Isotomidae of Japan and Asiatic part of Russia. II. The genus Tetracanthella of the Far East

Abstract The paper considers new and little-known species of the genus Tetracanthella distributed in the Far East of Russia and in Japan. Sensillar chaetotaxy and labial palp, two less known morphological characters for the genus, are discussed. Two new species T.annulatasp. nov. and T.tardokisp. nov. are described; T.manschurica Kutyreva, 1980 and T czernovae Kutyreva, 1980 are redescribed. For the latter species a lectotype and paralectotypes are designated. Remarks are provided for T.sylvatica Yosii, 1939. A second undescribed species is recorded for Japan. New records for T.orientalis Martynova, 1977 and T.sibirica Deharveng, 1987 are listed.


Introduction
Tetracanthella is a typically Holarctic genus and is one of largest in the family (Deharveng 1987). The maximal diversity is located in Europe where 80 species are known. The Asiatic fauna is less understood but is obviously not so rich. So far only 17 species are recorded in Asia. Our study is a result of examination of large collections coming from the Far East of Russia. In the area under study, the species of the genus is a rather rare and unpredictable component of Collembolan communities. Ecological niche of the genus is more limited here than in Europe: corticolous species absent, high mountain forms are very rare (T. tardoki sp. nov.). Most east-Asiatic species of Tetracanthella are damp litter dwellers. Taxonomically, the species belong to Asiatic or American groups ('sylvatica', 'stebaevae', and 'ethelae' groups). Few species (T. martynovae and T. sibirica) occurs only in the arctic zone of the Far East and belong to generally European 'wahlgreni' group. We list below all the species of the region, redescribe little-known species, and describe two new ones. This paper is our second special contribution to taxonomy of Asiatic species of Isotomidae of Russia and Japan (Potapov et al. 2018). Following our results, the fauna of Tetracanthella of the Far East of Russia and Japan is represented by eight and two species, respectively, including still undescribed forms.

Abbreviation used A, B, C, D, E
papillae of labial palp following notation of Fjellberg (1999)
abdominal segments Alt altitude Ant.
antennal segments Ap unpaired chaetae in anterior part of head B5, X chaetae on tibiotarsus 3 following notation of Deharveng (1983)  N. Kuznetsova p1, p3 chaetae of p-row on tergites PAO postantennal organ pc3 chaetae of p-row on head following notation of Deharveng (1987) pp chaetae of pp-row on head following notation of Deharveng (1987) s in the text and figures, macro s-chaeta(e) or s-chaeta(e) s' male s-chaeta on Ant.3 in lateral position Th.

Towards the knowledge of the taxonomic characters regarding the species of the Far East of Russia
S-chaetae on tergites. In his monograph Deharveng (1987) referred this character to be not of great value to identify the species of the genus. Number of s-chaetae is very conservative in the genus indeed, and probably all species obviously possess 3,3/2,2,2,2,4 s-chaetae, that should be confirmed since this is unknown for several species. The invariable set of s-chaetae is an additional confirmation of the monophyly of the genus. The only key taxonomic character of the genus is four anal spines on Abd.V that is not very safe if considering the independent appearance of spines in the evolution of the family (Deharveng 1978, Potapov et al. 2017 ('alpina','ethelae','cassagnaui',and 'wahlgreni' groups: Fig. 1) or between Mdl and Ml macrochaetae ('grinbergsi', 'stebaevae', and 'sylvatica' groups: Fig. 2). The former type is a characteristic of European and American groups while the latter one relates to Asiatic ones. This character was not indicated in the descriptions of all species of the genus and exceptions are possible. The European 'pilosa' group shows rather the "European-American" s-pattern although a few its members show considerable shift of medial s to lateral position. According to figures, at least, in T. doftana Fiera, Konikiewicz, Skarżyński, 2013, T. strenzkei Gisin, 1949, and T. gallica Deharveng, 1987, these s-chaetae are situated behind and lateral to Mdl on abdominal tergites (Deharveng 1987, Fiera et al. 2013). Labial palp. The character is poorly studied for the genus but appears to be promising at least at level of species group. After Fjellberg (1999, Fig. 51) and Smolis and Skarżyński (2006, Figs 12, 32) six Tetracanthella species ('wahlgreni', 'alpina', and 'pilosa' groups) from Scandinavia and Poland show reduced A(1)B(3)C(0)D(3)E(5) set in which four guards are lost. In East Asia the species of 'stebaevae' and 'sylvatica' groups have complete set [A(1)B(4)C(0)D(4)E(7)], and two young "afurcated" species of 'ethelae' group lost five guards giving A(1)B(3)C(0)D(3)E(4) formula (Figs 3 and 4, respectively).

