Holotype, female: EAST SIBERIA, YAKUTIYA, Khalerchinskaya Tundra, Chukochiya River, mosses in shrub tundra, IX 1975, leg. E.V. Gordeyeva (ZIRAS).

Paratypes: 5 females, 1 male, 2 N2, EAST SIBERIA, Suntar-Khayata Range, upper reaches of Kyubyume River, 63°13'N, 139°36'E, 1500 m a.s.l., spotty tundra, 29.VII 2002, leg. O.L. Makarova (CNC); 1 male, same district and collector, 1960 m a.s.l., snow bed near small stream, sedge turf, 1.VIII 2002; 1 female, delta of Indigirka River, Russko-Ustinskaya Protoka, hummocky Dryas-forb tundra, 14–16.VII 1994, leg. A.B. Babenko; NOVAYA ZEMLYA ARCHIPELAGO, 1 male, Southern Island, Pan’kova Zemlya, lower part of slope, sedge-willow-moss tundra, VIII 1995, leg. S.V. Goryachkin; 1 female, 1 male, Northern Island, Krestovaya Bay, tundra, VIII 1993, leg. V.I. Bulavintsev; 1 female, BOL’SHEZEMEL’SKAYA TUNDRA, Vorkuta Region, Sivaya Maska vicinity, tundra, 20.VII 1978, leg. N.A. Kuznetsova; 3 females, 1 male, KOLA PENINSULA, Khibiny Mountains, Eastern Petrelius Pass, Dryas-sedge-moss tundra, 28.VI 2006, leg. A.B. Babenko (ZIRAS).

Other material: NORTH-EAST ASIA, 5 females, 1 male, Chukotka, Daurkin Peninsula, lower reaches of Chegitun River, forb-sedge-moss tundra, 30.VIII 1996, leg. M.V. Berezin; 5 females, 3 males, Magadan Region, upper reaches of Kolyma River, Peak Aborigen vicinity, Pinus pumila litter, 24.VIII 2006, leg. A.V. Alfimov; 1 female, Olskoye Plateau, upper reaches of Ola River, 60°39'N, 151°16'E, 1220 m a.s.l., forb meadow, 11.VIII 2011, O.L. Makarova; EAST SIBERIA, delta of Kolyma River, Pokhodskaya Edoma, 1 female, litter in lower part of slope; 1 female, wet grassy meadow, 18.VII 1994, leg. A.B. Babenko; delta of Yana River, Shirokostan Peninsula, vicinity of Ledyanoye Lake, 72°25'N, 141°00'E, 1 female, south slope of valley, forb-grassy meadow; 1 male, high river terrace, Dryas-Carex tundra, 4–6.VIII 1994, leg. A.B. Babenko; 3 females, 1 N1, Yana Bay, Makar Island, 6.VIII 1985, leg. V.I. Bulavintsev; Suntar-Khayata Range, upper reaches of Kyubyume River, 3 males, 1 N2, 1300 m a.s.l., lichen Larix-forest, litter, 25.VII 2002, 2 females, 1 male, 1 N1, 1800 m a.s.l., snow bed, under Rhododendron aureum, 2 males, 5 N2, 2 N1, 1800 m a.s.l., hummocky Eriophorum-community, 28.VII 2002, 1 female, 1 N2, 1800 m a.s.l., lichen-willow tundra, 9.VII 2002, 1 female, 1 N2, 1 N1, 2000 m a.s.l., mossy tundra, 15.VII 2002, 4 females, 3 males, 2 N2, 1 N1, Dryas-Carex-Eriophorum bog, 19.VII 2002, leg. O.L. Makarova; 1 female, Cherskogo Range, Ust-Nera vicinity, 500 m a.s.l., grassy meadow near stream, 22.VII 1992, leg. N.A. Kuznetsova; TAYMYR PENINSULA, 1 male, N Taymyr, Khariton Laptev Coast, Opalovaya River, seaside sand-rubble terrace with Salix polaris, 15–17.VIII 1994, leg. A.B. Babenko; Taymyr Lake, Cape Blizhnyi, 2 females, Tetraplodon moss cushion in tundra, 28.VII 1994, 1 female, lemming hill within stony tundra on elevation, grassy turf, 10.VIII 1994, 2 females, lemming burrow, 17.VIII 1994, 5 females, 1 N1, lemming hill within spotty tundra, grassy turf, 17.VIII 1994, leg. O.L. Makarova; 1 female, NW Taymyr, upper reaches of Kolomejtseva River, lemming hill in spotty tundra, grassy turf, 21.VIII 1997, leg. O.L. Makarova; 2 females, 1 male, NW Taymyr, Ragozinka River, spotty tundra, 9.VII 1986, leg. A.B. Babenko; 1 female, NW Taymyr, mouth of the Tareya River, 73°15'N, 90°35'E, dry Dryas-community, 22.VII 2010, O.L. Makarova; 1 female, SW Taymyr, vicinity of Pyasino Lake, Nyapan’ Upland, zoogenic meadow on a hill, 16.VII 1999, O.L. Makarova; VAIGATCH ISLAND, 1 male, no other data, VII 1984, leg. V.I. Bulavintsev; NOVAYA ZEMLYA ARCHIPELAGO, 1 male, no other data, leg. V.I. Bulavintsev; BOL’SHEZEMEL’SKAYA TUNDRA, 3 females, Yugor Peninsula, Cape Belyi Nos, 25.VI 1983, leg. V.I. Bulavintsev; 1 female, Vorkuta city vicinity, IX 2009, leg. E.M. Perminova; 1 female, Pechora Bay, Kuznetskoye Lake, polar fox hill, litter under Cornus suecica, 25.VIII 1994, leg. A.B. Babenko; KOLGUEV ISLAND, 3 females, 1 male, 2 N2, 1 N1, tundra litter, VII 2011, leg. S.B. Rosenfeld; SOUTH SIBERIA, WEST SAYAN MOUNTAINS, 1 female, 50°23'N, 90°26'E, Tsagan-Shibetu Range, Mugur-Aksy vicinity, 2800 m a.s.l., tundra, litter, 22.VII 1993, leg. S.K. Stebaeva.

