Paravietnura gen. n., a new intriguing genus of Neanurini from the Caucasus (Collembola, Neanuridae, Neanurinae)

Abstract Paravietnura gen. n. is described and established for two new species of Neanurini from the Caucasus. The new genus is characterized by an unusual combination of features: the fusion of all lateral tubercles on the head into a single mass, the strong reduction of chaetae on the head, the fusion of cephalic tubercles Af and Oc into a transverse band, the absence of labial chaetae f, the presence of microchaetae on furca rudimentary, and the penultimate abdominal tergum with two tubercles separated along the midline. Paravietnura gen. n. strongly resembles Vietnura Deharveng & Bedos, 2000, a monotypic genus up to date known only from Vietnam. The main characteristics of Paravietnura notabilis sp. n. include an ogival labrum, the absence of chaetae A on the head, relatively thick and widely sheathed long macrochaetae, and minute microchaetae without chaetopores on furca rudimentary. Paravietnura insolita sp. n. differs from the previous species in chaetotaxic details, the size of furcal microchaetae, and the shape of the labrum. Short comments on the generic diversity of the tribe in the Western Palaearctic are also provided.


Introduction
The subfamily Neanurinae, with more than 800 recognized species, is certainly one of the richest and most diversified taxa among springtails (Collembola). It is also one of the most recognizable ones within Collembola as its members possess very characteristic cuticular tubercles on the dorsal side of the body and completely lack the furca, the organ typical for most described springtails. Regarding the taxonomy of Neanurinae, all species are classified into six established tribes (Cassagnau 1989). Among them, Neanurini is the second largest, after Paleonurini, with over 170 species belonging to 23 genera (Cassagnau 1989, Hopkin 1997, Deharveng and Bedos 2000, Deharveng et al. 2007, Smolis 2007, 2011, Mayvan et al. 2015. The number of eyes, the shape of mandibles, as well as the number and arrangement of tubercles on the body are typically used to separate the genera within the tribe. In the lateral part of the head, for instance, there are four main types of tubercle arrangements. The commonest situation is with lateral tubercles L and So fused but tubercle Dl separate. In turn, a complete fusion of all lateral tubercles seems to be the rarest option, only observed in two species of Neanurini, Vietnura caerulea Deharveng & Bedos, 2000, and Monobella cassagnaui Deharveng, 1981.
The examination of rich Neanurinae materials from the north-western Caucasus has revealed two unknown species. They belong to the mentioned tribe because of a presence of blue hypodermic pigment, the last abdominal segment bilobed and well developed tubercles on the body. Interestingly, both the undescribed taxa are characterized by the complete fusion of the outlined cuticular structures on the lateral part of head. This and other features of these species support proposal of a new genus within the tribe. Its detailed and illustrated description is provided with comments on the present stage of knowledge on Neanurini diversity in the Western Palaearctic region.

Materials and methods
The specimens were cleared in Nesbitt's fluid, subsequently mounted on slides in Phoera liquid and studied using a Nikon Eclipse E600 phase contrast microscope. Figures were drawn with camera lucida and prepared for publication using Adobe Photoshop CS3. Etymology. The name "Paravietnura" refers to its strong similarity to Vietnura. Remarks. The following characters: the presence of 2+2 eyes, the fusion of all lateral tubercles into a single mass on the head, the fusion of cephalic tubercles Af and Oc into a transverse band, the absence of cephalic chaetae Di2 and De2 and the presence of strong cryptopygy place Paravietnura gen. n. very close to Vietnura, the genus established by Deharveng and Bedos (2000) for a single Vietnamese species, V. caerulea Deharveng & Bedos, 2000. As both these genera are distributed in separate biogeographical regions, Paravietnura gen. n. in the Western Palaearctic and Vietnura in the Oriental region, it is recognized that this similarity is probably the result of convergence. Furthermore, they differ in a few essential characters, important from the taxonomic point of view: presence/absence of cephalic chaetae Ocp (in Paravietnura gen. n. present, in Vietnura absent), presence/absence of labial chaetae f (absent in Paravietnura gen. n., Figs 18, 19; present in Vietnura, Fig. 21), the number of tubercles on Abd. V (two tubercles (Di+De+Dl) in Paravietnura gen. n.; three tubercles: 2 (De+Dl) and (Di+Di) in Vietnura), and presence/absence of microchaetae on furca rudimentary (present in Paravietnura gen. n.; absent in Vietnura).