Species of the 'sylvatica' group
Description. Body length 0.9-1.5 mm. Body cylindrical, not narrowing ( Fig. 6). Coloration spotty, from dark to light grey, ventral side of corpus paler, often not pigmented. Pigmentation of antennae vary, paler than other parts of the body, sometimes colorless. Largest polygons much larger than mesochaeta sockets, canals between polygons broad, clearly marked (Fig. 51). No smooth fields. Dorsal mesochaetae long, not shortened in axial part of tergites, in posterior row of Abd. IV not longer than on other parts of body (Md : p1 = 1.8-2.5). Abd. IV with p3 longer than p1 (p3 : p1 = 1.2-1.8) (Fig. 10). Macrochaetae usually blunt and plain at tip.
Etymology. The species is characterized by annulated posterior side of dens. Distribution and ecology. The species is widely distributed in southern areas of the Far East of Russia (Primorsky Krai, Khabarovsky Krai and Amurskaya District), both in flatland and in the mountains (Fig. 58). It prefers rotten wood although occurs in forest litter.
Discussion. The new species belongs to 'sylvatica' group and differs from all species of the group by absence of chaetae on anterior side of dens. The disproportion of anterior and posterior number of chaetae on dens (0 vs. ~7), clear humps on posterior side of dens and grey coloration make T. annulata sp. nov. unmistakable in the area of its distribution. Yosii, 1939 Figs 3, 5, 21, 22, 52, 58 Material. Japan, Honshu, Kyoto, Kamigamo experimental forest in Kyoto University, 2011, leg. S. Fujii.
Discussion. The remarks to chaetotaxy of the species were given by Yosii (1961), the complete redescription was provided by Deharveng (1987). After our materials, the species has complete set of guards in labial palp (Fig. 3) that is common for the species of eastern groups ('sylvatica', 'stebaevae', 'grinbergsi'). Tetracanthella sylvatica, T. annulata sp. nov., and T. dorsoduplex Xie, Potapov, Sun, 2019 combine a natural group of species distributed in East Asia. They share long furca with annulated dorsal side of dens, well developed reticulation with broad canals between polygons (Figs 51-52), and few macrochaetae on body tergites. Tetracanthella sylvatica differs from other two species by better develop-ment of medial macrochaetae on body (Figs 5,21,22), from T. annulata sp. nov. by the presence of anterior chaeta on dens, from T. dorsoduplex by common position of lateral s on Abd.IV. The specimens from Kamigamo have 3+3 postlabial chaetae that could be an additional differentiated character of the species if confirmed by wider Japanese materials.
Coxa I without an external chaeta. Tibiotarsi with 1,2,2 long and clavate dorsal tenent hairs and without well developed ventral tenent hairs. Tibiotarsi with many additional chaetae on all legs, tibiotarsi I and II with 26-28 chaetae each, III with more than 30 chaetae (Fig. 34). Empodial appendage 0.7-0.8 as long as inner edge of claw, with apical filament.
Distribution and ecology. Tetracanthella manschurica occurs in the mountains of Sikhote-Alin Range (Fig. 58). It is a rare species preferring coniferous litter.
Discussion. Tetracanthella manschurica was described from Lazovski district of Primorski Krai. Afterwards, it was recorded once with few morphological remarks by Potapov (2001). The only known type individual of the species is probably lost and our redescription is based on 15 specimens from three different districts of the same region. These specimens share several peculiar features and generally fit to the original description. Some discrepancy between the text of the first description and our observations is probably due to the juvenile condition of the holotype (Kutyreva 1980). Tetracanthella manschurica belongs to the 'sylvatica' group and differs from other species of the group (T. sylvatica, T. annulata sp. nov., and T. dorsoduplex by three chaetae on anterior side of dens, absence of crenulation on posterior side of dens, thin reticulation of cuticle, dark blue colouration and few chaetae on posterior side of manubrium and posterior furcal subcoxa. Tettracanthella manschurica additionally has very peculiar mucro which appears to have three or four teeth due