Adults of middle size, yellowish or brownish, with idiosoma rather narrow and appendages of normal proportions. Idiosomal shields moderately sclerotized, very finely punctate (punctation rarely visible), with clearly reticulate ornamentation only on sternal, genital or sternitigenital shields; dorsal shield smooth with distinct sigillae. Most body setae of moderate size, needle-shaped. Some setae of distal leg segments, especially of leg IV, elongated and finely tipped, subapical setae av-1 and pv-1 on tarsi II-IV strongly formed and blunt.

Female. Idiosomal dorsum. Dorsal shield 528-616 × 248-304, narrowing posteriad, lD/wD ca 1.96–2.24, its maximal width at level of setae s4 (Fig. 1); lateral incisions of moderate length (30–44). Podonotal region normally with 17 pairs of simple setae (z3 present); z1 and s1 seldom asymmetrically absent; s2 usually on soft cuticle, sometimes asymmetrically on shield margin, sometimes symmetrically absent; s6 rarely off shield. Opisthonotal region with 13–14 pairs of setae (S2 always absent, S3 or S5 rarely asymmetrically off shield margin). Among podonotal setae, z1 (13–22) and s1 (20–35) distinguished by clearly shorter lengths, seta j1 30–36, j2 32–42, length of other setae 30–50. On opisthonotal region, setae J1-4 (25–35) slightly shorter than others (30–44), except J5 clearly shortest (9–17) and Z5 clearly longest (48–63); J(1–4)min/jmax 0.65–0.80, Z5/(Z5-Z5) 0.76–1.26; (J4-J4)/(J3-J3) 1.31–2.00. Dorsal shield with 5 pairs of gland pores: gdj4, gdj6, gdz5, gdz6, all poorly visible and gdZ3 distinct. All marginal setae on soft cuticle (their length 28–44, only r3 39–48); 4 (rarely 3) setae in series r, 5 setae in series R; marginal poroid Rp in usual position between setae R3 and R4.

