontomalota, and Oxypoda were produced. The samples were fixed as mentioned above, embedded in Spurr's medium (Electron Microscopy Sciences) and sectioned (1.5 μm) with an ultramicrotome (Leica, Reichert Ultracut S) using a diamond knife (Diatome, histo 45°). The mounted slices were stained with a combination of methylene blue and azure II (after Richardson et al. 1960).

Visualization of data

Because of the way SR-μCT operates, the virtual slices of the head capsule of each specimen are perfectly aligned. To perform the 3D-rendition of the sclerotized inner head skeleton (tentorium, internal sclerites), we used the software Amira® 4.1.0 (Mercury Computer Systems, Inc., Chelmsford, USA; hereafter called Amira®). The head capsule outlines in Figs 2–6 were displayed using the Voltex module of Amira®. Animated videos of the structures were generated using the CameraRotate and MovieMaker modules of Amira® (see animations of tentoria and hypopharyngeal and premental sclerites in the supplemental electronic material: movies 1–19; Data S1 - mp4 files). Additional information on the applied visualization methods is provided in the supplemental material (Data S1).

Other visualization procedures were performed with VGStudio Max (Volume Graphics, Heidelberg, Germany).

Labeling and coloring of images were performed using Adobe® Photoshop® CS4 (Adobe Systems Inc.).

Character mapping

To evaluate the possible groundplan of Aleocharinae with respect to the internal sclerites, suspensorium, feeding strategies, pseudomola, and selected characters of the tentorium (Tables 3 and 4), an interactive analysis of character evolution was implemented using Mesquite (v. 2.74, Maddison and Maddison 2010). For reconstructing ancestral character states, we applied the parsimony criterion of this software. Characters were coded as binary (0 for absence and 1 for presence) or multistate (Tables 3 and 4) and mapped on the phylogenetic schema shown in Fig. 12. Our analyses include, for outgroup comparison, the staphylinoid taxa Necrophilus (Agyrtidae) and Agathidium (Leiodidae) and the staphylinid taxa Lesteva and Omalium (Omaliinae), Anotylus (Oxytelinae), and Tachyporus (Tachyporinae).

Table 3. Explanation of the character state numbers used in Tables 4 and 7 and Fig. 12
Character/State codeATADTALTTBpsmSusp c: anterior elongationsSusp d: bridge(s)SCCCFS
  1. Abbreviations: see Table 2. −, absent; +, present. CC comprises the character mandibular pseudomolae plus both characters of the suspensorium (bridge[s] d1, d2, or d1 + d2, anterior elongations c, see figures).

0AbsentAbsent  AbsentAbsentAbsent   
1PresentPresentPresent: unfusedPresent: completePresentPresentPresent: anterior (d1)Fused: via structure ‘g’− psm, − Susp d, + Susp cNon-spore feeder
2  Present: fusedPresent: incomplete  Present: posterior (d2)Fused: via structure ‘h’+ psm, − Susp d, + Susp cObligate spore feeder
3      Present: anterior (d1) and posterior (d2)Unfused− psm, + Susp d1, − Susp cFacultative spore feeder
4        + psm, + Susp d1, + Susp c 
5        + psm, + Susp d2, + Susp c 
6        − psm, − Susp d, − Susp c 
7        − psm, +Susp d1 + d2, + Susp c 
8        − psm, + Susp d1 + d2, − Susp c 
9        − psm, + Susp d1, + Susp c 
Table 4. Data matrix used in the character-mapping analysis with respect to parts of the tentorium, to absence/presence of pseudomola and suspensorium elements, internal sclerite condition, character combination of mandibular pseudomolae plus both characters of the suspensorium and feeding strategies of the taxa and tribes of Aleocharinae investigated in this study
Family or tribeTaxon/CharacterATADTALTTBpsm*Susp c: anterior elongationsSusp d: bridge(s)SCCCFS
  1. For explanation of state numbers and abbreviations, see Tables 2 and 3; *According to Weide et al. 2010.

Agyrtidae Necrophilus subterraneus 1111001331
Leiodidae Agathidium mandibulare 1021013272
Anthophagini Lesteva longoelytrata 1111003381
Omaliini Omalium rivulare 0121011191
Oxytelini Anotylus sculpturatus 0121001131
Tachyporini Tachyporus chrysomelinus 1111003181
Aleocharini Aleochara haematoptera 1022010311
Autaliini Autalia impressa 1021112351
Hypocyphtini Oligota parva 1021001331
HomalotiniHomalota sp.1021111343
Eumicrota sp.1021112352
Gyrophaena fasciata 1021112352
Gyrophaena gentilis 1021112352
Stictalia sp.1021110323
PagliniPagla sp.1021010312
Athetini Atheta laticollis 1021110321
Pontomalota opaca 1021000361
Oxypodini Oxypoda alternans 1021010312
Polylobus sp.1021010312

In the absence of any comprehensive well-supported phylogeny for Aleocharinae, this schema has been used previously by Betz et al. (2003), Weide and Betz (2009), and Weide et al. (2010). The family-level relationships, that is, Agyrtidae plus Leiodidae as the sister group of Staphylinidae, are based on Lawrence and Newton (1982), Beutel and Molenda (1997), and Newton (1997, 1998); the relationships within Staphylinidae are based on Ashe and Newton (1993), Newton and Thayer (1995), and the consensus between the results of Elven et al. (2010, 2012) for the limited taxonomic overlap with ours (Note that in both of those studies, the taxa of interest in the present study are nearly all outgroups, so their relationships should be treated with caution.). The broadest analyses of Aleocharinae phylogeny to date, Ashe (2005) and Thomas (2009), were largely inconclusive concerning most intertribal relationships within higher Aleocharinae.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

The general arrangement of the inner head skeleton and the internal sclerites connected to the hypopharynx–prementum complex is shown in Fig. 1, using the aleocharine Gyrophaena fasciata as an example (for lateral views, see left-most images of Figs 2-6). For each species investigated, these endoskeletal structures are reconstructed three-dimensionally. The form and arrangement of the tentorium and the internal sclerites are illustrated in Figs 2-6 and 7-11, respectively.

image

Figure 1. Hand-drawn outline of the head capsule combined with 3D rendition of the premental (turquoise) and hypopharyngeal (blue) sclerites and tentorium (lilac) in Gyrophaena fasciata. Frontodorsal aspect. Visualization performed with Amira®. Scale bar = 200 μm.

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image

Figure 2. 3D rendition of the internal sclerites (i.e., premental and hypopharyngeal sclerite) and tentorium. A–C Necrophilus, D–F Agathidium, G–I Lesteva, J–L Omalium. —A, D, G, J. Lateral aspect with voltex (indication of the head capsule); —B, E, H, K. left frontolateral aspect of tentorium; —C, F, I, L. Dorsal aspect of tentorium. Anterior to left, posterior to right in all figures.

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image

Figure 3. 3D rendition of internal sclerites and tentorium. A–C Anotylus, D–F Tachyporus, G–I Aleochara, J–L Autalia. —A, D, G, J. Lateral aspect with voltex (indication of the head capsule); —B, E, H, K. Left frontolateral aspect of tentorium; —C, F, I, L. Dorsal aspect of tentorium. Anterior to left, posterior to right in all figures.

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image

Figure 4. 3D rendition of the internal sclerites and tentorium. A–C Oligota, D–F Homalota, G–I Eumicrota, J–L Gyrophaena fasciata. —A, D, G, J. Lateral aspect with voltex (indication of the head capsule); —B, E, H, K. Left frontolateral aspect of tentorium; —C, F, I, L. Dorsal aspect of tentorium. Anterior to left, posterior to right in all figures.

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image

Figure 5. 3D rendition of the hypopharynx–premental sclerite(s) and tentorium. A–C Gyrophaena gentilis, D–F Stictalia, G–I Pagla, J–L Atheta. —A, D, G, J. Lateral aspect with voltex (indication of the head capsule); —B, E, H, K. Left frontolateral aspect of tentorium; —C, F, I, L. Dorsal aspect of tentorium. Anterior to left, posterior to right in all figures.

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image

Figure 6. 3D rendition of the hypopharynx–premental sclerite(s) and tentorium. A–C Pontomalota, D–F Oxypoda, G–I Polylobus. —A, D, G. Lateral aspect with voltex (indication of the head capsule, see Visualization of data in 'Materials and Methods'); —B, E, H. left frontolateral aspect of tentorium; —C, F, I. dorsal aspect of tentorium. Anterior to left, posterior to right in all figures.

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image

Figure 7. 3D rendition of the hypopharyngeal (white) and premental (gray) sclerites in greater detail. A–C Necrophilus, D–F Agathidium, G–I Lesteva, J–L Omalium. —A, D, G, J. Lateral aspect; —B, E, H, K. Left frontolateral aspect; —C, F, I, L. Dorsal aspect. Anterior to left, posterior to right in all figures.

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image

Figure 8. 3D rendition of the hypopharyngeal (white) and premental (gray) sclerites in greater detail. A–C Anotylus, D–F Tachyporus, G–I Aleochara, J–L Autalia. —A, D, G, J. Lateral aspect; —B, E, H, K. Left frontolateral aspect; —C, F, I, L. Dorsal aspect. Anterior to left, posterior to right in all figures.

