Scutisotoma schisti Potapov & Babenko & Fjellberg 2006, sp.n.

Scutisotoma schisti sp.n.

Figs. 25, 87–98

Type material. Holotype: ♀, Russia, Siberia, Putorana plateau, Dynkengda Mt., vicinity of Sobach’e lake (69 o 19’N, 91 o 46’E), 700–750 m alt., alpine site with scarce vegetation, pitfall traps, 28.vii.– 13.viii.1996, leg. A. Babenko. Paratypes: 15 exx., same locality (MSPU).

Other material. 4 exx., Russia, Taimyr, Uboinaya River (73 o 37’N, 82 o 21’E), fresh flood debris in river bay, 09.viii.1990, leg. A. Fjellberg. Numerous specimens, Russia, Southern Siberia, SE Altai Mts., Kuray, Bol'shoy Kuraysky Range, 3100 m alt., on pools after the rain, 17.vi.1966, leg. S. Stebaeva. 5 exx., Russia, Southern Siberia, Tuva Republic, Mongun-Taiga Range, Mugur-Aksi, mountain tundra, 23.vii.1993, leg. S. Stebaeva. 6 exx., Russia, Southern Siberia, Tuva, Kup-Khol Lake, sedge bog, 20.vii.2001, leg. S. Stebaeva. 1 ex., Russia, Southern Siberia, West Sajan Mts., Usinsky Tract, Olen’ya River, mountain tundra, 31.v.1962, leg. S. Stebaeva (MSPU).

Description. Size 0.8–1.2 mm. Usually dark-violet, extremities paler. Cuticle with fine but visible primary granulation. Ocelli 8+8, G and H smaller. PAO elliptical, not constricted, about twice as long as ocellus diameter or 0.8–1.3 of inner unguis length (Fig. 90). Maxillary outer lobe with 4 sublobal hairs, maxillary palp bifurcate. Labral formula 4/ 554. Labium with a complete set of papillae and guards. Proximal field with 3 chaetae, basomedian field with 4 chaetae. Ventral side of a head with 4–6+4–6 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 5 distal s (AO and one lateral s). Guards of AO clearly longer than inner pair of sensilla (Fig. 25). Males with some additional sensilla on Ant.2–3. Sensilla on Ant.4 hardly differentiated, the shape of the subapical organite either big and spherical (Taimyr), or small (southern Siberia) (Figs. 93, 94).

All abdominal tergites clearly separated. Dorsal axial chaetom of Th.II–Abd.III as 9–10,6/5–6,5–6,5–6. Th.III with about 20 chaetae in p-row. Macrochaetae rather short, medial ones on Abd.V are 0.27–0.35 as long as tergite. Thorax without ventral axial chaetae. Sensillar formula 33/22224 (s) and 11/111 (ms). Medial sensillum on Abd.III set in posterior 1/3 of the tergite. On Abd.V the sensilla set in a line (Fig. 92).

Unguis simple, without inner tooth (Figs. 95, 96). Ti.1–3 usually with 21-21-25 chaetae, B-row complete. Tibiotarsal tenent chaetae (1-2-2) truncate or slightly clavate, about as long as unguis (U 3: t.ch. = 0.8–1.2: 1). Chaetae x and B 5 on Ti. 3 in male short and chaeta-like (Fig. 95). Ventral tube with 4+4 latero-distal and 5–7 posterior chaetae. Tenaculum usually with 3+3 teeth and one chaeta, sometimes with an additional small tooth on one side (Fig. 91). Anterior furcal subcoxa with 12–18(22) chaetae, posterior one with 6–10. Anterior side of manubrium with a pair of distal chaetae, posterior side with 17–21+17–21 on the main part and 4+4 on the laterobasal lobes. Dens with 12–14(16) anterior chaetae, basal 1/4–1/3 without chaetae (Figs. 87). Posterior side of dens crenulated, with 10–12(13) chaetae (5–6 basal, 2 (rarely 1 or 3) outer, 3 inner and 1 subapical chaeta near mucro (Figs. 88, 89). Mucro with 3 subequal teeth, lamellae hardly seen. Ratio of manubrium: dens: mucro = 5,2–7,4: 4,0–5,4: 1. Chaetotaxy of anal lobes as Figs. 97, 98.

Intraspecific variability. Specimens from Taimyr and southern Siberia differ in the size of the subapical organite (Figs. 93, 94), in the differentiation of preanal chaetae (Figs. 97, 98), and in some details of chaetotaxy of the posterior side of dens. We do not want to split the taxon at present, because of limited material.

Affinity. Using the recent key of Palaearctic Isotomidae (Potapov 2001) the new species may key out as Proisotoma christianseni Stach, described from high mountain regions of western Asia (Lebanon). About ten probable type specimens of christianseni (Lebanon, leg. Kowalski), deposited in the ISEA, Poland have been studied.

This species belongs to the genus Scutisotoma and differs from schisti by having simple maxillary palp, 5 basomedian chaetae on labium, very dense chaetal cover of Abd.V, and some other characters. The simple maxillary palp and modified chaetom of Abd.V indicate an affinity of S. christianseni not to S. schisti but to S. oirota and S. fjellbergi (for differences between these species see the key).

There is also one Nearctic species, Proisotoma bayouensis Mills, which has a furca similar to that of S. schisti (dens with 9–13 chaetae on anterior and posterior sides). Although the known differences of these species are rather minute or doubtful (4+4 tenacular teeth and a single tenent hair on each tibiotarsus in Proisotoma bayouensis), their different distributions indicate that they are separate species (bayouensis is only known from subtropical Louisiana).

Distribution. Scattered records from the high mountains in western Siberia.

Name derivation. The name reflects peculiarities of the type-locality — stony, alpine sites with schisty substrate.