Nienna chukotka sp. nov. (Protura, Acerentomidae, Nipponentominae) from the Arctic region, with a key to species of the genus

Abstract A new species of Nienna was collected in the most northern part of the Palearctic, inside the Arctic Circle. In possessing seta Pc on tergite VII and sternites VI–VII and a very long foretarsal sensillum a, Nienna chukotkasp. nov. is more similar to Alaskaentomon species than to the other Nienna species distributed in southern Siberia and northern China. The new species differs from nearly all other members of Nipponentominae in possessing five anterior setae on tergite VII and in the presence of posterolateral pores on tergite I, as in members of Hesperentomon (Hesperentomidae). An identification key to Nienna species is provided.


Introduction
The proturan genus Nienna Szeptycki, 1988 was created for Nienna parvula Szeptycki, 1988, described from the Altai mountains in southern Siberia (Szeptycki 1988). The genus differs from the 12 other genera of Nipponentominae Yin, 1983 in possessing a small, indistinctly granulated calyx and a short posterior filament on the maxillary gland, and in the small, nearly globular foretarsal sensillum t3. A second species, Nienna quinghaiensis Bu & Yin, 2008, was described from northern China. The diagnosis of the genus was recently updated (Galli et al. 2018). In the current paper, the description of a third species of Nienna is given. The type specimens, collected from the Arctic region, are the northernmost records for any Protura. A key to the species of Nienna is given.

Materials and methods
Protura specimens collected from western Chukotka in 2018 were extracted from soil samples with Berlese-Tullgren funnels into 95% ethanol. The specimens were mounted on glass slides in Faure's medium (Dunger and Fiedler 1989).

Results
The genus Nienna is characterized by three pairs of A-setae on the mesonotum and metanotum, small, indistinctly granulated appendices on the calyx and a short posterior filament on the maxillary gland. The foretarsal sensillum t1 is filiform, sensillum t3 is small and globular (lanceolate in N. quinghaiensis Bu &Yin, 2008), the position of sensillum d is close to the base of e, and seta β1 is setiform. Sensillum a' is distal to the base of t2. Sensillum b' is missing. The genus is similar to twelve other genera from the subfamily Nipponentominae in having abdominal legs with 2 nearly equal setae, 5 pairs of A-setae on tergites II-VI (except for Alaskaentomon Nosek, 1977 and Nanshanentulus Bu & Yin, 2007) and by the posterior position of seta P3 on abdominal tergites II-VI (except for A. fjellbergi Nosek, 1977) (Bu and Yin 2007;Bu et. al. 2013;Galli et al. 2018;Nosek 1977Nosek , 1981Shrubovych 2009Shrubovych , 2011Shrubovych , 2014Shrubovych and Smykla 2012;Shrubovych et al. 2014a, b, c). Diagnosis. Nienna chukotka is characterized by 3 pairs of A-setae on the mesonotum, metanotum and tergite VIII, 3 A-setae on sternites I-VII, absence of P1a setae on tergites I-VI, 5 pairs of A-setae on tergites II-VII, absence of A2 on prosternum, presence of seta Pc on tergite VII and sternites VI-VII, and presence of additional d6 setae on head. Foretarsal sensillum a is broadened, very long, surpassing the base of sensillum e. Posterolateral pores (pl) present on tergite I, psl pores present on tergites VI and VII, asymmetrical spsm pores present on sternites IV-VII.
Abdominal legs with 4, 2, 2 setae. Subapical and lateral apical setae on second and third pairs of abdominal legs nearly equal in length, 15 and 14 μm, respectively  Abdominal segment VIII with distinct striate band; tergite and sternite anteriorly with irregular small teeth (Figs 2 I, M). Pore psm without accompanying teeth. Posterior margin of sternite VIII and laterotergites smooth. Comb VIII with 9-10 small teeth (Fig. 1I). Seta 1a on tergite IX half the length of seta 1. Seta 2a on tergites IX and X shorter than other setae. Sternites IX-X with traces of striate band (Fig. 2N). Setae 1 and 2 on sternite IX of equal length, on sternite X seta 1 about half the length of seta 2 (Fig. 2N). Medial pore on dorsal lobe of segment XII and pair of sal pores on ventral lobe. Hind margin of dorsal lobe smooth, ventral lobe with fine serrations (Fig. 2O).
Chaetal variability. In the holotype, seta P4 is doubled asymmetrically on the mesonotum; in the paratype, seta A3 is absent symmetrically on tergite I and seta P2a is doubled on tergite VII.
Etymology. The species name is taken from the general locality where the specimens were collected.
Remarks. Nienna chukotka sp. nov. differs from N. parvula and N. quinghaiensis in the presence of seta Pc on tergite VII and sternites VI-VII (in N. quinghaiensis seta Pc is present on sternite VII only), the presence of 5 pairs of A-setae (4 pairs in the other two species) and P3a on tergite VII, the shape of the accessory setae on tergites and sternites I-VI (setiform in the new species, sensilliform in the other two species) and the shape of foretarsal sensilla a, c and e (in the other species sensillum a is shorter, reaching base of sensillum t2, sensilla c and e short and broad). The porotaxy of mesoand metanota and abdominal sternites also differs: Nienna chukotka has two pairs of sl and al pores on the meso-and metanota, and asymmetrical spsm pores on sternites IV-VII;], whereas N. parvula has a pair of sl pores on the meso-and metanota, and a simple spm pore on sternites VI-VII. Nienna quinghaiensis has al and l pores on the mesonotum, l pores on the metanotum, and an spm pore on sternite VII. The new species is more similar to N. parvula in possessing traces of a striate band on sternites IX-X and in the globular foretarsal sensillum t3. Nienna chukotka is characterized by the presence of pl on tergite I, which is the first report of posterolateral pores in Acerentomidae. Szeptycki (1988) previously described pl pores on Hesperentomon martynovae Szeptycki, 1988 (Hesperentomidae) collected in the Altai Mts. These pl pores have also been recorded in other Hesperentomon species: H. fopingense Bu, Shrubovych & Yin, 2011, H. nanshanensis Bu & Yin, 2007, H. xiningense Bu & Yin, 2007distributed in China, and H. tianshanicum Martynova, 1970(Shrubovych 2010.

