Slovenia, with an area of 20,273 km², lies at the junction of four major European geographical macro-regions: the south-eastern part of the Alps, the western margin of the Pannonian Basin, the north-western part of the Dinaric Alps and the northern margin of the Mediterranean (Perko et al. 2020). The country’s complex geological history has influenced the development of various landforms, topography, bedrock, soil types, hydrology, climate and vegetation. The climate ranges from sub-Mediterranean to temperate continental and montane (Perko et al. 2020). On average, there is 1750 mm of precipitation annually, which are spatially and temporally highly variable (Komac et al. 2020). The average annual temperature in Slovenia is between 10 and 13 °C (Komac et al. 2020). The primary vegetation in most parts of Slovenia is temperate forest (Andrič and Willis 2003; Šilc et al. 2020), which covered 58% of the country in 2020 (SURS 2021).

An important geographical feature of Slovenia is the karst, special type of landscape, where water soluble carbonate bedrock enables formation of fissures and caves (Gams 2004). There are over 14,000 caves registered in Slovenia, some of which are more than 1000 m deep (Karst Research Institute ZRC SAZU 2022; Ljubljana Cave Exploration Society 2022). Karst areas cover 43% of the country and are divided into three principal areas: Alpine karst, Isolated karst and Dinaric karst (Gams 2004; Zupan Hajna 2004).

Information on the occurrence of lithobiomorph species in caves was taken primarily from the database on subterranean biodiversity SubBioDB (managed by the Subterranean Biology Lab, Department of Biology, Biotechnical Faculty, University of Ljubljana), including data until the end of 2022. SubBioDB combines literature data and the data on individuals collected during various fieldwork expeditions. One record refers to the unique combination species-locality-source for literature data and species-locality-date for new (fieldwork) data.

Specimens reported as new findings were collected in the fieldwork by visual inspections and direct hand picking or, in some cases, by baited pit-fall traps. All individuals were preserved in 70% or 96% ethanol and stored in the Zoological collection of the Subterranean Biology Lab, Department of Biology, Biotechnical Faculty, University of Ljubljana. Some specimens were contributed to this collection from the collections of the Notranjska Museum in Postojna (Slavko Polak), or the speleobiological collection of the Department of Biology, Faculty of Natural Sciences and Mathematics, University of Maribor (Peter Kozel). In addition, some older material was taken from the “Chilobio” centipede collection of the Animal Ecology Group, University of Ljubljana (Ravnjak and Kos 2015). Specimens were examined under stereomicroscope Olympus SZH10 or Leica M165 C and morphologically determined using identification keys for lithobiomorph centipedes (e.g. Matic 1966; Koren 1992; Stoev et al. 2010). In cases where reliable species identification was not possible, e.g., due to damaged, juvenile or female individuals, specimens were identified only to a genus level.

Species taxonomy followed the one proposed in ChiloBase 2.0 (Bonato et al. 2016). We implemented this also for the already existing literature records, where names were curated to follow currently valid taxonomy. As an exception, Lithobius jugoslavicus Matic & Darabantu, 1968 was treated as a synonym of Lithobius stygius Latzel, 1880, following the taxonomic status of its substitute name Lithobius corneliae Stoev, 1997. Furthermore, Lithobius punctulatus was considered a synonym of Lithobius validus, following Eason (1972) and Kos (1987). Reassignments of taxa were made in four cases, where reports of species Eupolybothrus leostygis, Eupolybothrus fasciatus, Lithobius burzenlandicus wardaranus and Lithobius microps were assigned to E. obrovensis, E. grossipes, Lithobius sp. and Lithobius carinthiacus, respectively. The rationale behind these reassignments is explained in the discussion.

Geographic positions of cave entrances were taken from the Cave Registry of Slovenia (“Kataster jam Slovenije”), available at https://www.katasterjam.si/ (Ljubljana Cave Exploration Society 2022), or from GPS coordinates (in cases of caves whose registration is in progress and artificial tunnels). Few localities reported in literature were, due to dubious geographic position, assigned to wider geographic areas or region and not to individual caves. Distributional maps were produced using QGIS, ver. 3.16. (QGIS Development Team 2021).