Institutions of depository of materials
Because of the fusion of lateral tubercles on head, Paravietnura gen. n. resembles Monobella cassagnaui Deharveng, 1981, the species belonging to the European genus Monobella Cassagnau, 1979. Nevertheless, this similarity seems to be definitely superficial given the fact that M. cassagnaui differs from Paravietnura gen. n. in a number of characters e.g.: fusion of tubercles (Di+De) on head along midline (separate in Paravi-etnura gen. n.), presence of chaetae Di2 and De2 on head (absent in Paravietnura gen. n.), fusion of tubercles Di and De on Th. II-III and Abd. I-II (separate in Paravietnura gen. n.), fusion of tubercles Di on Abd. III along midline (separate in Paravietnura gen. n.) and presence of one tubercle (2Di+2De+2Dl) on Abd. V (two tubercles (Di+De+Dl) in Paravietnura gen. n.). Etymology. The name reflects the notable morphology of this new generotype species. Diagnosis. Body stumpy and relatively short. Macrochaetae long, thick and widely sheathed. Buccal cone long, labrum ogival. Labrum with two prelabral chaetae. Tubercle (Af +2Oc) on head with chaetae B, Ocm and Ocp, chaetae A absent. Tubercles (Dl+L+So) on head with ten chaetae, chaetae So2 absent. Furca rudimentary with minute and difficult to detect microchaetae, without chaetopores.
Etymology. Its name reflects a later discovery of another species within the genus (Latin word "insolita" means not lonely).
Legs. Chaetotaxy of legs as in Table 6. Claw without internal tooth. On tibiotarsi, chaeta M present and chaetae B4 and B5 relatively short and pointed.
Remarks. Paravietnura insolita sp. n. can be confused with the only other described species in the genus Paravietnura notabilis sp. n., which has thick and widely sheathed long macrochaetae (distinctly thinner and narrowly sheathed in insolita), an ogival labrum (non-ogival in insolita), chaetae A and So2 reduced on the head (present in insolita), chaetae L' reduced on Abd. V (present in insolita), and the furca rudimentary with minute microchaetae without chaetopores (longer microchaetae with distinct chaetopores in insolita).
Ecological note. Similarly to the previous species, Paravietnura insolita sp. n. seems to be very local and connected with specific climatic and vegetation conditions, probably with mountain xeric pine forest.
Variabilty. We observed an asymmetrical absence of chaeta A in the holotype (Fig. 9).

Discussion
As noted in the Introduction, the tribe Neanurini, containing more than 170 species in 23 genera, is the second largest within the subfamily Neanurinae. Paradoxically, in spite of such a large number of known species belonging to the tribe, knowledge about its distribution and diversity seems to be still incomplete and far from satisfactory. For example, the largest generic and species diversity of Neanurini occurs in the Western Palaearctic, where currently 18 genera and nearly 150 species are known. It should be mentioned that the tribe in this region absolutely dominates and constitutes more than 80% of all genera and species of Neanurinae. The other four Neanurinae tribes are represented only by single native species (Paranurini, Sensillanurini), single introduced species (Lobellini), and a few genera with relatively small numbers of species (Paleonurini). However, presently the picture of the generic distribution of Neanurini is highly uneven as its seven genera are known exclusively from Europe (Albanura Deharveng, 1982;Cansilianura Dallai & Fanciulli, 1983;Catalanura Deharveng, 1983;Lathriopyga Caroli, 1910;Monobella Cassagnau, 1979;Neanurella Cassagnau, 1968;Pumilinura Cassagnau, 1979), and the next seven taxa (Balkanura Cassagnau, 1979, Cryptonura Cassagnau, 1979Deutonura Cassagnau, 1979;Endonura Cassagnau, 1979;Neanura MacGillivray, 1893;Protanura Börner, 1906;Thaumanura Börner, 1932) are present both in Europe and in areas around it, e.g., Asia Minor, the Middle East, the Caucasus, or North Africa. That means that as many as 14 genera of the tribe, of 18 known in the western part of the Palaearctic, are present in Europe. Fortunately, to better understand the distributional pattern of Neanurini, a substantial number of studies dedicated to poorly investigated areas outside the continent have been undertaken during the last three decades. They have resulted the descriptions of four unknown genera, i.e. Caucasanura Kuznetsova & Potapov, 1988;Edoughnura Deharveng, Hamra-Kroua & Bedos, 2007;Ghirkanura Kuznetsova & Potapov, 1988; Persanura Mayvan, Shayanmehr, Smolis & Skarżyński, 2015; and many new species and records of known taxa (e.g., Kuznetsova and Potapov 1988, Deharveng et al. 2007, Smolis and Kaprus' 2009, Mayvan et al. 2015, 2012, 2016a, b, c, 2017. These papers, including the work presented here, show that the diversity of Neanurini in some regions of the Western Palaearctic is still underestimated, and that this diversity can be crucial to assessing the history of this megadiverse tribe of Nenurinae.