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image

Figure 9. 3D rendition of the hypopharyngeal (white) and premental (gray) sclerites in greater detail. A-C Oligota, D-F Homalota, G-I Eumicrota, J-L Gyrophaena fasciata. —A, D, G, J. Lateral aspect; —B, E, H, K. Left frontolateral aspect; —C, F, I, L. Dorsal aspect. Anterior to left, posterior to right in all figures.

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image

Figure 10. 3D rendition of the hypopharyngeal (white) and premental (gray) sclerites in greater detail. A–C Gyrophaena gentilis, D–F Stictalia, G–I Pagla, J–L Atheta. —A, D, G, J. Lateral aspect; —B, E, H, K. Left frontolateral aspect; —C, F, I, L. Dorsal aspect. Anterior to left, posterior to right in all figures.

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image

Figure 11. 3D rendition of the hypopharyngeal (white) and premental (gray) sclerite in greater detail. A–C Pontomalota, D–F Oxypoda, G–I Polylobus. —A, D, G. Lateral aspect; —B, E, H. Left frontolateral aspect; —C, F, I. Dorsal aspect. Anterior to left, posterior to right in all figures.

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Tentorium

In general organization, the tentorium of the species studied follows the groundplan of adult Staphylinidae (e.g., Naomi 1987; Thayer 2005): two anterior, two dorsal, and two posterior tentorial arms (PTA). Arising from the anterior tentorial arms (ATA) are flattened projections called laminatentoria (Lawrence et al. 2010), a term sometimes used in the singular (laminatentorium) for the fused condition (e.g., Stickney 1923; Naomi 1987). The posterior tentorial arms can be fused with the gular ridges to build a wall-like structure. Posteriorly, the ridges are connected by the tentorial bridge (Weide and Betz 2009; Weide et al. 2010; corpotentorium of Stickney 1923; Naomi 1987; Lawrence et al. 2010).

Reconstructions of the tentoria of the investigated species together with their positions within the head capsule are shown in Figs 2-6. For a complete overview of their 3D structure, animations of the tentoria are provided in the supplemental electronic material (Data S1 - movies 1–19).

The tentorium (lilac in Fig. 1) is the most posterior structure of the inner head skeleton; it provides for attachment of muscles and generally stabilizes the head capsule.

Non-staphylinid outgroups

In Necrophilus (Fig. 2A–C), the tentorium (Tent) differs slightly from the staphylinoid groundplan. The unfused laminatentoria (ULT; called ‘pedestal-like process’ in Weide and Betz 2009) do not meet medially. The dorsal tentorial arms (DTA) are indistinct outgrowths of the ATA. The PTA are fused with the gular ridges, posterior tentorial walls (PTW) are well-developed, and the PTW are connected by a massive tentorial bridge (TB). In Agathidium (Fig. 2D–F), the tentorium (Tent) shows a massive laminatentorium (LT) that bears a frontally directed medial vertical lamella (MVL); no DTA are present. In comparison with Necrophilus, the PTA extend further anteriorly. Also, the TB is not as massive as in Necrophilus.

Omaliine group

Lesteva (Fig. 2G–I) has ULT similar to Necrophilus. All tentorial arms (ATA, DTA, PTA) are present, although the PTA do not extend so far anteriorly as in Necrophilus and Agathidium. The PTW are connected via a TB. The median part of the TB is curved anteriorly, whereas its lateral parts are curved posteriorly. Omalium (Fig. 2J–L) has a massive LT with a frontally directed MVL. The ATA are lacking, and only the DTA and the PTA are well developed. The PTW and the TB are present, the latter being similar to that of Lesteva, though less robust.

Oxyteline group

In Anotylus (Fig. 3A–C), the Tent differs in many aspects from the groundplan. It shows a massive LT with an anteriorly directed MVL. While ATA are lacking, DTA are present. The small PTA are fused to each other and form an anteriorly directed unpaired projection. The PTW are fused at their ventral base; posteriorly, they are linked via a W-shaped TB.

Tachyporine group

Like the outgroup Necrophilus and the omaliine Lesteva, Tachyporus (Fig. 3D–F) has ULT (called ‘shovel-like process’ in Weide and Betz 2009). Otherwise, the tentorium in Tachyporus is similar to the groundplan, that is, all the tentorial arms are present (ATA, DTA, PTA). The tentorial arms and PTW are more weakly developed than in all preceding species, whereas the TB appears robust.

The tentoria of all the aleocharine species investigated (Figs 3G–6I) show a compact LT. Most (exception: Eumicrota, Fig. 4H,I) have a MVL. All studied aleocharine species lack DTA, but possess ATA and PTA, and the PTW are low compared to the outgroup taxa. In all but two aleocharine species, the PTW are spanned by a delicate W-shaped TB; the exceptions are Aleochara, where the bridge is incomplete (Fig. 3H,I), and Eumicrota (Fig. 4H,I), where the bridge is a very shallow arc.

Internal sclerites of the hypopharynx–prementum complex

For the first time in Coleoptera, the internal sclerites associated with the hypopharynx–prementum complex are analyzed in detail here, based on three-dimensional visualizations. An overview of the shape and arrangement of the internal sclerite(s) associated with the hypopharynx–prementum complex is shown in the leftmost images in Figs 2-6; detailed illustrations are given in Figs 7-11. Animations are presented as supplemental material (Data S1 - movies 1–19) to further clarify the 3D morphology of the internal sclerites and their relation to the tentorium.

In most cases, the internal sclerites are clearly divided into two parts (Scl1 and Scl2). The premental sclerite (Scl1, turquoise in Fig. 1, gray in Figs 7-11) is usually located more anteriorly, and the posterior hypopharyngeal sclerite (Scl2, blue in Fig. 1, white in Figs 7-11) is associated with the hypopharyngeal region (Figs 2-6 left images, 7-11). The premental sclerite (Scl1) is often more or less integrated into the premental wall or the region where the labial palpi originate. In some species (Omalium, Anotylus, and Tachyporus), sclerites Scl1 and Scl2 are fused via the posteriorly directed process ‘g’ and in Agathidium via the process ‘h’, thus forming a single sclerite (Scl1+2: Figs 2D,J, 3A,D). In the following, we put the recognizable subparts (gray or white in Figs 7D–F, J–L and 8A–F) in quotation marks, calling them ‘Scl1’ and ‘Scl2’ to distinguish them from separate Scl1 and Scl2 sclerites (i.e., without quotation marks). In the following descriptions, we use consistent labels for the parts of these sclerites that we consider homologous according to positional and structural criteria (Remane 1952).

The premental sclerite or premental part of the fused sclerite (Scl 1 or ‘Scl1’, respectively) is usually more complex than the hypopharyngeal one. Each outgroup species investigated has a different form, though with some underlying common features. Among the aleocharine species studied, there are strong similarities with respect to Scl1, except for dramatic reduction in minute species like Oligota and Eumicrota.

The hypopharyngeal sclerite or hypopharyngeal part of the fused sclerite (Scl 2 or ‘Scl2’, respectively), in both outgroup and ingroup species, in most cases consists of the following elements: an anterior (d1) and/or posterior (d2) bridge, one pair of ventrally directed (a), one pair of dorsoposteriorly directed (b), and one pair of anteriorly directed projections (c). Although its structure is quite variable among the outgroup species, there are strong similarities among the investigated aleocharine species.

Non-Staphylinidae outgroups

The premental sclerite (Scl1 in Fig. 2A) of Necrophilus (Agyrtidae) consists of paired structures anterolateral to the hypopharyngeal sclerite (Scl2). Scl2 (Figs 2A, white in 7A–C) forms a broad shelf-like area (d1 in Fig. 7A–C). From this area, two columns (a) branch out broadly ventrad and two additional projections (b) extend posterodorsad from the posterolateral edges of d1.

In Agathidium (Leiodidae), the single sclerite (Scl1+2 in Fig. 2D) is clearly divided (Fig. 7D–F) into an anterior (gray, f–h) and a posterior (white, a–d1, d2) part. ‘Scl1’ consists of a plate-like structure (h) that anteriorly bears an unpaired process (f). Dorsally f bears two short anterolaterally directed appendages, and posterolaterally, it bears a tube-like process on each side (g). However, as in this case, the fusion between ‘Scl1’ and ‘Scl2’ is not mediated by the process ‘g’ (as in all other species with fused condition of Scl1+2), but by the process ‘h’ (Fig. 7D–F), this fused state cannot be homologized with the fused condition in the other species (cf. Tables 3 and 4).

Two pairs of projections arise from this composite plate: one extending anteroventrally (a), the other dorsoposteriorly (b). The bridges are flanked by a strut-like structure (c) that connects d1 and d2 laterally.

Omaliine group

In Lesteva, the sclerite of the prementum (Scl1 in Fig. 2G, gray in Fig. 7G–I) forms an anteromedial plough-like unpaired process (f), which arises from a platform-like base branching out dorsolaterally at both sides (g). These sides broaden and are tilted toward the mediosagittal plane at an angle of 45°. The hypopharyngeal sclerite (Scl2 in Fig. 2G, white in Fig. 7G–I) is roughly rectangular and more planar than in all other taxa, with a circular opening in its center. Its anterior region is flat and bridge-like (d1); posteriorly, a second bridge-like structure (d2) bulges medioposteriorly. Bridge d2 has two dorsally directed projections (b) at its posterior end.

