Review of the genus Caucaseuma Strasser, 1970, with the description of a new cavernicolous species from the Western Caucasus and an updated key and distribution (Diplopoda, Chordeumatida, Anthroleucosomatidae)

. A new species of the genus Caucaseuma Strasser, 1970, Caucaseuma strasseri Antić sp. nov., is described from a cave in the Western Caucasus, representing the eighth species of the genus, and the fourth presumed troglobiontic Caucaseuma . An updated key to and a distribution map of all eight species of Caucaseuma is presented, including new records. In addition, the cave millipede fauna of the Caucasus is briefly discussed, with the inclusion of the most relevant references. Antić D. & Makarov S. 2022. Review of the genus Caucaseuma Strasser, 1970, with the description of a new cavernicolous species from the Western Caucasus and an updated key and distribution (Diplopoda, Chordeumatida, Anthroleucosomatidae). European Journal of Taxonomy 819: 90–107.


Introduction
The millipede genus Caucaseuma Strasser, 1970 was established as monospecific to accommodate the species C. lohmanderi Strasser, 1970. This species was the first cavernicolous chordeumatidan described from the Caucasus, and until recently the sole member of this mostly Western Caucasian genus. Almost half a century later, in their monographic study of Caucasian Anthroleucosomatidae Verhoeff, 1899, Antić & Makarov (2016 described another six species of Caucaseuma. Besides C. lohmanderi, known so far from more than 10 caves in the Sochi region, Antić & Makarov (2016) considered another two narrow endemics, viz., C. elephantum Antić & Makarov, 2016 and C. minellii Antić & Makarov, 2016, as "recent" troglobionts. All three species present some troglobiomorphic features such as completely or partially depigmented tegument, somewhat reduced pigmentation in ommatidia in some, elongated appendages and a larger, robust body (see Antić & Makarov 2016).
Here, we describe a new, fourth presumably troglobiontic species of the genus Caucaseuma, from the Kirovskaya (= Tigrovaya) Cave in the Sochi region, Western Caucasus. We also present an identification key to and a distribution map for all eight described species of the genus.

Material and methods Preservation, dissecting, imaging, map
Specimens preserved in 70% ethanol were examined with a Nikon SMZ 745T binocular stereo microscope. All taxonomically important structures were dissected and mounted in glycerine as temporary microscope preparations and observed with a Carl Zeiss Axioscope 40 microscope. Pictures of the specimens and legs were taken using a Nikon DS-Fi2 camera with a Nikon DS-L3 camera controller attached to a Nikon SMZ 1270 binocular stereo microscope. All pictures were stacked with a Zerene Stacker. Line drawings of the gonopods were obtained using tracing paper placed on a computer monitor displaying pictures of those structures made with a Canon PowerShot A80 digital camera connected to a Carl Zeiss Axioscope 40 microscope. For scanning electron microscopy (SEM) samples were (1) cleaned in an ultrasonic bath (50-60 Hz) for 5 to 10 seconds (maximum), (2) dehydrated in an ascending alcohol series (70%, 80%, 90%, 96% EtOH, 2 × 10-15 min each) and acetone; and (3) air dried. Samples were mounted on aluminium stubs equipped with sticky aluminium tape, coated with platinum (Leica EM SCD500) and studied with a JEOL JSM 6610-LV scanning electron microscope at an accelerating voltage of 15 kV (Natural History Museum Vienna, Austria). Figures were processed and assembled in Adobe Photoshop CS6.
The distribution map was created using Google Earth Pro (ver. 7.3.4.8248) and Adobe Photoshop CS6. The final images were processed with Adobe Photoshop CS6.

Abbreviations of gonopodal structures
Anterior gonopods aA = anterior part of angiocoxite bp = bone-like process of angiocoxite Cv = coxal vesicle pA = posterior part of angiocoxite pp = posterior projection of angiocoxite S = gonopodal sternum Sp = sternal process tp = triangular process of angiocoxite Posterior gonopods A = angiocoxite Cv = coxal vesicle Cx = coxite S = gonopodal sternum T = telopodite Abbreviations of indices of body ring 15 CIX = macrochaetal index = distance between exterior and median macrochaetae/distance between interior and median macrochaetae MA = macrochaetal angle = angle between the arm created by the median and exterior macrochaetae and the arm formed by the median and interior macrochaetae MIX = median index = distance between interior macrochaetae and axial suture/distance between interior and median macrochaetae PIX = paratergal index = (width of metazonite -width of prozonite) / (2 × length of paratergum) Genus Caucaseuma Strasser, 1970Caucaseuma Strasser, 1970.