The compilation of occurrence dataset inevitably relates to the current knowledge on lithobiomorph centipede taxonomy, which is incomplete, prone to temporal changes, and additionally complicated by varying taxonomic interpretations between the researchers as several taxa need revision (Zapparoli 2003; Bonato et al. 2011). Due to mentioned shortcomings the comparison of literature records is challenging and, the taxonomical curation of the dataset was necessary to include currently valid names and to address possible misidentifications originating from different taxonomic concept. As example, some specimens cited in Matic and Dărăbanţu, (1968) where re-examination of material was possible, were assigned to different species; reported finding of E. fasciatus and L. microps, correspond to E. grossipes and L. carinthiacus, respectively. Determinations of specimens reported as L. burzenlandicus wardaranus could not be conclusive due to damaged material, but the finding from Mala jama pri Veliki Groblji corresponds to a yet undescribed taxon Lithobius anici nomen nudum, reported from several surface sites in Slovenia (Kos 1995; Kuralt et al. 2022). Similarly, the finding from Vratnica likely belongs to Lithobius carinthiacus (due to presence of three spines on ventral side of 15^th prefemur), but specimen is too damaged to make conclusive determination. Furthermore, findings of Eupolybothrus leostygis, reported by some authors, were corrected to E. obrovensis as suggested by Verhoeff (1934) and Eason (1983). Interpretation of taxonomic status for Lithobius microporus Verhoeff, 1937, Lithobius jugoslavicus Matic & Dărăbanţu, 1968 and Lithobius glabratus fuscus Folkmanova, 1946 is treated as dubious. They are considered as probable synonyms of Lithobius lucifugus L. Koch, 1862, Lithobius stygius Latzel, 1880 and Lithobius melanops Newport, 1845 respectively (Stoev 2001; Bonato et al. 2016); however, their identity was not thoroughly studied. Similarly, synonymy of L. illyricus Latzel, 1880 with L. erythrocephalus C.L. Koch, 1847 is stated in ChiloBase 2.0, however it was never formally established and the species is considered valid by some authors (Kos 1992a; Stoev 1997; Stagl and Zapparoli 2006). On the other hand, Zapparoli (1989), who reported L. erythrocephalus from caves in Slovenia, considered that characters of L. illyricus are within the variation of L. erythrocephalus. Existence of potential new species, some of which might represent morphologically identical, cryptic species, additionally hinders complete overview of the species diversity. Their potential presence was detected among the examined individuals in our study and was also reported in literature (Zagmajster et al. 2021). This is expected as there have been descriptions of new species from the Dinaric caves in recent time (Stoev et al. 2015; Akkari et al. 2017; Stojanović et al. 2021) and cryptic diversity is a common problem in many centipede groups (Spelda et al. 2011; Del Latte et al. 2015; Kuralt et al. 2022; Peretti et al. 2022).

Taxonomic knowledge of several lithobiomorph species that were described from Slovenian caves changed over time. Eyeless centipede, Eupolybothrus obrovensis (Fig. 4) was described based on subadult male under the name Lithobius obrovensis. Species type locality is “Höhle von Obrovo“ (Verhoeff 1930), interpreted as Dimnice cave by most of the subsequent authors (Matic and Dărăbanţu 1968; Ozimec and Komerički 2009) or as Pečina v Borštu (Wolf 1938; this article). Later work by Verhoeff (1933) provides information on an adult male from Medvedjak and Matic & Daranbatu (1968) provided a description of a female from Dimnice. Nevertheless, a redescription of the species, based on the adult male and the consolidation of the type locality is needed. Attems (1908, 1929) found the species even before it was formally described in 1930, proclaiming it for Eupolybothrus leostygis, species which is otherwise narrowly distributed in Southern Croatia and Bosnia and Herzegovina. This error is mentioned already by Verhoeff (1934), however, several other authors (Manfredi 1932b; Wolf 1938) cited this finding, and species E. leostygis ended up even in the checklist of centipedes in Slovenia (Kos 1992a). In addition, the locality name Medvedova jama (referring to cave Medvedjak, Cad. nr. 881, “bear cave”) was translated into Italian “Grotta dell’Orso” and misinterpreted as a cave near Gabrovizza, San Primo in Italy (Manfredi 1932a, 1932b; Wolf 1938; Eason 1983). Eupolybothrus obrovensis was long known only from few caves, but the speleobiological surveys after the year 2000 yielded more findings, improving our knowledge on the species’s narrow distribution. The species occurs in Primorska and Istria regions in Slovenia and Croatia (Table 1; Ozimec and Komerički 2009). The species is listed as Vulnerable on the Slovenian and Croatian Red List (Kos 1992b; Wraber et al. 2002; Ozimec and Komerički 2009) and is protected species by the Croatian national legislation (Official Gazette of the Republic of Croatia No. 70/05, 139/08). Presumably, the species is vulnerable to changes in habitat and cave fauna, especially due to its narrow distribution and high position in the food chain (Ozimec and Komerički 2009).

Figure 4. 

Three lithobiomorph centipede species found in Slovenian caves. Subterranean centipede Lithobius stygius (upper left), surface species Lithobius nodulipes (upper right) and subterranean species Eupolybothrus obrovensis (bottom) (Photo: Teo Delić).

