The studied material consists of 61 specimens belonging to 35 species preserved in 70% or 95% EtOH (Table 1), deposited at the Natural History Museum Vienna (NHMW), the Natural History Museum London (BM/NHMUK) and the Hungarian Natural History Museum Budapest (HNHMB). All material was examined with light and scanning electron microscopy.

The epipharynx and hypopharynx were dissected from the preoral chamber as described in Koch and Edgecombe (2008) in one to four adult male or female individuals per species. Multifocus images of the sclerotized parts of the epipharynx and hypopharynx were obtained with a Nikon SMZ25 stereomicroscope equipped with a Nikon DS-F2.5 camera using NIS-Elements Microscope Imaging Software with an Extended Depth of Focus (EDF) patch. For scanning electron microscopy (SEM), the specimens were: (1) cleaned in an ultrasonic bath (50–60 Hz) for 5 to 10 seconds (maximum), occasionally in a solution of 15% hydrogen peroxide for 2 hours; (2) dehydrated in an ascending alcohol series (70%, 80%, 90%, 96% EtOH, 2 × 10-15 min each); (3) air dried overnight (or covered with HMDS) or critical point dried (Leica 300 CPD). Specimens were mounted on aluminium stubs equipped with a sticky aluminium tape, glued with conductive silver, coated with platinum (Leica EM SCD500) and studied with a JEOL JSM 6610-LV at an accelerating voltage of 15 kV. Figures were processed with Adobe Photoshop CS6 and assembled in Adobe InDesign CS6.

Terminology follows Koch and Edgecombe (2008).

bdb – labral bristles on distal bar; blf – labral bristles on lateral flap; bsc – ‘button-shaped’ sensilla; bu – single transverse bulge; bud – distal transverse bulge; bup – proximal transverse bulge; db – distal bar; gl – ‘bottle-shaped’ epidermal glandular shafts; hb – hypopharyngeal bar; hsp – hypopharyngeal spine field; lf – lateral flap; lsp – lateral spine field; lmc – paired lips forming median crest; mo – mouth opening; msc – median sensilla cluster; msp – median spine field; nsc – cluster of ‘nipple-shaped’ sensilla; pb – proximal bar; pp – pharyngeal plate; smc – spines flanking median crest; tu – tuft of bristles; tub – tubercles on distal bar; vlb – ventrolateral bar.

In the following, eight peristomatic characters are proposed for the genus Lithobius, three of which are newly described (see char. 4, 6, 7). Additionally, we verified the consistency of two characters (see char. 2, 8) and adjusted three (see char. 1, 3, 5) from those indicated by Koch and Edgecombe (2008). Codings are provided in Appendix 1.

Epipharynx

1. ‘Bottle-shaped’ glandular shafts at the border between labral and clypeal part of epipharynx: (0) one distinct regular row; (1) more than one regular or irregular row.

All the investigated lithobiomorph species possess ‘bottle-shaped’ epidermal glandular shafts at the border between the labral and clypeal parts of the epipharynx. The latter can be in one regular row (Figs 2B, D–F, 5A) or with a variable arrangement, e.g. one regular row medially, which expands to two or three regular or irregular rows laterally (Figs 2C, 3A, 4A, 5B). A regular or irregular arrangement of consistently two or more rows along the whole width is present, for example, for D. loricatus and L. piceus (Figs 2A, 3B). Both states were identified across all subgenera of Lithobius with state (0) being underrepresented in the subgenus Lithobius (6 of 23 examined species). N. aztecus, D. loricatus (Fig. 2A) and E. grossipes (Fig. 3D) share state (1).

2. Labral bristle bands of epipharynx: (0) bristle bands continuous across transverse bulge; (1) distinct break in bristle bands proximal and distal to transverse bulge. (Character 31 in Koch and Edgecombe 2008).

The subgenera of Lithobius and other lithobiid genera show labral bristle bands that are either continuous (Fig. 4D) or are interrupted at the transverse bulge (Fig. 4E–F). All studied species of the subgenus Sigibius share state (1) (e.g. Fig. 2D).

3. Number of transverse bulge(s) at border between labral and clypeal parts of epipharynx: (0) none; (1) one; (2) two.

The presence of one or two transverse bulges is common for the genera Lithobius, Neolithobius and Eupolybothrus (e.g. Figs 2B–F, 3). The bulges are absent only in the genus Disphaerobius (Fig. 2A). Two bulges are shared by L. calcaratus, L. lucifugus, L. tenebrosus and E. grossipes only (e.g. Fig. 3C–D).

4. Direction of distal and proximal furrowed margins of transverse bulge or transverse bulges on epipharynx: (0) parallel; (1) non-parallel.

Both states occur in all studied genera, state (0) e.g. in L. pyrenaicus, L. fagei, L. lucifugus and E. grossipes (Figs 2B–C, 3C–D) or state (1) e.g. in L. microps, L. mutabilis, L. aeruginosus, L. macilentus and L. piceus (Figs 2D–F, 3A–B). All species of the subgenus Sigibius share a non-parallel alignment (state (1)) of the transverse bulge margins (e.g. L. microps; Fig. 2D).

