Subisotoma multisensillata Potapov, Babenko, Fjellberg & Greenslade, 2009, sp. nov.

Subisotoma multisensillata sp. nov.

Folsomides pusillus: Stebaeva 1973 Figs 8, 51–57, Tab. 2

Type material. Holotype, male, Russia, northern Altai, right bank of Katun’ river, Ust’–Sema [51° 39’N, 85° 45’E], plant debris on sandy bank, 27.vii. 2000, leg. S. Matalin (MSPU). Paratypes: 52 specimens, Southeastern Altai, Bolshoi Kuraiskii Mt. Range, 3,100 m. alt., mountain tundra, 19.viii.1963; 10 specimens, same region, 19.vi.1964; 15 specimens, same region, 27.vii.1963; 27 specimens, same region, 12.vii.1964, leg. S. Stebaeva; 1 male, Mongolia, somon Khank, Eastern Sayan, 2800 m alt., steppe meadow with Festuca sp, leg. A. Druk (MSPU).

Description. Size up to 1.5 mm. Colour dark, almost-black, with numerous lighter spots. Cuticle with strong pits (larger on head) (Figs 53, 57). Ocelli 8+8, H and G clearly smaller (Fig. 52). PAO elliptic, without constriction, about 1.5 as long as ocellus diameter and 0.6–0.8 as long as inner unguis. Maxillary palp simple with 4 sublobal chaetae. Labrum with 2/554 chaetae. Labium with all papillae A–E present, papilla E with 6 guard chaetae (e7 absent), proximal chaetae 3. Basomedial field of labium with 4 chaetae, basolateral with 5. Ventral side of head usually with 4+4 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 in АО. Inner sensilla of АО clearly broadened. Dorsal side of Ant.3 with 2–4 additional sensilla set in two separate groups (Fig. 55). Ant.4 with numerous poorly differentiated sensilla, subapical organite small, microsensillum present.

All terga with dense cover of short uniform chaetae, macrochaetae poorly developed. Longest chaetae on Abd.V 0.15–0.22 as long as tergal length. Dorsal axial chaetom of Th.II–Abd.III as 10–11,8–9/7,7–8,7–8. Tergal macrosensilla rather short, clearly differentiated from ordinary chaetae. The total number of sensilla is not stable and may vary even in a single specimen: 4–7,6–12/4–5,3–6,3–5,4,5–11, grouped into clusters corresponding to usual formula 33/22224 (Figs 8, 51) (holotype with 77/46548 on one side). The number and position of microsensilla as usual: 10/101. Ventral side of Th.III always with 1+1 chaetae.

Unguis with small tooth in distal 2/3 of inner edge. Ti.1–3 with 21, 21, 25 chaetae, slightly variable only on Ti.3 (24–26). Tibiotarsal tenent chaetae 1–2–2, clearly clavate, longer than inner edge of unguis (U3: t.ch. = 0.55–0.72). Chaetae x and B5 on Ti.3 slightly modified. Ventral tube with 4+4 lateral and 4 posterior chaetae. Tenaculum with 3+3(4) teeth and one chaeta. Anterior furcal subcoxae with 9–11, posterior ones with 7–9 chaetae. Manubrium without anterior chaetae, posterior side with 10–14+10–14 chaetae, laterobasal lobes with 3(4) chaetae (Fig. 54). Dens with one anterior and (3)4 posterior strong chaetae (Fig. 56). Mucro narrow and rather long, with 2 teeth, not clearly set off from dens. Ratio of manubrium: dens: mucro = 4.3–6.5: 2.4– 3.8: 1. Each anal lobe with 3 tiny equal setulae.

Affinity. The most notable features of S multisensillata sp. nov. are the number and position of tergal sensilla, while other morphological characters are of a more general nature. The sensillar pattern of this species is a result of the duplication or triplication of each sensillum of common sensillar set. This pattern is unique among isotomids. Only Anurophorus koreanus Potapov possibly shows the same trend in particular groups of sensilla (see Fig. 14 in: Potapov 1997). Other ‘polysensillar’ species considered in this paper, e.g. Isotopenola delicata sp.n. and Isotopenola loftyensis, have a normal pattern of scattered sensilla.

Distribution. The species is common in different parts of Altai–Sayan Mountains, being a permanent inhabitant of tundra-like communities of the alpine belt, more rarely found in lowlands. Name derivation. The name reflects a conspicuous feature of the new species, the increased number of tergal sensilla.