Discussion
The foretarsus of N. chukotka sp. nov. has a very long sensillum a, surpassing the base of sensillum e, and filiform foretarsal sensillum t1, characters shared with two species of Alaskaentomon (A. fjellbergi, A. condei). These two Alaskaentomon species possess seta Pc on tergite VII and sternites VI-VII. Alaskaentomon spp. differ from N. chukotka sp. nov. in having two pairs of A-setae on the meso-and metanota and large granulated appendices on the calyx of the maxillary gland (Shrubovych et al. 2014c). In notal chaetotaxy (three pairs of A-setae) and the filiform sensillum t1, the genus Nienna is similar to the genera Callientomon Yin, 1980, Noldo Szeptycki, 1988, Paracerella Imadaté, 1980and Verrucoentomon Rusek, 1974. However, Nienna differs from all of them in possessing small, indistinctly granulated appendices on the calyx of the maxillary gland and in the small, nearly globular foretarsal sensillum t3 (Shrubovych et al. 2014a). The new species differs from nearly all species of Nipponentominae in possessing a pair of A1 setae on tergite VII (five pairs of A-setae), while nearly all other nipponentomines have four pairs of A-setae (except Nipponentomon macleani Nosek, 1977 from Alaska, which also has 5 pairs of Asetae). Therefore, N. chukotka sp. nov. from Chukotka is more similar in body chaetotaxy and in foretarsal sensilla pattern to members of other genera distributed in Alaska than to the other Nienna species distributed in more southern regions of the Palearctic. This peculiarity could be an effect of subsequent allopatric speciation resulting from successive closings of the Bering Strait and cooling of the Arctic Ocean during the Pliocene-Pleistocene. Another interesting fact is that species recorded on the northern edge of proturan distribution (only a few Protura species are known from the Arctic region) possess a larger number of setae on the body than species with a more southern distribution.