Subisotoma tenuis (Dunger, 1982)

Figs 1, 29, 32, Tab. 2

Proisotoma (s.str.) tenuis Dunger, 1982: 51

Material. 2 paratypes, Mongolia, Central Aimak, sample 760 in Dunger (1982) (SMNG), 11 specimens, Russia, Buryatia, Vitim Plateau, Telemba, birch forest mixed with larch, 06.i.2008, leg. А. Chimitova; 7 specimens, Russia, Irkutsk Province, W Khamar–Daban Range, Cherskogo Peak, rocks, steep northern slope, cold mossy shelves in permanent shadow, 2000 m alt., 26.viii.2008, leg. M. Potapov, 1 specimen, Russia, Yakutia, middle reaches of Indigirka river, Ust'–Nera, 800 m alt., mosses and litter on stones, 21.vii.1992, leg. M. Potapov (MSPU); 1 specimen, Russia, Yakutia, Suntar–Khayata Mt. Range, Kyubyume River, 1250 m alt., bog, 30.vii.2002, leg. O. Makarova (MSPU).

Redescription. Size 1.2–1.3 mm. Colour almost-black, with numerous light spots. Ventral side and appendices also coloured. Cuticular big pits regularly scattered all over body, larger on head (as in Fig. 53). Ocelli 8+8, H and G clearly smaller. PAO small, narrow elliptical, without constriction, 1.5–2.0 as long as ocellus diameter and 0.6–0.8 as long as inner unguis. Maxillary palp simple, 4 sublobal chaetae present. Labral formula as 2/554. Labium with papillae A–E present, papilla Е with 6 guard chaetae (е 7 absent), proximal part of labium with 3 chaetae, basomedian and basolateral fields with 4 and 5 chaetae as usual. Ventral side of head with (4)5+5 postlabial chaetae. Ant.1 with 2 bms, dorsal and ventral, and 2 ventral sensilla (s), Ant.2 with 3 bms and 1 distal s, Ant.3 with 1 bms and 5 distal s, including 1 lateral (= АО), “inner” sensilla of АО clearly broadened. One additional sensillum present on dorsal side of Ant.3 (Fig. 29). Ant.4 with numerous poorly differentiated sensilla, subapical organite small, microsensillum present.

All terga with dense cover of short, uniform chaetae, macrochaetae not developed. Dorsal axial chaetom as 11–13,9–11/7–8,7–8,7–9. Tergal sensilla shorter and finer than ordinary chaetae. Sensillar formulas 33/ 22224 (s) and 10/101 (ms). Arrangement of sensilla and microsensilla as on Fig. 1. On Abd.I–III medial sensilla set well in front of the p-row. Ventral side of Th.III with 1+1 strong chaetae (Fig. 32).

Unguis normally with a weak tooth in the distal 2/3 of the inner edge. Ti.1–3 with additional proximal chaetae: 22–25, 24–27, 25–30 respectively. Tenent chaetae on Ti.1–3 1–2–2, long and clearly clavate, longer than inner unguis (U3: t.ch. = 0.55–0.70). The long ‘ ventrale Spürhaare’ on Ti.1–2 (chaeta B4) and on femora which was mentioned by Dunger (1982), are present as in the majority of other species of the genus. Chaetae x and B5 on Ti.3 undifferentiated in the studied males. Ventral tube with 4+4 laterodistal and 4 caudal chaetae. Tenaculum with 3+3 teeth and one chaeta. Anterior furcal subcoxae with 12–15 chaetae, posterior ones with 7–9. Anterior side of manubrium without chaetae, posterior side with 11–12+11–12 chaetae, laterobasal lobes usually with 4 chaetae. Dens rather long with one anterior and 4 posterior chaetae. Mucro clearly set off from dens, narrow and long, with two teeth and more or less clear lateral lamella. Ratio of manubrium: dens: mucro = 4.5–7.5: 2.5–4.0: 1. Each anal lobe with 3 tiny equal setulae.

Affinity. A study of the two paratypes of S. tenuis from Mongolia revealed that sensillar formula given in the original description (3,2/2,1(2),1,2,3) is not complete and the species is characterized by the common (for Anurophorinae) number and arrangement of s and ms. The presence of only 1+1 ventral chaetae on Th.III and the only additional sensillum on Ant.3 separate S. tenuis from other species of the group which have a similar number of macro- and microsensilla ( S. bisensillata sp. nov. and S. guzeriplica sp. nov.).

Distribution. Probably widespread in Inner Asia being recorded from central and southern parts of Mongolia, Buryatia, and Yakutia.