Omalium has a single sclerite (Scl1+2 in Fig. 2J), with a plate-like structure (h in Fig. 7K–L) at the posterodorsal end of ‘Scl1’. Distally, ‘Scl1’ bears a plough-like unpaired process (f in Fig. 7J–L) that is fused posteriorly to h and flanked laterally by two sickle-shaped processes (i). Each of these projections (i) continues posteriorly into a process (g) that extends posterodorsally and connects ‘Scl1’ to ‘Scl2’. The latter appears wedge-shaped in lateral view (Fig. 7J), and its sides are linked anteriorly via a bridge-like connection (d1) that sends out paired ventrally directed, posteriorly directed, and anteriorly directed processes (a, b, and c, respectively).

Oxyteline group

The premental part of Scl1+2 of Anotylus (Fig. 3A; gray in Fig. 8A–C) consists of an unpaired slender bar (f) that bears two unpaired processes at its tip, one directed anterodorsally and one anteroventrally, and also sends out a short dorsal projection (not visible in Fig. 8). At the front of the hypopharyngeal sclerite ‘Scl2’ (white in Fig. 8A–C), the posterior end of the premental sclerite ‘Scl1’ splits into two posteriorly directed arms that merge with a distinct plate-like region (d1) that we interpret as belonging to the hypopharyngeal sclerite. A prominent lateral process that extends far anteriorly (g), and seemingly belongs to the premental sclerite, merges posteriorly with the hypopharyngeal sclerite. This process (g) sends off an anterodorsally directed outgrowth (i) further distally. The plate-like region of the hypopharyngeal sclerite merges posteriorly into a bridge-like region (d1), from whose posterior margin arise two ventrally directed and two posterodorsally directed processes (a and b, respectively).

Tachyporine group

Tachyporus has a single sclerite (Scl1+2 in Fig. 3D, Fig. 8D–F). ‘Scl1’ has a plate (h) that sends out an anteriorly directed laminar process (f) and paired anteriorly directed processes (i). From the lateral base of i, paired appendages (g) extend posteroventrally and further posteriorly connect to ‘Scl2’. ‘Scl2’ has a distinct plate anterior to a bridge-like region (d1) at whose posterior end there is a circular opening bounded posteriorly by a second bridge-like structure (d2). At the posterior edges of d2 are paired projections extending almost directly ventrad (a) and dorsad (b).

In the Aleocharinae studied (Figs 3G,J, 4A,D,G,J, 5A,D,G,J, 6A,D,G, 8G–L, 9, 10, 11), the premental sclerite (Scl1) is composed of a plate-like ventral region (p) with two anteriorly oriented projections (n). Often, the lateral margins of p are cambered upward, and a process can be present on each side at the anterior (o) and/or posterior region (m). In Pagla, the upward-cambering lateral margins are fused dorsally, forming a tube-like structure (Fig. 10H), and in nearly all others there is at least a hint of such a tubular form (Figs 8H,K, 9E,K, 10B,E,K, 11B,E,H). At the posterior end of the plate-like region, two usually long lateral projections (r) arise and extend further posteriorly. The hypopharyngeal sclerite (Scl2) of Aleocharinae is quite variable in form, consisting of either a pair of large structures or a single structure with its sides united by an anterior (d1) or posterior (d2) bridge; in either case, ventral (a), dorsal (b), and anterior (c) processes are present on each side.

Aleochara (Fig. 3G, 8G–I) has a relatively slender typical aleocharine premental sclerite (Scl1 in Fig. 3G, gray in Fig. 8G–I), consisting of a plate-like ventral region (p) with two anteriorly oriented processes (n). The lateral margins of p are cambered upward and form short projections anteriorly (o) and longer projections posteriorly (m). From p, there arise two pairs of posteriorly oriented processes, of which the dorsal (r) extends much further posteriorly than the ventral (not labeled). The hypopharyngeal sclerite (Scl2 in Fig. 3G, white in Fig. 8G–I) is divided into two parts lying lateral to the premental sclerite (Scl1), each of which has a lateral column (a) that dorsally bears extensions anteriorly (c) and posteriorly (b) and medially bears possible remnants or rudiments of bridges d1 and d2 (not labeled).

The premental sclerite in Autalia (Scl1 in Fig. 3J) is much more robust than, but otherwise similar to, that of Aleochara. Its plate-like ventral region (p in Fig. 8K,L) is more extensive, the anterior processes (n) are relatively shorter, and the posterior processes (r) are much shorter. The hypopharyngeal sclerite (Scl2) is more robust than that of Aleochara and has a posterior dorsal bridge (d2) and lateral columns (a) that extend dorsally into anteriorly directed processes (c) only.

Oligota shows a highly reduced premental sclerite (Scl1 in Fig. 4A), consisting only of two bars (r in Fig. 9A–C) that are connected to the lateral wall of the prementum. As in the other aleocharines, the hypopharyngeal sclerite (Scl2 in Fig. 4A) consists of an anterior bridge (d1) with sturdy lateral columns (a) with projections (b) at their posterior margins.

In Homalota, the plate-like ventral region (p) of the premental sclerite (Scl1 in Fig. 4D) shows anteriorly oriented processes (n in Fig. 9D–F) that merge at their anterior ends. There are posteriorly oriented projections (m) at the dorsolateral margin of the upward-cambered lateral walls of p. The hypopharyngeal sclerite (Scl2 in Fig. 4D) possesses an anterior dorsal bridge-like structure (d1) with broad lateral columns (a). Two pairs of processes are present at the dorsal anterior (c) and posterior (b) margins of a.

Eumicrota, like Oligota, has Scl1 consisting of only a (shorter) pair of sclerites (r in Fig. 9G–I) connected to the premental wall. Posteriorly, the hypopharyngeal sclerite (Scl2 in Fig. 4G) has a dorsal bridge-like element (d2) with lateral supporting columns (a). These columns have anteriorly (c) and posteriorly (b) oriented processes.

In Gyrophaena fasciata, the premental sclerite (Scl1 in Fig. 4J, gray in Fig. 9J–L) consists of a central plate-like structure (p). In the medial plane of p, there are anterior processes (n) that branch distally. The lateral parts of p are cambered upward to form shovel-like dorsolateral extensions (o). Posterolaterally, p extends into long bars (r) that cross posteriorly without fusing. The hypopharyngeal sclerite (Scl2 in Fig. 4J, white in Fig. 9J–L) is a shelf-like structure (d2) whose lateral columns (a) each bear an anteriorly directed process (c) dorsally.

The premental sclerite in G. gentilis (Scl1 in Fig. 5A, gray in Fig. 10A–C) is very similar to that of G. fasciata, although the dorsolateral extensions (o) are less distinct. The hypopharyngeal sclerite (Scl2 in Fig. 5A, white in Fig. 10A–C) is also very similar, differing only in the lateral supporting columns (a) extending further anteriorly at their ventral base.

In Stictalia (Figs 5D and 10D–F), the premental sclerite (Scl1) is similar to those described previously, with parts n, o, p, and r. The hypopharyngeal sclerite (Scl2 in Fig. 5D, white in Fig. 10D–F) is also generally similar to those of the aleocharines described previously, although lacking a bridge connecting the sides. Each half of Scl2 consists of a stout lateral column (a) that dorsally bears posterodorsal (b) and short anterior (c) processes and medially possible remnants or rudiments of bridges d1 and d2 (not labeled).

Pagla has a premental sclerite (Scl1 in Fig. 5G, gray in Fig. 10G–I) with characteristic structures of aleocharines (n, p, r), but the anterior parts of the posterior processes (r) extend dorsad and fuse into a tube-like structure that protrudes posteriorly as two short projections (t). The hypopharyngeal sclerite (Scl2 in Fig. 5G, white in Fig. 10G–I) is highly reduced, consisting of paired structures dorsolateral to Scl1, each consisting of three processes: a short ventrally directed one (a), an anteriorly directed one (c), and a dorsally projecting one (b).

In Atheta (Figs 5J and 10J–L), the premental sclerite (Scl1 in Fig. 5J, gray in Fig. 10J–L) is much like that of Pagla, except that the posteriorly directed lateral bars (r) are not fused dorsally and the projections (t) are lacking. The hypopharyngeal sclerite (Scl2 in Fig. 5J, white in Fig. 10J–L) consists of a pair of structures flanking the anterior region of Scl1. Each structure has a lateral column (a) that posterodorsally has a process extending steeply dorsad (b) and anterodorsally extends into a small process (c).

The premental sclerite of Pontomalota (Scl1 in Fig. 6A, gray in Fig. 11A–C) is robust, with elements n, o, p, and r present. The hypopharyngeal sclerite (Scl2 in Fig. 6A, white in Fig. 11A–C), in contrast, is relatively reduced, consisting of two parts posterolateral to Scl1, each consisting of a lateral column (a) with a prominent posterodorsal dorsal process (b).