Diagnosis
Differs from all other genera of the family Anthroleucosomatidae, except the genus Heterocaucaseuma Antić & Makarov, 2016, by the presence of two pairs of flagelliform processes originating from the base of the angiocoxites of the posterior gonopods. From the genus Heterocaucaseuma, it differs by the presence of more complicated anterior gonopods, characterized by angiocoxites clearly differentiated into anterior and posterior parts. The anterior parts are shield-like, separated, partially fused or completely fused, with a visible furrow. The genus Heterocaucaseuma is characterized by somewhat simplified anterior gonopods with angiocoxites differentiated into a medial part, fully fused, and two lateral parts.

Distribution
The genus Caucaseuma includes mainly narrow endemics confined to the Western and North Western Caucasus, with the exception of C. variabile with a rather disjunct distribution in the western, northern and central Caucasus (Fig. 1).

Diagnosis
Caucaseuma strasseri Antić sp. nov. is the only species in the genus characterized by the presence of anterior triangular processes on the angiocoxites of the anterior gonopods.

Etymology
The new species is named after the late Carlo Strasser (1903Strasser ( -1981, a well-known and always inspiring diplopodologist, who made an immeasurable contribution to the knowledge of European millipedes. At the same time, Strasser described the first cave chordeumatidan taxon from the Caucasus, Caucaseuma lohmanderi. A noun in the genitive case. Material examined (  Paratypes RUSSIA • 1 ♂ (used for SEM); same collection data as for holotype; NHMW MY10260 • 4 ♂♂ (2 without posterior body rings); same collection data as for holotype; ZMUM • 1 ♂ (without posterior body rings); same collection data as for holotype; IZB • 1 ♀; same collection data as for holotype; ZMUM.
anterior gonopodS (Figs 5a-C, 6a-f, 7H). Gonopodal sternum (S) well-developed, wide, with anterior triangular, long sternal process (Sp) covered with hair-like outgrowths. Anterior parts (aA) of angiocoxites shield-like, medially completely fused, only with anterior furrow, almost fully rounded in anterior view; subdistally with a pair of notches and short, lateral, bone-like processes (bp); distally with medial structure mushroom-shaped in anterior view with lateral margins curved posteriad; posteriorly, at site of fusion of anterior parts of angiocoxites, with a posterior projection (pp), denticulate distally, and extending between posterior parts of angiocoxites. A pair of characteristic anterior triangular (tp) processes present on angiocoxites. Posterior parts (pA) of angiocoxites in form of elongate levers in posterior view, with distal parts curved anteriad; anteriorly with additional lamellar and setose projections. Between posterior parts of angiocoxites coxal vesicles (Cv) visible.

Remarks
This species is known only from a cave and shows certain troglobiomorphic features such as reduced body and ommatidia pigmentation, elongated antennae and walking legs, as well as a larger and more robust body compared to epigean congeners.

Remarks and distribution
A troglophile. So far known only from the type locality, Fanagoriyskaya Cave, Krasnodar Province in Russia (Fig. 1).

Remarks and distribution
Epigean species. So far known only from the type series described from two localities at ca 2000 m a.s.l. in the Teberda Nature Reserve in Russia (Antić & Makarov 2016) (Fig. 1).