Idiosomal venter. Base of tritosternum narrow (20–35 × 16–22); laciniae with sparse large barbs, free for ca 0.8–0.9 of lengths, their fused basal area fimbriated anteriorly (Fig. 20); rarely barbs are hardly separated (Fig. 21); length of laciniae free part 60–80. Presternal platelets large, consolidated with each other and sternal shield, lineate and clearly punctate (Fig. 6). Sternal shield commonly longer (146–164) than wide (124–160), lSt/wSt 1.00–1.23, merged with endopodal platelets between coxae II-III. Endopodal projections between coxae I-II small, nearly bacilliform, commonly separated, their lateral part encompassing opening of gland gvb. Sternal shield entirely reticulated; anterior margin straight or slightly concave, posterior margin straight. Sternal shield with typical setae st1-3 (32–44), lyrifissures iv1-3, and with vestiges of gland gv1 on posterior margin. Setae st4 (24–34) on soft cuticle. Endopodal strips between coxae III and IV free, rather narrow, partly hidden under epigynal flap. Epigynal shield (140–164 × 72–82) distinctly reticulated (Fig. 9), broadly axe- or flask-shaped, with evenly convex hyaline flap well distant from sternal shield, and posterior margin broadly convex; lateral margins widening behind level of setae st5, but st5 (26–36) and paragenital poroids iv5 remain on soft cuticle. Opening of gland gv2 on soft cuticle close beside end of peritrematal-exopodal strip behind coxa IV. Two pairs of postgenital platelets in fold of soft cuticle adjacent to posterior margin of epigynal shield, another pair between setae JV1 and ZV1. Anal shield rather small, ovate, weakly reticulated (Fig. 10), longer (94–112) than wide (76–93), lAn/wAn 1.08–1.30; paranal setae (36–46) variably inserted, usually between mid-level of anus and its posterior margin, and often nearly as long as postanal seta (42–64); opening of gland gv3 inconspicuous; cribrum ordinary. Opisthogastral setae (8 pairs, JV1–5, ZV1, 2, 4) of moderate length (most setae 30–40), JV5 longest (35–52), ZV1 shortest (20–34). Anterior metapodal platelets small, of variable form; posterior metapodal platelets elongate, often bacilliform (24–39 × 4–7). Exopodal platelets between coxae II and III, as well as between coxae III and IV, small, triangular, rarely visible in ventral aspect. Peritrematal shield reduced, of uneven width, with angular extension at level of coxa I; its anterior end abutting dorsal shield; its posterior edge connecting with exopodal strip enveloping coxa IV posteriorly; lyrifissures ip1–3 and glands gp1, 2 present. Peritreme shortened (128–156 × 8–11), extending to mid-level of coxa II anteriorly. Spermathecal apparatus seldom visible, with very narrow tubuli, thin-walled sperm receptacle and sperm duct strongly expanded medially (Fig. 8).

Gnathosoma. Gnathotectum (28–39 × 40–56) triramous, with projections of almost equal length, sometimes divided apically (Figs 23, 24); middle process sometimes narrower basally than lateral ones. Subcapitulum always slightly longer (116–128) than wide (104–120) (Fig. 7). Deutosternum with 7 (rarely 8) rows of denticles (15–22 denticles in each row); groove delineated laterally, its width 15–19 (Fig. 7). Hypostomatic pair hp3 (46–58) longer than other subcapitular setae (hp1 31–40, sc26–42, hp2 18–26); all setae simple, attenuate. Corniculi somewhat elongated, 46–53 × 15–19; lCo/lD 7.66–9.56. Internal malae slightly shorter than corniculi, gradually tapering to tip, with lateral margins roughly fimbriated basally. Chelicera medium-sized, length of middle article 132–156; movable digit moderate sized (50–56, lCh/lD 8.65–9.71), slightly longer than corniculus (lCh/lCo 1.03–1.17). Fixed digit of chela ending in apical trident, masticatory surface with a row of 5–6 teeth in paraxial position and pilus dentilis in antaxial position (Figs 2–4). Movable digit bidentate. Palp length 180–204; internal seta of trochanter (39–43) slightly longer than external seta (34–38); palpi with typically specialized setae on femur (al) and genu (al1, al2) large, thick, with oblique tip.