In Oxypoda, the premental sclerite (Scl1 in Fig. 6D, gray in 11D–F) consists of the elements n, o, p, and r. The divided hypopharyngeal sclerite (Scl2 in Fig. 6D, white in Fig. 11D–F) is again somewhat reduced, with a lateral column (a) having an anteriorly (c) and a short posteriorly oriented process (b) at its antero- and posterodorsal margins, respectively.

In Polylobus, the premental sclerite (Scl1 in Fig. 6G, gray in Fig. 11G–I) is very similar to those of Pontomalota and Stictalia. The divided hypopharyngeal sclerite (Scl2 in Fig. 6G, white in Fig. 11G–I) is also similar to those of Pontomalota and Stictalia, consisting of a lateral column (a) with a steeply dorsally extending projection (b) posterodorsally and a distinct anteriorly directed process (c) anterodorsally.

Character mapping analysis

Based on our tree-mapping analysis of the tentorial characters (Tables 3 and 4, Fig. 12), in the following section, we present tentorial characters that vary across the investigated staphylinids to infer tentative groundplan states for Aleocharinae.

image

Figure 12. Character mapping of parts of the tentorium and the hypopharynx, internal sclerite condition, mandibular pseudomolae, and feeding styles, using the data matrix shown in Table 4. The characters are mapped on a phylogenetic schema based on Fig. 21 in Betz et al. (2003) that shows the relative relationships of the species under study. Groundplan states of Staphylinoidea for characters that vary are shown at the base of the tree; posterior tentorial arms and posterior tentorial walls are present throughout. Abbreviations: -‒, non-spore feeder; ~, facultative spore feeder; +, obligate spore feeder; ?, equivocal state of character; numerals are character states defined in Table 3.

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Anterior tentorial arms

The ATA probably also belong to the groundplan of Staphylinoidea, as they are present in all the species studied except Omalium (Omaliini; characteristic of that tribe, Thayer 1992) and Anotylus (Oxytelini) (Tables 3 and 4, Fig. 12). In the species investigated in this study, the ATA (if present) are fused to the head capsule at the lateral area of the antennal base, as noted by Blackwelder (1936) and Naomi (1987). There are no anterior tentorial pits visible externally in the studied Staphylinidae, in accordance with Naomi (1987).

Dorsal tentorial arms

Our analysis suggests that the DTA are part of the Staphylinoidea groundplan (Tables 3 and 4, Fig. 12). Although the DTA are present in the groundplan of adult Coleoptera (Beutel 1997), it is known that they have been reduced (to varying degrees) several times independently in insects (Snodgrass 1993). Naomi (1987) suggested the absence of DTA in Aleocharinae as a possible autapomorphy, which our study supports, since all investigated aleocharine species lack DTA. This character was not mentioned by Ashe (2005) nor Thayer (2005). DTA are also missing in the leiodid Agathidium, which might be of phylogenetic significance within Leiodidae (Newton 2005).

Laminatentorium

All staphylinoid species studied have a LT (Tables 3 and 4), as in the groundplan of adult Coleoptera (Beutel 1997, 2003). In most cases, the LT is built up by the ATA that merge into a transverse bar-like structure (e.g., Fig. 2F; Tables 2 and 3). This fused structure seems to represent a groundplan feature of Staphylinoidea (Fig. 12: LT2).

The unfused condition in Necrophilus, Lesteva, and Tachyporus thus appears to be secondary (Fig. 12: LT1).

Posterior tentorial arms

The PTA are present in all species studied. Their presence is considered to be a groundplan character in Coleoptera (Beutel 1997) and apparently also is for Staphylinoidea.

Posterior tentorial walls

The PTW are present in all the species studied and probably are a groundplan feature of Staphylinoidea, as for Coleoptera (Beutel 1997). They vary with respect to dorsoventral extension (i.e., height) among the species studied. They are substantially lower in comparison with the depth of the head in Lesteva, Omalium, and all the aleocharines investigated. As this character state is a quantitative one, we refrained from assigning discrete coding for it.

Tentorial bridge

Our analysis confirms that the TB is present in all Staphylinoidea studied (Tables 3 and 4, Fig. 12) and should therefore be regarded as a groundplan character. The TB is usually located in the posterior third of the PTW. The uniquely incomplete TB in Aleochara (Fig. 3I) seems to be secondary (Fig. 12).

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

Weide and Betz (2009) and Weide et al. (2010) provided comparative datasets on the head anatomy of selected adult staphylinid species. These studies evaluated both the muscle configuration and the general organization and extent of the hypopharynx–prementum complex with regard to sporophagous and non-sporophagous members of the Staphylinidae. Groundplan features of the head muscles of Staphylinoidea were proposed in Weide and Betz (2009). Within Staphylinidae, the absence of the retractor of the hypopharynx, M. tentoriobuccalis anterior (M. 48; cited as M. 42 by Weide and Betz 2009), was proposed as a possible autapomorphy of Aleocharinae or a subgroup thereof. The same holds for locations of several muscle origins (Mm. 1, 2, 17, 18, 28, 29, 30; see Table 5 for muscle names). Within Aleocharinae, the origins of the labial muscles (Mm. 28–30) have shifted posteriorly to the gula, which might enhance the retractability of the hypopharynx and partly compensate for the loss of M. 48. In several Aleocharinae investigated, a dorsal elevation of the hypopharynx (cf. Betz et al. 2003; Weide et al. 2010) connected with a strong retractability of the prementum appears to be the main factor enabling the mortar-like processing of spore material between the ventral mandibular pseudomola and the dorsal side of the hypopharynx (Weide et al. 2010). However, close examination of the hypopharynx–prementum complex and its associated muscles showed no obvious characteristics separating species with different feeding habits, especially spore- versus non-spore feeders (Weide et al. 2010).

Table 5. Muscles cited in text
(Abbreviated) name of muscle after von Kelér 1963Abbreviated name of muscle after Wipfler et al. 2011
(M. 1)M. tentorioscapalis anterior0an1
(M. 2)M. tentorioscapalis posterior0an2
(M. 4)M. tentorioscapalis medialis0an4
(M. 11)M. craniomandibularis internus0md1
(M. 17)M. tentoriocardinalis0mx3
(M. 18)M. tentoriostipitalis0mx4, 0mx5
(M. 19)M. craniolacinialis0mx2
(M. 28)M. submentopraementalis0la8
(M. 29)M. tentoriopraementalis inferior0la5
(M. 30)M. tentoriopraementalis superior0la6
(M. 34)M. prementopalpalis externus0la14
(M. 37)M. hypopharyngosalivarialisa0hy12
(M. 38)M. praementosalivarialis anteriora0hy7
(M. 39)M. praementosalivarialis posteriora0hy8
(M. 40)M. anularis salivariia0hy13
(M. 41)M. frontohypopharyngalis0hy1
(M. 42)M. tentoriohypopharyngalis0hy3
(M. 48)M. tentoriobuccalis anteriorb0bu5
(M. 50)M. tentoriobuccalis posterior0bu6
(Abbreviated) name of muscle after e.g., Weide and Betz 2009Abbreviated name of muscle after Wipfler et al. 2011
  1. a

    Absent in Staphylinoidea studied.

  2. b

    Not present in Aleocharinae, erroneously cited as M. 42 in some previous works (e.g., Anton and Beutel 2004; Weide and Betz 2009) and corrected in Beutel et al. (2008) and Weide et al. (2010). Abbreviation: –, not cited by the author; M., musculus.

(M. U1)M. of unknown homology associated with antenna

Although the general structure of the tentorium is well known among insects, a thorough comparative analysis within Coleoptera or its subgroups is still lacking; a few tentorial characters were included in a recent morphological phylogenetic analysis of Coleoptera (Lawrence et al. 2011). Far less attention has been given to the more delicate and obscure hypopharyngeal and premental sclerites, which are expected to be functionally connected to the design of the hypopharynx–prementum complex of some mycophagous aleocharine species. The present study uses new techniques yielding 3D reconstructions to describe and visualize these structures for the first time for 19 species of Staphylinoidea, providing a first step toward better understanding their variation in Coleoptera.

In the following, we discuss aspects of the tentorium and the hypopharynx–prementum complex that vary across the investigated staphylinids and might be correlated with feeding mode. To assess that possibility, we performed a tree-mapping analysis of characters of these structures. For the hypopharynx–prementum complex, we discuss the reconstructed sclerite structures in terms of their variation between in- and outgroups and within Aleocharinae.

Tentorium

General functions of the tentorium include mechanical support of the head capsule (DuPorte 1957), stabilizing and protecting the supra- and subesophageal ganglia (Snodgrass 1993), supporting the pharynx dorsal to the tentorial bridge (Snodgrass 1993), and providing attachment areas for head muscles (von Kelér 1963; Snodgrass 1993).

In pterygote insects, the tentorium generally consists of paired anterior and posterior tentorial arms formed by invaginations of the head capsule marked externally by the anterior and posterior tentorial pits. The dorsal arms, in contrast, are considered to be secondary outgrowths of the anterior arms (Snodgrass 1993), not invaginations of the head capsule as suggested earlier (e.g., Crampton 1921). Across insects, the tentorium has undergone many alterations whose functional and phylogenetic significance are yet to be explored (Klass and Eulitz 2007; Zimmermann et al. 2011).