Discussion
Just over 70 years ago, the pioneer of biospeleological research in the former USSR, Birstein (1950), recognized the biological potential of karst in the Western Caucasus. A few decades later, Culver et al. (2006) were of the opinion that there are potential subterranean hotspots of biodiversity in the Western Caucasus. Numerous descriptions of new subterranean taxa in this region during the last decade are very much in favour of these assumptions.
In terms of cave millipedes of the Caucasus, from Birsetein's (1950) pioneering study until the end of the second and the beginning of the third decade of the 21 st century, only a few species have been described (see also Golovatch 1985). As mentioned above, the first cavernicolous chordeumatidan from the Caucasus, C. lohmanderi, was described by Strasser (1970) based on samples found in two caves in the Sochi region. From then until recently, only one other cavernicolous taxon of Chordeumatida has been described from the Caucasus, a monospecific Ratcheuma Golovach, 1985, known from only one cave in the Racha region in Georgia. In the same paper, Golovatch (1985) added new cave localities for C. lohmanderi, but also mentioned some other, at that time still undescribed, cave chordeumatidans. These and other species have only recently been examined by Antić & Makarov (2016), Golovatch & Makarov  Strasser, 1970. G. C. variabile Antić & Makarov, 2016. H. C. strasseri Antić sp. nov., paratype, ♂ (IZB). A-E, G after Antić & Makarov (2016), F after Strasser (1970. Scale bar = 0.3 mm for all, except for F (not to scale).
Species of the genus Caucaseuma include both cavernicolous and epigean forms. It is interesting that none of, not only Caucaseuma, but also other cave chordeumatidans from the Caucasus, are without ommatidia, while some cave-dwelling species have a large number of ommatidia. This was precisely why Golovatch (1985) considered them as probably troglophiles. However, with the increase in the number of described chordeumatidans from the Caucasus, both cave and epigean, it has become clear that most exclusively cave-dwelling taxa present certain troglobiontic features. These adaptations are reflected in the reduction in pigment of both body and ommatidia, in the reduction of the number of ommatidia in some, in the elongation of the antennae and/or walking legs, and a more robust and larger habitus (cave gigantism) (cf. Antić & Makarov 2016;Liu et al. 2017;Antić et al. 2018). These are best seen in cases where subterranean and epigean species occur within the same genus, as is the case with the genus Caucaseuma. All four presumed troglobiontic species (including C. strasseri Antić sp. nov.) are larger and more robust when compared to the four epigean congeners (16-20 mm vs 8-15 mm); they also have pale bodies and somewhat elongated legs and antennae. The number of ommatidia does not differ significantly between cavernicolous and epigean Caucaseuma, but it seems that in some cavedwellers they are, although darkly pigmented, still less pigmented compared to epigean forms. Within these presumed troglobiontic Caucaseuma, it appears that three species, C. lohmanderi, C. minellii and C. strasseri, show a similar degree of subterranean adaptation, which is mostly reflected in the partially depigmented body and elongated antennae and legs. On the other hand, C. elephantum has even more pronounced troglomorphic adaptations, which in addition to the above includes a densely setose gnathochilarium, as well as a reduced number of ommatidia compared to the remaining three cavernicolous congeners (11-12 vs 20-25) (see also Antić & Makarov 2016). Recently collected material of C. minellii showed that all adults have a brownish pigment dorsally, while the ventral parts are yellowish, and dark brown to almost black ommatidia. Thus, differences in the degree of adaptation can be noticed within the cavernicolous Caucaseuma species. A similar pattern of hypothetical gradual evolution in cave-dwelling Caucaseuma can be observed in the genus Heterocaucaseuma (see Antić et al. 2018). All this suggests that they are probably neotroglobionts, in the process of subterranean adaptation, rather than troglophiles. This is supported by the fact that these robust animals were not found on the surface, as well as that some populations were relatively large.
In addition to several representatives of the order Chordeumatida and the family Anthroleucosomatidae, there are a few other families of millipedes with cavernicolous members in the Caucasus. These are mainly individual troglobiontic representatives from the families Glomeridae Brandt, 1833, Glomeridellidae Cook, 1896, Trichopolydesmidae Verhoeff, 1910 and Blaniulidae C.L. Koch, 1847 (see Golovatch 1976Golovatch , 1985Enghoff 1984;Golovatch & Turbanov 2017. In the Caucasus, the only millipede family that can match the Anthroleucosomatidae in the number of troglobiontic species is the family Julidae Leach, 1814. Probably the most iconic genus of cave millipedes in this region is Leucogeorgia Verhoeff, 1930. Until recently, this genus included four species, three of which were known exclusively from caves (see Verhoeff 1930;Lohmander 1936;Golovatch 1983). Antić & Reip (2020) increased the number of species in the genus to 15, describing 11 new species, of which as many as 10 are troglobiontic. The genus Leucogeorgia, as well as the monospecific genus Martvilia Antić & Reip, 2020, includes some interesting forms that are characterized by highly modified mouthparts, and have a semiaquatic life, occasionally entering cave waters where they can presumably feed by filtering (Antić & Reip 2020). In addition, the genus Leucogeoria includes the world's deepest terrestrial arthropod ever found, Leucogeorgia profunda Antić & Reip, 2020, with one specimen collected in Veryovkina Cave at a depth of -2204 meters (Antić 2021).
In the last decade, the number of newly described and discovered troglobiontic millipedes in the Caucasus has increased significantly. This not only provides a greater understanding concerning regional subterranean fauna, but has also led to additional questions concerning the ecology and evolution in underground habitats. The description of another cavernicolous millipede in this paper clearly indicates that the number of cave representatives of this group in the Caucasus is far from fully known, and that we can expect new discoveries and surprises in the future.