Legs. Legs of moderate length (I 512–624, II 432–480, III 352–424, IV 512–592); leg I similar in length to dorsal shield, lExI/lD 0.91–1.09. Length of tarsi I 152–180, II 108–134, III 96–132, IV 160–192; tarsi of normal proportions: lTIV/lD 0.28–0.34, lTIII/wTIII 3.25–4.10. Leg II significantly thicker than others (width of genu II 56–68, genu I 36–40, genua III, IV 44–48). Leg chaetome in general as described for genus (Lindquist and Evans 1965); tibia IV with 7 setae, pl present. Most leg setae simple, of moderate lengths; length of pd2 on genu II (42–48) by a third shorter than width of segment (55–64). Tarsi II–IV each with some rather long setae (up to 100 on tarsus IV), many of them with fine tips (Figs 12, 14, 16); with lateral subapical setae al1 and pl1 needle-shaped, strong, rather long (24–32 on leg II, 21–39 on leg III, 36–56 on leg IV), two times more distant from tarsus apex and much thinner than robust blunt ventral subapical setae av1 and pv1 (14–19, 12–20, 16–27 respectively) (Figs 11–16); av2 on tarsus II and pv2 on tarsus III also thick, sometimes shortened, with blunt tips (Figs 11, 13). Ambulacrum I on pedicellate base, claws I (12–13) smaller than claws II-IV (16–21). Tarsus I distally with 6 rod-like solenidia, 4 of them inserted apically; length of sensillum with lanceolate apex 36–42. Ambulacra of legs II-IV (length 30–40) with rather long paradactyli (14–20 on legs II and III; 18–26 on leg IV) extending clearly beyond apices of claws. Tarsi II-IV with apical setae ad1 and pd1 (24–39 on legs II, III; 30–40 on leg IV) subequally as long as ambulacrum.

Male. Idiosomal dorsum. Dorsal shield 446–552 × 228–260, more narrowed posteriad than in female (lD/wD ca 2.05–2.17); lateral incisions on average shorter than in female (20-40); chaetome in general as in female, but setae commonly slightly shorter (by 5–15 %); 9 pairs of marginal setae (r2-r5, R1-R5) on soft cuticle; s2 usually on soft cuticle, rarely on shield margin or absent.

Idiosomal venter. Tritosternum base (16–20 × 12–16) slightly smaller than in female (Fig. 17). Presternal platelets connected with sternitigenital shield. Sternitigenital shield united with endopodal platelets developed between coxae II-III, partly separated from ones situated between coxae III-IV and only abut ones developed between coxae I-II; its posterior margin slightly convex or truncate; length of shield 212–248, width 124–140, its narrowest part (44–48) between coxae IV. Sternitigenital shield without reticulation in the central and posterior areas, with setae st1–3 (30–38) longer than setae st4, 5 (20–28) and distinct lyrifissures iv1–3; vestiges of glands gv1 not discernible. Ventrianal shield well separated from sternitigenital and peritrematal shields, subtriangular, often with small anterior ledge, fully reticulated, its length 188–224, width 184–240, lVa/wVa 0.83–1.09, normally consolidated with metapodal platelet sigillae laterally; its anterolateral margins evenly convex, cribrum formed as in female. Ventrianal shield with 6 pairs of opisthogastral setae (JV1–4, ZV1, 2), 2–3 pairs of poroids and gland opening gv3, sometimes ZV1 asymmetrically absent, JV4 rarely asymmetrically off shield margin; ZV1, 2 (20–28) somewhat shorter than others (26–35), paranals 32–42, postanal seta 40–52; seta JV5 (34–40) on soft cuticle.