Beutel (1997) suggested that in the groundplan of adult Coleoptera, the tentorium consists of anterior, dorsal, and posterior arms and a laminatentorium; the results of Lawrence et al. (2011) suggest some ambiguity concerning the presence of the laminatentorium in the groundplan. For Staphylinoidea, Naomi (1987) defined the anterior tentorial arm as originating ‘near the inner margin of antennal fossa’, the dorsal tentorial arm as reaching the vertex, and the posterior tentorial arm as originating from the gular suture(s). Naomi (1987) defined posterior tentorial walls and used the term ‘corpotentorium’ for the connection between them. Blackwelder (1936) described the tentorium of Staphylinidae as consisting of anterior, dorsal and posterior tentorial arms, and body of the tentorium (apparently meaning the laminatentorium of Stickney 1923 and Naomi 1987). In the present study, we largely follow Naomi's nomenclature, except for calling the corpotentorium sensu Naomi (1987) the tentorial bridge, following Beutel (1997).

Functional aspects of the tentorium in Aleocharinae and its associated muscles

von Kelér (1963) listed ten antennal, maxillary, labial, hypopharyngeal, and pharyngeal muscles that originate from the tentorium in many pterygote insects. The full names of muscles (after von Kelér 1963) cited in the text are shown in Table 5. We observed all but one of these originating from the tentorium in one or another species of Staphylinidae (Weide and Betz 2009; Weide et al. 2010). The exception, M. 42, was not seen in any species examined and seems to be missing from the Staphylinoidea groundplan.

Anterior tentorial arms

In pterygote insects, antennal muscles Mm. 1 and 2, parts of mandibular muscle M. 11, and maxillary muscles Mm. 17 and 18 usually originate from the ATA (von Kelér 1963). We showed earlier that in Staphylinoidea, Mm. 1, 2, 4, U1, and (partly) 18 arise from the ATA (Weide and Betz 2009: tables 4 and 5; Weide et al. 2010: tables 2 and 3). In the Aleocharinae investigated there and here, M. 1 did not originate at the ATA in most species, but rather at the laminatentorium (LT) and its median vertical lamella (MVL) (Pontomalota), ATA and MVL (Autalia, Pagla, Polylobus, and Stictalia), or the MVL only (remaining taxa). In Pagla, Pontomalota, and Polylobus, M. 4 originates partly from ATA, whereas in the other taxa studied its origin is on the posterior tentorial wall (PTW) or the merging area of PTW and PTA (Weide and Betz 2009; Weide et al. 2010).

In Omalium and Anotylus, where the ATA are reduced, Mm. 1, 2, 4 have shifted their origins to the DTA (Weide and Betz 2009; wrong coding of Mm. 1, 2, and 4 for Anotylus in table 2 and fig. 8 of Weide et al. 2010, see corrections in the electronic appendix of this publication), and M. 18 originates from LT and its MVL.

Dorsal tentorial arms

In pterygote insects, the DTA, if present, can also be the origin of antennal muscles Mm. 1, 2, and 4 (von Kelér 1963) as in most of the outgroup species of this study, but in all Aleocharinae examined the DTA are absent. One can also find U1 (a muscle of unknown homology, associated with the antennae) arising from the DTA in Necrophilus (Agyrtidae) (Weide and Betz 2009; Weide et al. 2010).

In our studies, in the species that lack DTA (Agathidium and Aleocharinae), these muscles have shifted their origins to the ridge-like structure of the unfused LT (Agathidium) or MVL at the LT (in Aleocharinae), to the LT itself, to the ATA, or to regions at the PTA and the PTW (for details, see Weide and Betz 2009; Weide et al. 2010).

Laminatentorium

According to von Kelér (1963), in pterygote insects, the LT can be the region of origin of Mm. 1, 17, 18, and 48. In different aleocharine species studied, the LT plus its median vertical lamella (MVL) is the origin of (parts of) Mm. 1, U1, 17, 18, and 19 (tables 2 and 3 in Weide et al. 2010).

In all investigated species that possess a fused LT, one finds a structure called the ‘median vertical lamella of LT’ by Anton and Beutel (2004) and ‘ridge-like structure’ (RLS) in our earlier studies (Weide and Betz 2009; Weide et al. 2010). In this study, we follow Anton and Beutel (2004) and use median vertical lamella (MVL). This structure provides an attachment area for M. 18 in Helophorus spp. (Anton and Beutel 2004). We found a situation like that described by Anton and Beutel (2004): varying in size among species, the MVL (Figs 2E,F,K,L, 3B,C,H,I,K,L, 4B,C,E,F,K,L, 5B,C,E,F,H,I,K,L, 6B,C,E,F,H,I) functions in most species as (one) origin of M. 18 (Weide et al. 2010: tables 2 and 3: codes 2, 5–7; exceptions: Necrophilus, Lesteva, Tachyporus, Oligota, Eumicrota).

In species lacking a fused LT with a MVL, other (paired) structures are present (Weide and Betz 2009: pedestal-like processes, PLP, in Necrophilus, Lesteva; or Weide and Betz 2009: shovel-like processes, SLP, in Tachyporus). These formations seem to functionally replace a probably homologous fused LT plus its associated MVL, forming the attachment area of M. 18 (Weide and Betz 2009: electronic appendix). Generally, this muscle is of special importance for the process of feeding, as it moves the stipes posteriorly while pressing it against the lateral region of the hypopharynx (von Kelér 1963). Compared to the investigated outgroup taxa, the enlarged area of origin of this muscle in Aleocharinae could significantly increase its power output, which might be of special importance in aleocharine spore feeders that use the maxillae for sweeping in fine material such as spores from fungal hymenium (Betz et al. 2003).

According to von Kelér (1963), in many pterygote insects, the LT or the PTA also form the site of origin of M. 48, the retractor of the hypopharynx, which is part of the groundplan of both Coleoptera and Staphylinoidea (Weide and Betz 2009: cited as M. 42; Weide et al. 2010). Aleocharinae, however, lack this muscle (Weide and Betz 2009; Weide et al. 2010), which might be connected with the transformation of the hypopharynx–prementum complex into an elevated formation where the hypopharynx forms a ‘second layer’ dorsal to the prementum that interacts directly with the pseudomolae (secondary trituration surfaces; discussion in Weide and Betz 2009). Further arguments are presented in the section on the hypopharynx sclerites.

Posterior tentorial arms

In many pterygote insects, the PTA are the areas of origin of Mm. 1, 2, 4, 29, 30, 48, and 50 (von Kelér 1963). For M. 50, this point of origin is the only known position, and the PTA also seem to be the general origination site for Mm. 29 and 30 (von Kelér 1963). In Staphylinidae, the PTA usually function as the attachment site for (parts of) Mm. 2, 4, U1, 17, and 18 (Weide and Betz 2009; Weide et al. 2010). In the aleocharines Atheta, Oligota, Oxypoda, and Stictalia, at least parts of M. 2 originate at the PTA (Weide et al. 2010). In Autalia and Stictalia, M. 4 has its origin entirely from a region reaching from the fusion area of PTA to the posterior tentorial wall (PTW); in Pagla, Pontomalota, and Polylobus, only parts of this muscle originate at that region (Weide et al. 2010).

Posterior tentorial walls

In pterygote insects, antennal muscles Mm. 1, 2, 4, maxillary muscle M. 18, and labial muscle M. 29 can originate at the PTW (von Kelér 1963). According to our studies (Weide and Betz 2009; Weide et al. 2010), this is the case for Mm. 2 and 4 in several species of the tachyporine group, whereas only parts of M. 18 originate from a projection of the PTW in Tachyporus and only parts of M. 18 from the PTW in Gyrophaena gentilis. Parts of Mm. 17, 19, and 30 also arise from the PTW in some non-aleocharine staphylinoid species (Weide and Betz 2009; Weide et al. 2010).

Whereas M. 17 has shifted its origin to the posteroventral region of the head capsule in all Aleocharinae studied (Weide et al. 2010), in all outgroup species the PTW are the origin for only parts of this muscle (Weide and Betz 2009). Muscle M. 29 originates partly at PTW in Lesteva and from a process of the PTW in Agathidium and Tachyporus (Weide and Betz 2009). Both Mm. 29 and 30 shifted their origins from (parts of) PTW to the gula in all Aleocharinae studied except Pagla, where they originate from a bar-like connection between the gular ridges (Weide et al. 2010).

Tentorial bridge

von Kelér (1963) did not note any origin of muscles at the TB and in fact did not mention that structure. Beutel (1997) considered a fully developed TB a groundplan character of adult Coleoptera. According to our studies (Weide and Betz 2009; Weide et al. 2010), the TB forms the attachment site of M. 48 (cited as M. 42 in Weide and Betz 2009) and M. 50 in (at least) non-aleocharine Staphylinoidea. These muscles have shifted their origins from the LT (M. 48) or PTA (Mm. 48, 50) (von Kelér 1963) to the TB (Anton and Beutel 2004: M. 48 cited as M. 42; Beutel et al. 2008). In all aleocharine species studied, M. 48 is missing (Weide et al. 2010).