5. Median field of branching spines immediately proximal to the border between labral and clypeal parts of epipharynx: (0) rhomboid, widening medially; (1) widening laterally; (2) subequal width medially and laterally.

State (2) is most common throughout the subgenus Lithobius and occurs in the other subgenera of Lithobius, e.g. L. microps, L. piceus and L. electus (Figs 2D, 3B, 5D). The genera Eupolybothrus, Disphaerobius and Neolithobius share state (0) but show variation in the number of rows of branching spines (e.g. Figs 2A, 3D). All states occur with a narrower or wider band having a few or several rows of branching spines.

6. Shape of branching bristles on labral flap of epipharynx: (0) lateral to medial transition from plumose to ‘fan-shaped’ bristles; (1) ‘fan-shaped’ bristles only; (2) plumose bristles only; (3) simple bristles only.

A transition of branching bristles from plumose laterally to ‘fan-shaped’ medially is the most common state (0) across the genus Lithobius, and also pertains to Neolithobius and Eupolybothrus (e.g. Fig. 9A–C). State (1) was observed in L. cyrtopus (Fig. 9E), L. lucifugus, L. pelidnus and L. microps. State (2) was present in L. peregrinus (Fig. 9D), L. piceus and L. tricuspis, and state (3) in D. loricatus only (Fig. 9F).

7. Lateral expansion of median sensilla cluster of epipharynx: (0) isolated from the lateral spine fields; (1) partly overlapping with the lateral spine fields.

In all Lithobius species we examined (except for L. tricuspis and L. nodulipes for which the samples were damaged), the median sensilla cluster is bordered laterally by fields of spines (state (0); Fig. 10A–B). The sensilla in D. loricatus and N. aztecus slightly overlap with the lateral spine fields medially (state (1); e.g. Fig. 10C). In E. grossipes the sensilla of the median sensilla cluster strongly overlap with the lateral spine fields proximolaterally (state (1); Fig. 10D).

8. Differentiation of spines flanking median crest of hypopharynx: (0) intergrading rows of branching bristles; (1) single outer row of simple flattened spines with abrupt transition to multifurcating inner rows of branching bristles. (Character 39 in Koch and Edgecombe 2008)

Species of the subgenus Monotarsobius always display state (1) (e.g. L. franciscorum; Fig. 15B). The Ezembius species L. electus studied here displays state (1), which differs from Lithobius (Ezembius) giganteus Sseliwanoff, 1881, stated by Koch and Edgecombe (2008). State (1) (Fig. 15A–C, F) is more common throughout the other subgenera of Lithobius and species of the other examined genera compared to state (0) (Fig. 15D–E).

While studying the peristomatic structures of Lithobiomorpha and Scutigeromorpha, Koch and Edgecombe (2008) compared the presence of the ‘bottle-shaped’ epidermal glandular shafts between the labral and clypeal part of the epipharynx. These glandular shafts were reported to be constantly present in Lithobiomorpha (Koch and Edgecombe 2008) and absent in other chilopods (Koch and Edgecombe 2006, 2008, 2012, Edgecombe and Koch 2008). We confirmed the presence of glandular shafts in the specimens we examined in the lithobiid genera Lithobius, Neolithobius, Eupolybothrus and Disphaerobius and further recorded differences in number and regularity of rows (character 1).

The same authors (Koch and Edgecombe 2008) described the presence of a transverse bulge dividing the labral and clypeal part on the epipharynx for all Lithobiomorpha except for Hessebius plumatus Zalesskaja, 1978 and L. (Ezembius) giganteus displaying no bulge at all. This study confirms the absence of the bulge in the species D. loricatus (Fig. 2A) and for the first time the presence of a second bulge (distal transverse bulge) as recorded for the species L. calcaratus, L. lucifugus, L. tenebrosus and E. grossipes as well as E. fasciatus (Newport, 1845) (specimens used by Koch and Edgecombe 2008). The alignment of the bulges is further described and proposed as an additional character state (character 4).

The examination of additional taxa within Lithobiidae revealed more variation in the shape of the median spine field than previously described and having surveyed more species we include additional character states to those already described by Koch and Edgecombe (2008) (character 5).

Although differences in shape of the bristles on the labral flap were briefly mentioned by Koch and Edgecombe (2008), our study unveiled four consistent states in the shape of bristles and transition of those from laterally to medially, which serves as a new multistate character for Lithobiidae (character 6). A transition of bristles from plumose to ‘fan-shaped’ was described for Pleurolithobius patriarchalis (Berlese, 1894) (Koch and Edgecombe 2008), as in the majority of the investigated species in the present study. In contrast, only ‘fan-shaped’ bristles are observed in the lithobiid Harpolithobius anodus (Latzel, 1880) and the henicopid Lamyctes (Lamyctes) emarginatus (Newport, 1844). On the other hand, the interpretation that Lithobius (Monotarsobius) holstii (Pocock, 1895) possesses only ‘fan-shaped’ bristles (Fig. 6E in Koch and Edgecombe 2008) seems erroneous as their figure reveals a pattern in accordance with the other examined Monotarsobius-species, which exhibit a transition from plumose to ‘fan-shaped’ bristles (e.g. L. aeruginosus, Fig. 9C).

