Figs. 114–118
Material. Holotype: ♀, Chukotka, Magadan Province, Snow Valley, moss/lichens on rock, 20.viii.1979, leg. A. Fjellberg. Paratypes: 10 exx., same sample (MSPU).
Description. Size 0.5–0.6 mm. Colour unknown (only cleared specimens seen). Cuticle with fine but visible granulation. Ocelli 6+6, of subequal size, G and H absent. PAO wide elliptical, sometimes with unclear constriction, slightly longer than U 3 (1.0–1.2) (Fig. 116). Maxillary outer lobe with 4 sublobal hairs and bifurcate maxillary palp. Labral formula 4/554. Labium with a full set of papillae and guards. Proximal field with 3 chaetae, basomedian field with 4. Ventral side of a head usually with 4+4 postlabial chaetae. Ant.1 with 2 basal microsensilla (bms, dorsal and ventral) and 2 ventral sensilla (s). Ant.2 with 3 bms and one laterodistal s, Ant.3 with one bms and 6 distal s (AO and two lateral s). Guards of AO clearly longer than inner sensilla. Additional sensilla on Ant.2–3 present in males. Sensilla on Ant.4 hardly differentiated, subapical organite small.
All abdominal tergites clearly separated. Dorsal axial chaetom of Th.II–Abd.III as 5,5–6/3,3–4,4. Thorax without ventral axial chaetae. Macrochaetae practically undifferentiated, medial ones on Abd.V are as long as 0.3–0.4 of tergal length. Sensilla on tergites almost as long as ordinary chaetae (Fig. 118). Sensillar formula 33/22224 (s) and 11/111 (ms). Microsensillum on Abd.III set close to lateral sensillum (Fig. 117). Sensilla on Abd.I–III in mid-tergal position (medial sensillum on Abd.III in posterior quarter of the tergite). Sensilla on Abd.V set in a line, medial pair (accp1 and accp2) often slightly longer (Fig. 118).
Unguis simple, without inner tooth. Ti.1–3 with the basic set of chaetae (21-21-25). Brow on Ti.1–2 with 7 chaetae. Chaetae x and B 5 on Ti. 3 in males are short and stick-like. Tibiotarsal tenent chaetae (1-2-2) more or less clavate, slightly longer than unguis (U 3: t.ch. = 0.8–1.0: 1). Ventral tube with 4+4 laterodistal and 5–6 posterior chaetae. Tenaculum with one chaeta and usually 3+3 teeth, but additional tooth sometimes present on one or both sides. Anterior furcal subcoxa with 7–9, posterior with 5 chaetae. Furca rather short. Anterior side of manubrium with 1+1 proximal chaetae, its posterior side with 8–12+8–12 chaetae on main part and 3+3 chaetae on lateral lobes (Fig. 114). Dens with one subapical chaeta on anterior side and 3(2) posterior ones (Figs. 114, 115). Mucro with two subequal teeth and a clear lateral lamella. Ratio of manubrium: dens: mucro = 4.6–5.7:2.3–2.4:1.
Affinity. Due to reduced number of ocelli, a large, broad PAO and short furca with a few chaetae on dens S. dodecocellata has an isolated position in the Proisotoma complex. It is similar to Weberacantha by the long mucro in relation to dens (mucro/dens ratio 0.4–0.5:1). The only significant differences between Weberacantha octa Christiansen and our new species are the long and narrow PAO, spine-like chaetae on Abd.V and more than two mucronal teeth in W. octa. The only other Asian species of the complex with 6+6 ocelli is Weberacantha beckeri (Stebaeva), comb.n., which differs by having different furcal chaetotaxy and a narrow PAO. Proisotoma brevidens Stach, the European species with the shortest furca, is characterised by 8+8 ocelli, more dental chaetae (6/3), complete absence of microsensilla and a different position of sensilla which is typical for the genus Proisotoma.
The unrelated S. indigirka sp.n. has a similar chaetotaxy of dens but a different mucro, 3 prelabral chaetae, and a simple palp on maxillary outer lobe.
Proisotoma intermedia Izarra (Southern America) is characterised by the same number of anterior chaetae on dens and manubrium as the new species. It has 5 ocelli and a much slender dens.
Distribution. Known only from the type locality in the Russian Far East.
Name derivation. The name reflects number of ocelli.