General description of the internal sclerites of the hypopharynx–prementum complex

Snodgrass (1993) presented an overview of the internal sclerites connected to the insect hypopharynx. His schema was extended by several authors such as Matsuda (1965) and Honomichl (1975). In Periplaneta americana (Blattodea), two so-called lingual sclerites support the lateral and ventral hypopharyngeal surfaces (Matsuda 1965: fig. 42). These sclerites merge proximally to form the fulcrum. In the more posterior region of the hypopharynx, two lateral linear sclerites (fused anteriorly) extend toward the mouth angles and form the hypopharyngeal suspensorium. At half of their linear extension, they give rise to ventrally directed processes named loral arms. A depression at the dorsal surface of the hypopharynx that supports the food processing toward the mouth is called the sitophore (e.g., Matsuda 1965). Jeannel (1911: 21–24; 1949: 781–782) figured and described the hypopharyngeal skeleton of a leiodid beetle (though not particularly close to Agathidium, studied here); the apparent correspondences of the Snodgrass/Matsuda terminology (1993/1965) with Jeannel's can be found in Table 6.

Table 6. Described areas of the hypopharyngeal sclerites mentioned in the text and their possible homologies
Properties in examined taxaTerminology after Jeannel (1911)Jeannel (1949)Matsuda (1965)Snodgrass (1993)
  1. Abbreviations: see Table 2. –, not shown by the author; ?, unclear.

a(unnamed process?)(unnamed process?)Loral armsBasal plate or bar
bLames poststomachiquesApophyses tergalesOral armsProximal ends of the suspensoria
image Trabécules internes SuspensoriumSuspensoria
d2Lames laterales
h?StylesTiges hypopharyngiennes

Matsuda (1965) referred to muscles in Periplaneta americana associated with the salivarium, which are often used as points of reference for the spatial arrangement of the hypopharyngeal sclerites, but most Coleoptera do not have a salivarium (Beutel 2003), and the muscles associated with it in other pterygote insects (Mm. 37–40; von Kelér 1963) are lacking in the coleopteran groundplan (Weide and Betz 2009).

In contrast to the hypopharyngeal sclerite, literature on the premental sclerites in insects is nonexistent, presumably because these structures are very delicate and easily overlooked using conventional techniques (e.g., clearing agents such as KOH). As they are often fused to the outer walls of the prementum itself, they are easily overlooked even by thin-sectioning.

As far as we know, this is the first comparative study of these structures, as they have not been systematically investigated in insects.

Our character-mapping analyses revealed the internal sclerite condition as an equivocal character state in the groundplan condition of both Staphylinoidea and Staphylinidae (SC? in Fig. 12). The division into two distinct premental and hypopharyngeal sclerites (SC3: Tables 3 and 4, Fig. 12) in Aleocharinae might form another synapomorphy for this taxon. The divided state also occurs, however, in Necrophilus and Lesteva, so the division is not unique to Aleocharinae, and further sampling is needed to clarify its distribution.

Hypopharyngeal sclerites

The hypopharyngeal sclerites in the species investigated are diverse, and reference points for assessing homology are arguable. Although it is somewhat speculative to homologize the sclerites described here with those of the very remotely related P. americana studied by Matsuda (1965), it seems more reliable to homologize sclerites among the species we studied (as labeled with identical small letters in Figs 7-11) using positional and structural criteria (Remane 1952).

Although hypopharyngeal sclerites have been described in some Coleoptera, as noted previously, they have not been described in detail nor discussed with respect to their possible functions or homologies to those of other insects, and there is currently no system of terminology applicable across Coleoptera. It is striking to see from our limited study how great the variation in these sclerites can be even among closely related taxa.

In our study, the hypopharyngeal sclerite is labeled Scl2 in Figs 2-6 (white in Figs 7-11) or is part of Scl1+2 when fused with the premental sclerite. Matsuda (1965) suggested the suspensoria might originally have been paired in insects, representing remnants of the superlinguae. In the present study, the unpaired bridges (d1, d2) plus the paired anterior processes (c) can be identified as together forming the suspensorium (Matsuda 1965; Honomichl 1975; Snodgrass 1993). Within the species studied, the anterior processes (c in Figs 7D–F,J–L, 8G–L, 9D–L, 10A–L, 11D–I) of the suspensoria on both sides seem to be fused to form bridge-like structure(s) (d1, d2 in Figs 7A–C,F,G–I,K,L, 8A–F,J–L, 9B,C,F,H–L, 10B,C) dorsally supporting the base of the hypopharynx, which may bear dorsally a sitophore (the trough-like channel that conveys the food toward the mouth; Gordh and Headrick 2001). This arrangement is similar to other Coleoptera such as Gyrinus substriatus (Honomichl 1975) or Coccinella septempunctata (Pradhan 1938).

The lateral supporting processes ‘a’ (Figs 7A–F,J–L, 8A–H,J,K, 9A,B,D,E,G,H,J,K, 10A,B,D,E,G–K, 11A–H) (‘suspensorial struts’ of Doyen 1966) extend laterally from both sides of the suspensorium in a dorsoventral (vertical) direction, resting ventrally on the mentum. It remains to be clarified whether these structures are homologous to the ‘loral arms’ sensu Matsuda (1965) and the ‘basal plates or bars’ sensu Snodgrass (1993). Alternatively, they might correspond to the lingual sclerite complex described by Matsuda (1965) to strengthen the lateral surface of the hypopharynx.

Snodgrass (1993) reported that the proximal ends of the suspensoria (i.e., the oral arms sensu Matsuda 1965) extend toward the mouth angles (sensu von Kelér 1963; i.e., at the dorsal base of the hypopharynx between the inner edges of the mandibles); these arms correspond with our structure ‘b’ in Figs 7A–L, 8A–I, 9A–I, 10D–L, 11A–I. According to Matsuda (1965), the oral arms can be defined by the attachment of the ‘frontal muscle’ that corresponds to M. 41 (von Kelér 1963; cf. Table 6). Muscle 41 is present in all species studied (Weide and Betz 2009; Weide et al. 2010); it is the levator of the hypopharyngeal base and the mouth angle, with its origin at the front. In the species studied, we did not detect any sclerotized formation like, for example, a boomerang-shaped supporting structure mentioned by Weinreich (1968), to which the muscle is attached. Its insertion in the examined species is directly at the (more or less membranous) mouth angles; we never observed this muscle to be attached to the oral arms (b, see above; Weide and Betz 2009; Weide et al. 2010).

Premental sclerites

The premental area of the sclerotized hypopharynx–prementum complex of the non-aleocharine outgroups can be homologized through several structural aspects. In Agathidium, Omalium, Anotylus, and Tachyporus, the premental and the hypopharyngeal sclerites are fused (Scl1+2), that is, the premental structures ‘g’ (Omalium, Anotylus, and Tachyporus: Figs 7J–L, 8A–F) or ‘h’ (Agathidium: Fig. 7D–F) connect the two sclerites. In species that show an unfused condition of Scl1 and Scl2, ‘g’ can also be present, but is not fused with the hypopharyngeal sclerite (Fig. 7A–C, G–I). Anteriorly, a plough-like unpaired projection (f in Figs 7D–L, 8A–F) is present. In some species, this structure is flanked by two sickle-shaped lateral processes (i in Figs 7J–L, 8A–F).

The premental sclerites of the Aleocharinae studied show strong overall structural similarity, but are not easily homologized with the structures described previously in the outgroups. Hence, we use different labels (m-p, r, t) for their description. There is a plate-like structure p (Figs 8H,I,K,L, 9E,F,K,L, 10B,C,E,F–I,K,L, 11B,C,E,F,H,I). This plate bears two anteriorly directed processes n (Figs 8G–L, 9D–F, J–L, 10A–L, 11A–I) and, in some species, two more dorsally oriented processes o (Figs 8G–L, 9J,K, 10A–E, 11A–H). In most species, two elongate posterior structures r (Figs 8G–L, 9A–L, 10A–J,L, 11A–I) are present. They often extend far posteriorly into the region of the hypopharynx.

Parts of these premental sclerites may support the origin of M. 34 (cf. Table 5: Musculus praementopalpalis externus). According to von Kelér (1963), this muscle, when present, originates from the prementum. Within the investigated outgroup species, M. 34 has different origins, but without detailed knowledge of the sclerites of the hypopharynx–prementum complex, the origin of M. 34 was unclear in our previous studies (e.g., sclerotized region/sclerotized processus in Weide and Betz 2009; Weide et al. 2010). We can now describe it more precisely: in all six non-aleocharine species, the muscle arises from the median side of ‘g’ (Figs 7A–L, 8A–F). In Aleocharinae, we earlier (Weide and Betz 2009; Weide et al. 2010) considered M. 34 to originate at a projection of the lateral premental wall or (Atheta, Gyrophaena fasciata) at a sclerotized structure ventral to the hypopharynx. The present study confirms this and shows more precisely that the process is part of the premental sclerite: M. 34 originates from the middle to posterior region of process r (Figs 8G–L, 9A–L, 10A–J,L, 11A–I).