Generally, the median sensilla cluster borders or overlaps marginally with the lateral field of spines in Lithobiomorpha (Koch and Edgecombe 2008). However, we observed a median sensilla cluster considerably expanding along the length of the lateral spine fields on the epipharynx in E. grossipes for Lithobiomorpha (Fig. 10D). This was also verified in E. fasciatus (specimens used by Koch and Edgecombe 2008), which also displays a large but partial overlap.

As mentioned in the introduction, the hypopharynx as a short outgrowth with a median crest is an apomorphic character for Lithobiomorpha. This is verified in all examined lithobiid species. Moreover, the median crest margin of all studied species of the subgenus Monotarsobius displays flattened spines (character 8) as previously described for L. holstii (Koch and Edgecombe 2008).

Besides the well-defined characters listed in the previous paragraph, our investigation also yielded several structures with high variability in appearance and/or intermediate forms between and even within species. For example, the branching bristles of (i) the labral bristle band on the distal bar, (ii) the spines of the median spine field of the epipharynx and (iii) the branching bristles as a tuft on the hypopharynx occur with several non-definable forms. Koch and Edgecombe (2008) described a smooth transverse bulge for Lithobiidae, which we confirmed for most of the examined species. However, we also observed a longitudinal striation of the whole bulge surface or at least on the lateral parts of the bulge for some species (Figs 3C, 4D). A similar description of the latter state was observed for the henicopid Lamyctes emarginatus, where more defined longitudinal grooves occur (Koch and Edgecombe 2008).

The paired oblique rows of elongated lateral spines on the clypeal part of the epipharynx were also considered as an apomorphic character for Lithobiidae (Koch and Edgecombe 2008). This is also confirmed in all examined lithobiid species we studied. However, the proposed character states, i.e. (2): oblique rows of single spines and (3): a few small groups of branching spines for the lateral field of spines on Lithobiidae were not consistent across the species we examined and showed many intermediate states. On this basis we excluded the character for conclusions on the systematics in Lithobiidae, especially Lithobius, in our study.

Koch and Edgecombe (2008) recorded groups of lateral fields of spines in the subgenus Monotarsobius in contrast to pairs of oblique rows in the rest of Lithobiidae (character 32, state (3)). These spines seem to be arranged in oblique rows as in the rest of Lithobiidae in the species L. (Monotarsobius) aeruginosus and L. (Monotarsobius) curtipes (Fig. 10F–G).

A correlation between the number of ‘bottle-shaped’ epidermal glandular shafts of Lithobiomorpha and body size was also mentioned by Koch and Edgecombe (2008), implying that larger species tend to have higher numbers. Here, we suggest the same for the number of glandular shafts, sensilla in the median sensilla cluster and the ‘nipple-shaped’ sensilla cluster, lateral spines, and the hypopharyngeal spines. This size correlation needs to be confirmed by morphometrics and statistical analysis but the phylogenetic significance of these characters is cast into doubt.

The peristomatic structures of H. plumatus and L. (Ezembius) giganteus described by Koch and Edgecombe (2008) and D. loricatus examined in this study, i.e. a missing transverse bulge (character 3), simple bristles on the labral flap of the epipharynx (character 6) and scales on the distal tips of the lips of the hypopharynx (Fig. 17E), differ from all other studied species of Lithobius, including L. (Ezembius) electus, even if the latter is correctly placed in the subgenus Ezembius. Several taxa in Central Asia, also species of the giganteus-group of Lithobius (Eason 1983, 1986) and of the genus Hessebius Verhoeff, 1941 share some morphological characters with the genus Disphaerobius Attems, 1926, as mentioned by Farzalieva et al. (2017): “… functionally biarticulated tarsi of leg 1–13, the antennae composed of 20 antennomeres, the rounded posterior angles of all tergites, the 1-segmented male gonopods, and Tömösváry’s organ being equal in size to the nearest ocellus or smaller.” In contrast to the three other species of the giganteus-group of Lithobius, L. (Ezembius) giganteus displays secondary sexual modifications of the tergites in males similar to Disphaerobius (Farzalieva et al. 2017). Here, we assume that the epipharyngeal and hypopharyngeal structures may confirm a closer relationship of L. (Ezembius) giganteus to D. loricatus than to L. (Ezembius) electus. This relationship is inconsistent with the classification of Disphaerobius as a separate subfamily, Pterygoterginae Verhoeff, 1933, because that classification would render Lithobiinae, as well as Lithobius and L. (Ezembius) as paraphyletic groups.