Gnathosoma. Gnathotectum as in female. Corniculi slightly thinner and more parallel than in female (Fig. 5). Internal malae unlike female finely and densely fimbriate. Cheliceral digits 46–52 (lCh/lD 8.58–10.94), longer than corniculus (42–47 × 12–15, lCh/lCo 1.02–1.30, lCo/lD 7.52–9.40). Fixed digit with dentition similar to female, but apical trident not developed and one denticle present subapically in antaxial position, apart from paraxial row of 5–6 denticles. Movable digit with one large denticle and tightly (at a sharp angle) sinuate spermatodactyl (length of free part 26–29), half protruding beyond tip of digit (Fig. 18).

Legs. Length of legs I to IV 492–596 (lExI/lD 0.96–1.13), 364–432, 336–416, and 472–568 respectively; length of tarsi I to IV 140–168, 96–124, 104–122, and 140–170 respectively. Leg II stouter than other legs, with dimorphically modified setae. Three setae on leg II, namely v2 on femur, av2 and mv on telotarsus, remarkably stout basally and attenuate apically, on elevated bases; opposable ventral setae v-1 on femur II and av on genu II and tibia II also usually slightly spinelike (Fig. 22). Other leg structure and setation generally as in female.

Adults of Arctoseius koltschaki sp. n. are similar to those of Arctoseius multidentatus Evans, 1955, but the dorsal shield in Arctoseius koltschaki sp. n. is often narrower (lD/wD in female 1.96–2.24, in male 2.05–2.17 instead of 1.91–2.05 and 1.85–2.03 respectively), and tarsi II-IV have subapical setae av1 and pv1 more stout and blunt (Figs 11–15). In male of Arctoseius koltschaki sp. n., the ventrianal shield is relatively wide (Fig. 17), subtriangular (narrowly oval in Arctoseius multidentatus), incorporating the metapodal platelets (free in Arctoseius multidentatus); the spermatodactyl is more tightly or sharply sinuate, and more angled apically (not as gently sinuate, and digitiform apically as in Arctoseius multidentatus) (cf. Figs 18 and 19). In female of Arctoseius koltschaki sp. n., the sternal shield is uniformly reticulated (whereas in Arctoseius multidentatus, the reticulation is developed laterally, and separately within an elongate posteromedial “window”). Female of Arctoseius koltschaki sp. n. is often smaller (dorsal shield length 528–616 instead of 566–688 in Arctoseius multidentatus).

The species named in honor of Admiral Alexander Koltschak, the prominent Arctic hydrologist and outstanding Citizen of Russia.

At present known from the Russian Arctic and Siberian mountain regions (Fig. 25).

Recorded from dry and humid tundra sites, various meadows including zoogenic ones (e.g. polar fox and lemming hills), snow-beds, bogs, larch forests and shrub (Salix spp., Rhododendron aureum, Pinus pumila) thickets.

The species is rather stable morphologically. The form of female epigynal and anal shields varies a little (Figs 6, 9, 10). The variability concerns mainly the form of tectum (Figs 23, 24), the position of setae s2, s6, S3, S5 (on or off shield margin), and the form and size of leg subapical setae (Figs 11–16). The robust subapical setae av1 and pv1 sometimes may be acuminate, more often on leg IV (Fig. 16). The deutosternum rarely has 8 (instead normal 7) rows of denticles. The lacinia of trito-sternum seldom has adnate barbs (Fig. 21). One male (from 23 ones) originating from Chukotka has free baculiform metapodal platelets (26 × 4).