Two new species of the millipede genus Metonomastus Attems, 1937 from the Balkan Peninsula (Diplopoda, Polydesmida, Paradoxosomatidae)

Abstract In addition to the eleven previously known species of the Mediterranean genus Metonomastus, two more species are described: M.petrovisp. n., from the Rhodopi Mts. and Bunardzhik Hill in Bulgaria, and M.radjaisp. n., from the island of Mljet in Croatia. The relationships between the congeners and their distributions are briefly discussed. All 13 species of the genus are keyed.


Introduction
In the latest taxonomic survey of the genus Metonomastus Attems, 1937, Golovatch andStoev (2004) recognized eleven species. Among these, only one species, M. bosniensis (Verhoeff, 1901), was considered dubious as it had been described from a single female (holo-type) coming from within the distribution area of M. albus (Verhoeff, 1901), a fairly common and abundant congener. This Mediterranean genus includes only small-sized forms (< 10 mm long) ranging from the Apennine and Balkan peninsulas in the west to northwestern Anatolia in the east (Hoffman and Lohmander 1968;Golovatch and Stoev 2004).
The present paper is devoted to descriptions of two new species of Metonomastus found in the Rhodopi Mountains and Bunardzhik Hill in Bulgaria and the Dinaric island of Mljet in Croatia.

Preservation, dissecting, imaging, and terminology
Specimens preserved in 70% ethanol were examined with Nikon SMZ 745T and Zeiss Stemi 2000-C binocular stereo microscopes. All taxonomically important structures were dissected and mounted in glycerine as temporary microscopic preparations and observed with a Carl Zeiss Axioscope 40 and an Olympus BX51 light microscope. Pictures of legs were taken with a Canon PowerShot A80 digital camera connected to the Axioscope 40 microscope, and with an Olympus XC30 digital camera connected to the Olympus BX51 microscope. Line drawings of gonopods were made using tracing paper placed on a computer monitor showing pictures of those structures. Pictures of habitus structures were taken using a Nikon DS-Fi2 camera with a Nikon DS-L3 camera controller attached to a Nikon SMZ 1270 binocular stereo microscope, and with a Nikon Coolpix S3700 attached to one eyepiece of a Carl Zeiss Discovery.V8 binocular stereo microscope. Focal stacking was completed with Zerene Stacker software. Some relevant structures were investigated with JEOL JSM-6460LV (University Centre for Electron Microscopy, Department of Biology and Ecology, University of Novi Sad, Serbia) and JEOL JSM-5510 (Faculty of Chemistry, Sofia University) scanning electron microscopes.
The final images were processed with Adobe Photoshop CS6. Abbreviations used to denote gonopodal structures are explained directly in figure captions.

IZB
Institute of Zoology, University of Belgrade -Faculty of Biology, Belgrade, Serbia NHMSC Natural History Museum, Split, Croatia NMNHS National Museum of Natural History, Sofia, Bulgaria
Diagnosis. The new species belongs to the Metonomastus group of species with three acropodital branches, but clearly differs from both previously described species in this group, M. strasseri Hoffman &Lohmander, 1968, andM. pomak Golovatch &Stoev, 2004, by the presence of a strongly developed, beak-shaped solenomere, by the completely reduced solenophore and by the development of a strongly arched, microspiculate, ventromesal, acropodital process.
Collum with 3 rows of setae; two rows of similar setae per postcollum metatergum: one frontally, the other caudally, setae long and simple. Metaterga (Figure 2A, B) with a weak, mid-dorsal, transverse sulcus on rings 5 to 17; sulcus absent from 18 th . Paraterga (Figure 2A, B) laterally extremely poorly developed, being very faintly delimited by a shallow sulcus only dorsally. Certain midbody rings occasionally with visibly more or less strongly developed paranota compared to neighboring rings. Ozopores indistinct, located near posterior margin of paraterga; pore formula normal. Strictures between pro-and metazonae very faintly striolate, deep and narrow. Pleurosternal carinae present on ring 2 as small lobes, thereafter missing. Limbus faintly microcrenulate. Epiproct long, slender, nearly half as long as telson height. Hypoproct semi-circular, 1+1 strongly separated caudal setae borne on minute knobs. Sterna broad and weakly impressed.

Metonomastus radjai
Etymology. The new species is named after the collector, Tonći Rađa, a renowned Croatian biospeleologist who discovered numerous new or interesting arthropods. Noun in genitive case.