Functional aspects of the hypopharyngeal and premental sclerites in Aleocharinae

Hypopharyngeal sclerite (Scl2)

From its spatial arrangement in relation to the head capsule, one can infer that the adult hypopharyngeal sclerite has a stabilizing function, supporting the hypopharynx ventrolaterally. This holds especially for the parts lying closely beneath the dorsal wall of the hypopharynx (Fig. 1, blue: hypopharyngeal sclerite). In microphagous aleocharines (e.g., those feeding on spores or pollen), this construction suggests a mortar-like function of the hypopharynx in interaction with other mouthparts. According to our earlier interpretation (Weide et al. 2010), the parts identified as suspensoria, namely suspensorial bridge(s) d1 and d2 (Figs 8J–L, 9B,C, F,H–L, 10B,C) and anterior elongations c (Figs 8G–L, 9D–L, 10A–L, 11D–I), serve as an abutment for the pseudomola, a secondary grinding structure on the ventral side of each mandible (psm in Weide et al. 2010: Figs 4F,O, 5C,F,I,L,O). The bowl-like depression of the hypopharynx (e.g., Weide et al. 2010; Fig. 5E), into which the fine particulate food material (spores or pollen) is concentrated (cf. Betz et al. 2003: Fig. 12 d,f) by the action of the maxillae, seems to be little sclerotized with a soft surface and is not supported by the sclerotized hypopharyngeal suspensorium; its lateral margins bear hair-like structures that appear to keep the food material in the center. It seems more likely that the actual grinding process takes place at the margins of this depression, as that area is reinforced from beneath by the anterior elongation of the suspensorium (c in Figs 8G–L, 9D–L, 10A–L, 11D–I). This hypothesis is supported by the position of the pseudomolae (when present) corresponding to this area (see Fig. 13E, showing the contiguity of the anterior elongation of the suspensorium (asterisks) and the pseudomolae (arrows) of the mandibles in the facultative spore feeder Homalota). The pseudomolae seem to shred the fine particulate material (e.g., soft spores or pollen) rather than fragmenting it with high vertical forces as in a mortar. After processing, the spores are transferred into the digestive tract via hypopharyngeal retraction by contraction of muscles 28, 29, and 30 (prementum retractors, cf. Table 5), whose origins have shifted further posterad to the gula and thereby increased the motility of the prementum (see discussion in Weide et al. 2010). The rake-like mesal margins of the pseudomolae and the rasp-like structures at the cibarial roof also canalize the food toward the mouth opening (see Betz et al. 2003: fig. 17E). All these structures and muscles are supported by M. 41 (cf. Table 5), which is the levator of the mouth angles and therefore indirectly moves the hypopharyngeal base.

image

Figure 13. Virtual transverse sections through the heads of four species investigated by synchrotron X-ray microtomography (SR-μCT). Region anterior to mandibular base and area of hypopharynx to show different character combinations (CC) (Tables 3 and 4). —A. CC 1 without pseudomolae, with suspensorium (asterisks) having anterior elongations but no suspensorial bridge, as found in Aleochara haematoptera (white in Fig. 88G\x96I), —B. CC 2 with both pseudomolae (arrows) and suspensorium (not shown) having anterior elongations but no suspensorial bridge, as found in Stictalia (white in Fig. 1010D\x96F), —C, D. CC 3 without pseudomolae (C) and with suspensorium having anterior suspensorial bridge (asterisks in D) but no anterior elongations, as found in Oligota (white in Fig. 99A\x96C), Fig. C lies further anterior compared to Fig. D, —E, F. CC 4 with both pseudomolae (arrows in E) and fully developed suspensorium (i.e., having both anterior suspensorial bridge and anterior elongations; asterisks in E and F), as found in Homalota (white in Fig. 9D–F), Fig. E lies further anterior compared to Fig. F. Scale bars = 100 μm.

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Whereas a ventral mandibular pseudomola seems to be restricted to (some) Aleocharinae, the stabilizing elements of the hypopharyngeal sclerites (c, d1, d2) are not—they are also present in non-aleocharine staphylinoids (Figs 7A–L, 8A–F), and further investigation of their role in the feeding process is needed.

Premental sclerite (Scl1)

The primary function of the premental sclerites seems to be stabilization of the prementum and to serve, for instance, as an attachment site for M. 34 (labial palp abductor muscle). Moreover, the sclerites within the prementum seem to facilitate the high retractability of the prementum back into the head. When the prementum is being moved in- and outward, this sclerite probably keeps it dimensionally stable and therefore assists the movements of the other participating mouthparts during the feeding process.

Another function of the premental sclerite is especially obvious in some species of the tribe Homalotini. According to our present analysis and two previous studies (Weide and Betz 2009; Weide et al. 2010), the dorsal elevation of the hypopharynx, which is especially well developed in spore-feeding members of the tribe Homalotini (Weide et al. 2010: figs 4M, 5A, D, G, J), facilitates movement of the dorsal surface of the hypopharynx toward the pseudomolae on the ventral sides of the mandibles. This movement may be supported by muscles Mm. 28–30, which move the prementum posteriorly so that it becomes sandwiched between the postmentum and the hypopharynx (e.g., Weide et al. 2010: fig. 4M). Whereas in staphylinids (or possibly staphylinoids) generally, the plesiomorphic origin of M. 29 is the posterior submentum (Weide et al. 2010) and for M. 30 it is ambiguous (Weide et al. 2010); in all the aleocharines studied, the origins of these muscles have shifted toward the ventral side of the head capsule (gula, except for Pagla, see Weide et al. 2010). This shift might enhance the retraction of the prementum and thus partly compensate for the loss of M. 48 in Aleocharinae. Previous studies (Weide and Betz 2009; Weide et al. 2010) found that the origin of M. 50 is the tentorial bridge in all staphylinoid species examined; this muscle may be able to perform an intense retraction of the hypopharynx, and in Aleocharinae, it may thus take over the function of the absent M. 48 (see discussion in Weide et al. 2010). Additionally, M. 41 seems to sustain the approximation of the hypopharyngeal region to the pseudomolae by lifting the mouth opening dorsad. Both these movements lift the hypopharynx upward and decrease the space between the mandibles and the hypopharyngeal surface, so that fine particulate food material such as spores can be more easily processed by the pseudomolae before being moved onward into the digestive tract.

Phylogenetic context of feeding structures associated with the hypopharynx and the mandibles

In the Aleocharinae studied, we found six character combinations (CC) of presence or absence of a ventral pseudomola (Weide et al. 2010) and presence or absence of some parts of the suspensorium: bridges d1, d2, and anterior elongations c (Tables 3, 4, 7, Fig. 13: CC 1–CC 6), out of a theoretically possible 16 combinations. Those found were:

Table 7. Character combinations and feeding strategies of the species under study
 FS 1: non-spore feederFS 2: obligate spore (or pollen) feederFS 3: facultative spore feeder
  1. Abbreviations: see Table 2. For meaning of character combinations CC1-CC9, see Table 3. Species of Aleocharinae are written in boldface. Underlined species have anterior elongations of suspensoria.

CC 1 Aleochara haematoptera

Oxypoda alternans

Pagla sp. (pollen)

Polylobus sp. (pollen)

 
CC 2 Atheta laticollis   Stictalia sp.
CC 3

Necrophilus subterraneus

Anotylus sculpturatus

Oligota parva

  
CC 4   Homalota sp.
CC 5 Autalia impressa

Eumicrota sp.

Gyrophaena fasciata

Gyrophaena gentilis

 
CC 6 Pontomalota opaca   
CC 7  Agathidium mandibulare  
CC 8

Lesteva longoelytrata

Tachyporus chrysomelinus

  
CC 9 Omalium rivulare   

CC 1 (Fig. 13A, Table 7): Pseudomolae lacking, suspensorium lacking a bridge (d1, d2) but with anterior elongations (c; Tables 3 and 4). CC 1 was seen in both non-spore feeders (Aleochara, Aleocharini, Fig. 8G–I) and obligate spore feeders (Pagla, Paglini; Oxypoda and Polylobus, Oxypodini, Figs 10G–I and 11G–I), belonging to three different tribes (Table 1). For the spore feeders, there is no information available on the extent to which the ingested spores are comminuted before passing into the digestive tract in the absence of pseudomolar grinding structures.

CC 2 (Fig. 13B, Table 7): Pseudomolae present, suspensorium lacking a bridge (d1, d2) but with anterior elongations (c; not visible in Fig. 13B). CC 2 was found in Atheta (Athetini, non-spore feeder, Fig. 10J–L) and Stictalia (Homalotini, facultative spore feeder, Fig. 10D–F) (Tables 3 and 4). As a facultative spore feeder with a pseudomola (Table 4), Stictalia might represent a functionally more derived situation than CC 1.

CC 3 (Fig. 13C,D, Table 7): Pseudomolae absent (Fig. 13C), suspensorium with anterior bridge (d1, asterisks in Fig. 13D), but without anterior elongations (c). CC 3, seen only in Oligota (Hypocyphtini, Fig. 9A–C), may reflect the unique feeding habits of the representatives of that genus—predation on spider mites (Shimoda et al. 1997; Atanasov 1998)—compared to all other species studied (Tables 3 and 4).