to one or two lamellae. Regardless the lamellae, mucro of this species keeps the general bidentate pattern known in the genus. Population from the northern part of Sikhote-Alin Range (Vaninski district) show longer meso-and macrochaetae (Fig. 24), clearly clavate tenent hairs on legs, longer claws (dens:claw III = 3.1-3.6) and mucro (dens : mucro = 4.6-5.1 : 1) (Figs 30, 31). More southern populations (Fig. 58) (correspond better to the type specimens morphology because of shorter meso-and macrochaetae (Fig. 25), slightly (vs. clearly) clavate tenent hairs, short claws (dens : claw III = 4.1-4.8) and short mucro (dens : mucro = 6.7-7.1 : 1) (Fig. 29). We include both forms to the diagnosis of T. manschurica in view of the possible ecomorphic nature of the differences. One individual of unclear status from Kunashir Island (Alekhino, leg. I. Volonikhina) differs from continental populations by much shorter dens.
Distribution. The species is known only from type locality, by two specimens. Discussion. Tetracanthella czernovae belongs to the 'stebaevae' group due to chaeta on coxa I missing and complete set of macrochaetae on tergites. The species how-ever shares many essential characters, incl. appearance, with T. manschurica ('sylvatica' group). Tetracanthella czernovae was briefly redescribed by Potapov (2001) from two specimens supposed to be syntypes. These two specimens from the collection of E. Kutyreva did not have labels indicating type status. We designate two specimens collected by L. Kutyreva as lectotype and paralectotype. Tetracanthella czernovae resembles T. wui Xie, Potapov, Sun, 2019 but differs by having more setae on the dens (2-3/6 vs. 1/5).
One individual from central Honshu (Japan, Nagano Prefecture: Chino, leg. M.P. and N.K.) is close to T. czernovae but obviously represents a new species differing by absence of additional macrochaetae on body and presence of additional chaetae on Tibiotarsi I and II. It is the second species of the genus Tetracanthella occurring in Japan.
Etymology. The species is named after the type locality. Distribution and ecology. It is known only from the Tardoki-Yany mountain massive (central part of Sikhote-Alin Range) where it occurs in all samples from alpine sites which we have examined (Fig. 58).
Discussion. The new species belongs to the 'ethelae' group by absence of chaeta on coxa I, three sublobal hairs, two prelabral chaetae and other characters. Together with T. orientalis they are the only representatives of this Nearctic group in Palearctic. The two species share several apomorphic characteristics unknown in North American species: absence of furca, presence of the third macrochaetae in p-position on thorax, low number of axial chaetae, short empodium. Tetracanthella tardoki sp. nov. differs from T. orientalis by the presence of Md macrochaetae on Abd.II resulting in formula 2,3,3 (vs. 2,2,3) on abdomen. Martynova, 1977in Martynova et al. 1977 Material. Magadanskaya region: vicinities of Magadan, Snow Valley, 18.09.1974. It is the type locality of the species although the type specimens were not seen by us and are probably lost. Description. Body length 1.2-1.6 mm. Body slender, continuously narrowing (Fig. 40). Coloration dark, including antennae. Polygons large, canals between polygons well marked. Smooth fields usually present on all tergites of body (Fig. 49, 50). Posterior edge of head with smooth fields in lateral position (Fig. 50) or two groups of larger polygons (Fig. 57) in associated places. Area between ASi often with small smooth field. Dorsal mesochaetae short, slightly shortened in axial part of tergites (Fig.  49), in posterior row of Abd. IV not longer than on other parts of body (Md:p1 = 3.7-5.7). Abd. IV with p3 much longer than p1 (p3 : p1 = 2.3-4.4). Macrochaetae long.
Distribution. Tetracanthella orientalis is widely distributed in northern part of the Far East of Russia (Fig. 58). In Magadanskaya region and Kamchatka it is the only known species of the genus. In Chukotka, T. orientalis can be recorded together with T. sibirica which has the similar appearance but belongs to another group of species.
Discussion. See the remarks to T. tardoki sp. nov.