Species with the highest number of occurrences, Lithobius stygius (Fig. 4), was described from Postojnska jama by Latzel (1880) and reported in caves through the Balkans (Stoev 2001). The existing reports are subjected to some taxonomical unclarities, emphasising the need for its further study. Examples from the area of Slovenia include reported species L. erythrocephalus and L. illyricus, surface species similar to L. stygius, from several caves (Attems 1929; Matic and Dărăbanţu 1968; Zapparoli 1989; Gasparo 1995, 1998). It is unclear whether all of them are truly correct identifications or possible misidentifications, and refer to findings of L. stygius or another, similar, and potentially new species. On the other hand, species Lithobius jugoslavicus Matic & Dărăbanţu, 1968 is one of the possible junior synonyms of L. stygius, where revision is needed to clarify its status (Stoev 2001). It is very likely that the species was described based on an abnormal individual. In some of the works it was referred also as Lithobius corneliae Stoev, 1997 in attempt to distinguish it from Lithobius jugoslavicus (Hoffer, 1937) (now a synonym of Lithobius matulici Verhoeff, 1899, occurring in Bosnia and Herzegovina) (Stoev 1997; Dányi et al. 2019). Preliminary genetic analyses covering the North-western Dinarides showed that the species L. stygius is occurring in the south-eastern part of Slovenia and north-western Croatia, while there exists also morphologically similar, but genetically distinct species (Kos A., unpublished). Furthermore, there have been records of synonyms Lithobius cerberi Verhoeff, 1943 and Oligobothrus luciani Folkmanová, 1935 in the Balkans (Folkmanova 1935; Verhoeff 1943; Eason 1983; Stoev 2001), as well as possible synonym L. temnensis Verhoeff, 1943, which was reported from caves as well as surface (Zapparoli 2002).

Lithobius zveri is eyeless centipede and was described by Matic and Stenzer (1977). Even though it was found in one of the most surveyed caves, Planinska jama, it is known only from a single specimen. This might be due to its rarity, our inability to sample its main habitat, small size or lack of targeted sampling. The species is morphologically well separated from other species (Stojanović et al. 2021), however further findings on this elusive and presumably highly endemic species are needed to better understand its taxonomic position.

Polybothrus cerberus Verhoeff, 1929, Lithobius microporus Verhoeff, 1937 and Lithobius glabratus fuscus Folkmanova, 1946 were also described from Slovenian caves, but are now considered as synonyms of Eupolybothrus tridentinus (Fanzago, 1874), Lithobius lucifugus L. Koch, 1862 and Lithobius melanops Newport, 1845, respectively (Eason 1983; Zapparoli 1989; Stoev 1997). To make it even more complicated the identity of the type locality for the subspecies Lithobius glabratus fuscus, described as Lok. 776 Vranja peć, is unclear. Stoev (1997) suggested that the locality might be in Croatia. Naming of the localities follows the collection Biospeleologica balcanica, enabling a direct comparison with other literature. Willmann (1941) describes this locality as Vranja peć bei Saxenstein, Carniola. Based on an examination of old names for toponyms and settlements in Slovenia and Croatia, we concluded that this cave is most likely Raja peč (Cad. nr. 371, one of its synonyms is Vranja pečina) near town Boštanj, which was referred to as Savenstein in German (the name was apparently misspelt by researchers).

Occurrence and distribution of lithobiomorph centipedes in Slovenia open many interesting questions and directions for further studies. The frequent findings of different surface centipede species underground open question on the connectivity of subterranean and surface habitats. It remains open to investigate, how the species composition in caves is related to the overall centipede species diversity in the region. Further on, lithobiomorph centipedes may present an interesting group to study processes related to colonization of subterranean habitats, especially as there are groups of species that can be found in the surface-subterranean ecotone as well as deep in caves.

As top predators and K-strategists (Albert 1983; Vahtera et al. 2020), lithobiomorph centipedes might have an specific position in subterranean ecosystems, however, their role in communities is still understudied. Different surface species might be part of different communities and are presumably more interconnected with community at cave entrances, while exclusively subterranean species are likely part of the communities deeper in caves (Novak et al. 2012; Cuff et al. 2021). In the Dinarides, there are several centipede species that were found in deep parts of caves, with geophilid centipede Geophilus hadesi Stoev, Akkari, Komerički, Edgecombe & Bonato, 2015 holding depth record with finding in cave Lukina jama – Trojama cave system, at depth 1100 m (Stoev et al. 2015). Given the lack of ecological data, further studies would be needed to investigate the ecology of centipedes and their role in subterranean communities.

This paper represents an important contribution to the knowledge of diversity of centipedes in the caves of Slovenia. It is clear that centipedes are a diverse group of animals that, despite the rich history of research require further faunistic and ecological studies to gain at least a basic understanding of their role in communities within subterranean realm. With the data at hand, the understanding of their taxonomy is also far from complete, as is evident from the existence of numerous taxonomic issues. Therefore, further work should aim to resolve taxonomic status, include molecular information on the specimens to improve the resolution of centipede species diversity and their phylogenetic relationships, and to set a solid framework for future ecological studies.