CC 4 (Fig. 13E,F, Table 7): Pseudomolae (arrows in Fig. 13E), anterior suspensorial bridge (d1; asterisks in Fig. 13F), and anterior elongations of the suspensorium (c) present. CC 4 was seen only in Homalota (Homalotini, Fig. 9D–F), a facultative spore feeder (Tables 3 and 4).

CC 5 (Table 7): Pseudomolae, posterior suspensorial bridge (d2), and anterior elongations of the suspensorium (c) present. CC 5 was shared by the three studied Homalotini: Gyrophaenina (Eumicrota, Gyrophaena fasciata, and G. gentilis, Figs 9G–L, 10A–C), all obligate sporophages, but (as for CC 1) a non-spore feeder, Autalia (Autaliini), also has this combination (Tables 3 and 4).

CC 6 (Table 7): Pseudomolae, suspensorial bridges (d1, d2), and anterior suspensorial elongations (c) lacking. This was found only in Pontomalota (Athetini; non-spore feeder, Tables 3 and 4, Fig. 11A–C).

CC 7–CC 9 (Table 7) were found only within the outgroups and therefore are not discussed in detail here (Tables 3 and 4).

Our analysis shows that there are few clear-cut correlations between these character combinations and feeding types. The absence of anterior suspensorial elongations (Susp c in Fig. 12) is the plesiomorphic character state in the Staphylinoidea studied. The plesiomorphic state of the suspensorial bridge(s) is unclear (Susp d in Fig. 12). Susp c has evolved at least three times independently within Staphylinoidea and might form an autapomorphy for Aleocharinae (Fig. 12), though lost in two of our study taxa. Notably in spore feeders, the suspensorial elongations placed ventral to the hypopharynx can be functionally interpreted to be an abutment to the pseudomola. Whereas the absence of suspensorial bridges might form an autapomorphy for Aleocharinae (Fig. 12), in obligate spore feeders within the tribe Homalotini (i.e., subtribe Gyrophaenina), the hypopharyngeal region is stabilized by the posterior suspensorial bridge (Fig. 12: Susp d2), also present in non-spore-feeding Autalia.

In obligate spore feeders among Aleocharinae, we found two alternative character combinations (Table 7): (CC 1) suspensoria with anterior elongations (c) but without a bridge (d1, d2), plus mandibles without pseudomolar grinding structures (Fig. 13A) and (CC 5) well-developed suspensoria (with both bridge(s) and anterior elongations) interacting with the ventral surfaces of mandibles that show well-differentiated pseudomolae. These structural differences, reinforced by their distribution on the phylogenetic schema (Fig. 12), suggest at least three separate origins of obligate sporophagy in Aleocharinae: within Oxypodini (not all Oxypodini are obligate sporophages), in Gyrophaenina (tribe Homalotini), and in Paglini. Additionally, our analysis suggests that within the tribe Homalotini, obligate spore feeding evolved from facultative spore feeding, in contrast to two origins of obligate spore feeding from non-spore feeding (Tables 3 and 4, Fig. 12).

It is intriguing that among the obligate sporophages in Aleocharinae (Table 7), (1) two of the three lacking the suspensorial bridge and showing anterior elongations are pollen feeders (Pagla and Polylobus, Thayer 2005) rather than fungal spore feeders, and (2) all three species having the suspensorial bridge and elongations (Eumicrota, Gyrophaena spp.) are closely related fungal spore feeders (Ashe 1993). The obligate pollen feeders Pagla and Polylobus also share a unique pair of changes in muscle attachments (see Weide et al. 2010: Fig. 8: M. 1,3 to 1,5 and M. 4,3 to 4,4). These spore- (sensu stricto) and pollen-feeding lineages presumably—with different hypopharyngeal structures—process their different microparticulate foods in different ways, and behavioral investigations are needed to characterize the functional differences. Understanding how their feeding habits evolved also requires new information about the identity and habits of their sister groups.

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

A total of nineteen species of staphylinoid beetles, comprising selected spore- and non-spore-feeding Aleocharinae and outgroups, were investigated comparatively with respect to their inner head skeletons, that is, tentoria and the sclerites associated with the hypopharynx and prementum. Although the general tentorial morphology of the species studied agrees with the groundplan of adult Coleoptera (Beutel 1997), we found conspicuous variations in single characters in several species (Fig. 12), for example, the DTA having been independently reduced in Agathidium (Leiodidae) and all Aleocharinae studied.

The muscles that originate on the tentorium, as described by von Kelér (1963) for pterygote insects, are also found in the species studied. Within the non-aleocharine outgroups, antennal muscles Mm. 1, 2, and 4, maxillary muscles Mm. 17–19 (M. 19 not from the tentorium according to von Kelér 1963), labial muscles Mm. 29 and 30, and pharyngeal muscles Mm. 48 and 50 can originate from the inner head skeleton (Weide and Betz 2009; Weide et al. 2010). In Aleocharinae, Mm. 1, 2, 4, 18, and 50 likewise originate from the tentorium, but M. 19 arises from the ventral posterior part of the head capsule, and Mm. 29 and 30 from various parts of the gula (Weide et al. 2010). Muscle U1 (if present) also has its origin at the tentorium (most often at the ATA, rarely at DTA or the merging area of LT and ATA/PTA) in the staphylinoid species studied (Weide and Betz 2009; Weide et al. 2010). The established tentorial differences in our analysis were not correlated with spore- or non-spore feeding and may be more closely connected to the phylogenetic histories of the investigated taxa.

Some parts of the sclerites associated with the hypopharynx can be homologized with structures mentioned in broader insect morphology literature, that is, the loral arms, oral arms, and suspensorium. The last of these seems to play a role in processing food particles, as it supports the dorsal hypopharynx in interacting with the pseudomola (a ventral grinding surface of the mandibles) to break up fine particulate material such as spores or pollen. Aside from secondary reductions (e.g., Oligota, Eumicrota), Aleocharinae have a characteristic structure of the premental sclerites, which are variations on an apparent aleocharine groundplan distinct from the conditions in the outgroups. In contrast, the hypopharyngeal sclerites vary widely in the Aleocharinae studied. In general, they are built up from similar components, but in the majority (seven in thirteen) connecting structure(s) (d1 and/or d2) are lacking. In the investigated Aleocharinae, there are no clear-cut differences between spore- and non-spore feeders in the structure of the hypopharynx–premental sclerites (Tables 3, 4, 7; Fig. 13). Leschen (1993) found that shifts within trophic systems were not paralleled by changes in mouthpart morphology in selected Staphylinoidea. Our findings likewise suggest that there is not only one morphological solution to become an (obligate) spore feeder, but that multiple morphologies of the internal feeding apparatus (cf., CC1, CC2, CC4, CC5, CC7 in Tables 3 and 7) can lead to the same general function of grinding the spore material (cf. principle of many-to-one mapping of form to function: Wainwright 2007; Wainwright et al. 2005). Actually, as in other complex morphological systems (e.g., fish jaws: Wainwright et al. 2005), such an evolutionary principle of redundancy might generate morphological diversity in ecologically similar species. The different internal morphologies we found might not, however, be equal with respect to their final performance capacity, that is, some combinations and formations of the various elements might be more efficient in terms of processing spore material to the gut compared to others. Notably, the distinct stabilization of the hypopharyngeal region in obligate spore feeders within the tribe Homalotini (i.e., subtribe Gyrophaenina) in combination with an extensive ventral grinding structure (pseudomola) of the mandibles might be considered a particularly advanced adaptation to spore feeding. It is true that the non-spore feeding Autalia has the same character combination (CC5: Table 7), but in this case the lateral corners of the suspensorium are less sclerotized (Fig. 8J–L), and the pseudomola is less extensive (Weide et al. 2010: cf. Figs 4F and 5F,I,L). This example demonstrates how postadaptational functional improvements of a more general microphagous groundplan might have evolved after a functional shift toward more specialized mycophagy such as spore feeding.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

This work was supported by the Deutsche Forschungsgemeinschaft (BE-2233/6-1 and 6-2) and the Landesgraduiertenförderung Baden-Württemberg. The analyses of the species were enabled by Ch. Bückle (Tübingen, Germany), who collected and determined most of the beetles collected in Germany. Prof. K. Peschke (Freiburg, Germany) provided Aleochara haematoptera, Dr. K. Wolf-Schwenninger (Stuttgart, Germany) contributed Omalium rivulare, and Prof. P. Zwick (Schlitz, Germany) provided Necrophilus subterraneus. We thank Dr. P. Cloetens and Dr. L. Helfen (ESRF, Grenoble, France) for their assistance at the beamline ID 19. We gratefully acknowledge S. Schmelzle, A. Dieterich, and Dr. M. Laumann (all Tübingen, Germany) for their help with the model visualization performed with Amira®. We thank Dr. A. F. Newton for helpful comments and discussions and allowing M. K. Thayer access to his invaluable slide collection.

Three anonymous reviewers helped to improve the manuscript and are gratefully acknowledged.

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  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information
FilenameFormatSizeDescription
azo12011-sup-0001-DataS1.docxWord document12KData S1. Additional information concerning synchrotron X-ray micro-tomography and additional information concerning visualization of data.
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azo12011-sup-0016-Pagla-sp.mp4MPEG-4 video1981K 
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