Diagnosis.
The new species belongs to the Metonomastus group of species with two postfemoral branches, but clearly differs from all of these by the presence of a well-developed, broad, lamellar solenophore (= tibiotarsus) directed strongly mesad, and proximally curved ventrad. The solenomere is without additional processes and is also directed strongly mesad.
Remarks. This species was found under stones next to a dry stone wall close to the seashore.

Discussion
Based on gonopod characters, species of Metonomastus may provisionally be divided into two groups. The first group is characterized by a two-branched acropodite, while the second group includes species with three branches. Such a division is also supported biogeographically. Taxa with two acropodital branches appear in the central Mediterranean: in northeastern and central Italy and along the Adriatic coast of the western Balkans. This group contains ten species, including Metonomastus radjai sp. n. Among these congeners, M. capreae (Verhoeff, 1942), M. hirtellus (Silvestri, 1903), M. mariae (Strasser, 1965), M. patrizii Manfredi, 1950, andM. romanus (Verhoeff, 1951) are all known from central Italy [M. hirtellus was recently found also in Croatia (Antić et al. 2018)], also showing a similar pattern of gonopodal structure. The prefemorite seems to be somewhat elongated, at least equal in length to the acropodite, the solenomere is a well-developed and more or less erect outer branch, with an additional small process in M. mariae and M. patrizii (this could also be true for the other three species?), while the inner branch, the solenophore (= tibiotarsus) is curved mesad. On the other hand, the very common and abundant M. albus, known from Slovenia, Croatia and Bosnia and Herzegovina, has gonopods with a much shorter prefemorite, while the solenophore is broad, lamellar, erect, obviously more strongly developed than the solenomere, with a deep rift between the two latter structures. Although our new island species, M. radjai sp. n., occurs geographically much closer to the coastinhabiting M. albus, it seems to show more similarities to central Italian forms, viz., a somewhat elongated prefemorite which is almost equal in length to the acropodite, the solenophore curved mesad, and a shallow rift present between the solenophore and solenomere. However, M. radjai sp. n. clearly differs by both acropodital branches being strongly curved mesad. In addition, the solenophore is very broad, lamellar, and with a ventrally curved proximal edge. Similarities in the gonopodal structure between M. radjai sp. n. and the mostly central Italian Metonomastus species could be evidence of a trans-Adriatic distribution of this (sub)group. Such a distribution pattern is already known not only in some millipedes, but also in such arthropods as isopods or endogean beetles (see Antić et al. 2018). Even if it has a habitus usual for the genus, the Albanian M. petrelensis Mauriès, Golovatch & Stoev, 1997 has gonopods with significantly different postfemoral branches from those of the above species. Both solenomere and solenophore are simple and slender, whereas the solenomere is mesal in position and spinulose distally. This species differs also from the above ones by the absence of adenostyles on the pregonopodal femora (Mauriès et al. 1997). If we consider the habitus, one species in this group is clearly distinguished from the others: M. saetosus (Strasser, 1960). It is characterized by very abundant and irregular metatergal pilosity, contrary to the presence of two or three transverse rows of setae in the other known Metonomastus species (the exception is M. bosniensis, reported to lack setae, but this observation requires verification). Originally, M. saetosus was placed in the genus Microdesminus Strasser, 1960, but it was synonymized under Metonomastus by Golovatch and Stoev (2004).
The second group within Metonomastus includes three species, viz. M. pomak, M. strasseri, and M. petrovi sp. n. All three species are characterized by the presence of three acropodital branches. Unlike the previous, central Mediterranean group, these taxa inhabit the eastern Mediterranean within Greece and northwestern Anatolia (M. strasseri) or the Rhodopian part of Bulgaria (M. pomak and M. petrovi sp. n.). In both previously described species, the solenophore appears as the main, long and curved branch above the solenomere which can be long and slender in M. strasseri or lamelliform and broad in M. pomak. The new species clearly differs from both of them by the presence of a strongly developed, robust and somewhat beak-shaped solenomere which appears as the main branch, while a solenophore is fully reduced.
As already stated by Golovatch and Stoev (2004), the somewhat disjunct distribution of the representatives of Metonomastus may be evidence of the group being ancient and relict. On the other hand, the very small bodies and a cryptic life in the soil or caves can also account for such a sporadic known distribution. Future research in the Mediterranean and southeastern Europe, including the application of various collecting techniques, will surely result in a considerable increase in the knowledge of millipede diversity, including further progress in our knowledge of the species richness and distribution of the genus Metonomastus.