Holotype ♂ (ZMUM, Rc 7867): Russia, southwestern Siberia, Altai Province, Krasnoshchiokovo District, near Tigirek village, buffer zone of the Tigirek State Nature Reserve, foot of W slope of Mt. Kozyr, 51°09'26.54"N, 83°00'01.59"E, thicket of Padus avium, Lonicera tatarica and Viburnum opulus, 600 m a.s.l., soil sample (30–40 cm deep), 29–30.08.2016, leg. T.M. Krugova, L.Yu. Gruntova, V.V. Zelensky, K.V. Smirnova, A.E. Pupkova, M.N. Terioshkina and R.V. Scherbakova.

1 ♂ (ZMUM, Rc 7868), 1 ♀ (ZMUM, Rc 7869), together with holotype, soil samples (30–40 cm deep and 0–10 cm deep, respectively); 1 ♀ (PSU-612), Russia, southwestern Siberia, Altai Province, Krasnoshchiokovo District, near Tigirek village, Tigirek State Nature Reserve, Khankhara Site, right bank of Khankhara River, upper part of N slope, 51°11'35.36"N, 82°58'48.26"E, Larix sibirica forest with Betula pendula, 920 m a.s.l., soil sample (20–30 cm deep), 19.08.2016, leg. T.M. Krugova, L.Yu. Gruntova, V.V. Zelensky, K.V. Smirnova, A.E. Pupkova, M.N. Terioshkina and R.V. Scherbakova.

1 ♂ (PSU-536), [Russia, southwestern Siberia, Republic of Altai, Turochak District], near Lake Teletskoye, environs of Artybash village [51.814745N, 87.278742E, ca 450 m a.s.l.], Betula pendula, Abies sibirica and Pinus sibirica forest, litter, 4–13.07.1982, leg. S.I. Golovatch.

The specific epithet refers to Altai, the locus typicus.

A Shikokuobius species with the body 4–6 mm long, the antennae composed of 15–17 antennomeres, commonly 15; coxosternal teeth large, separated from each other by distances less than width at the base of a tooth; P, F and T of 15 leg relatively short and thick, 15 C with a prominent, acute, mesodistal process; the number of coxal pores varying from 1 to 2 on 12–15 CC (formula 1,1,1,1 in the male and 1,1,1,2 in the female); 15 t and 15 P with bifurcate ventral spines (seldom on legs 13 and 14); at least each leg 15 with a bifurcate spine at the distodorsal end of P; 1–10 tibiae with a distal spinose projection (Figs 7–8, 24); 1–12 tarsi clearly unipartite, claws of 1–14 legs with two accessory spines, claw of legs 15 with a single accessory spine; 1^st female gonopodal segment with 2+2 coniform spurs and eight long setae, 2^nd with four setae, 3^rd with a single seta on the external face; terminal claw simple; male gonopod with four segments including terminal filament.

(Fig. 46). Altai Province and Republic of Altai, southwestern Siberia, Russia.

Holotype ♂. Body ca 4.0 mm long, ca 0.4 mm wide (in 70% alcohol); colour yellow. Tergites: almost smooth, with relatively long and sparse setae, as in Figs 32–33; T 15 indistinct; posterior margin of TT 1, 3, 5, 8, 10, 12 and 14 slightly sinuate; TT 2, 4, 6 7 9 11 and 13 almost straight; intermediate T slightly elongated, as in Fig. 5. Cephalic plate: width/length ratio 0.8 (width 0.4 mm, length 0.5 mm). Antennae short, reaching the middle of T3, composed of 15+15 short moniliform articles (Fig. 13). Ocelli absent; Tömösváry’s organ very large, oval (Fig. 2). The sides of the labrum with poorly-expressed fringes of bristles; a pair of setae projecting across the labral midpiece present (Figs 14–15). Gnathal edge of mandible with 4 pairs of well-developed teeth and 3–4 rather thick aciculae (Fig. 6). First maxillae: edge with 5–6 plumose bristles and simple setae as well (Figs 28, 36). Second maxillary telopodite with simple and plumose bristles on the tip (two plumose bristles on the left and right parts, respectively) (Fig. 20). Forcipulae: dental margin of coxosternite almost straight, with 3+3 teeth and long setiform porodonts, teeth relatively large, separated from each other by distances less than width at the base of a tooth, median diastema V-shaped; shoulders of coxosternite strongly sloping, as in Figs 1, 4; claw as in Figs 18–19.

Figures 1–6. 

Shikokuobius altaicus sp. n., male paratype (1, 3, 6) and female paratype (2, 4, 5). 1 forcipulae, ventral view 2 head, ventral view 3 12–14 sternites and coxae, ventral view 4 dental margin of forcipular coxosternite 5 13–16 tergites, dorsal view 6 mandibula, ventrolateral view. Abbreviations: TO Tömösváry’s organ, CO coxal pore. Scale bars: 0.1 mm (1–5), 0.05 mm (6).

Tarsal articulation of legs 1–12 indistinct, tarsi distinctly longer than tibiae. 1–10 tibiae with a distal spinose projection, as in Figs 7–8, 24. 1–14 legs with two accessory spines. 14 and 15 legs not incrassate, with long setae (Figs 11–12). 15 leg: P, F and T relatively short and thick (Fig. 11); C ventrally with a long process (Fig. 26); t and P ventrally with bifurcate spines (Figs 11, 21–22); tarsus 2 with a small distodorsal projection (Fig. 11, shown by arrow). 14–13 legs with the same, but less strongly expressed distodorsal projections. Accessory spines on 15 leg small, poorly-developed (Figs 16–17). At least 13–15 legs dorsally with trace of a broken spine or process (Figs 9, 11–12). A single coxal pore on each of 12–15 legs small and rounded (Fig. 3). Gonopods 4-segmented including terminal filament; 1^st segment with three, 2^nd segment with four long setae on the external face (Fig. 25).

Figures 7–13. 

Shikokuobius altaicus sp. n., male paratype, lateral views. 7 leg 1 8 leg 3 9 leg 14 10 leg 12 11 leg 15 12 leg 13 13 5 terminal antennomeres. Scale bars: 0.1 mm (7, 9–13), 0.5 mm (8).

Figures 14–19. 

Shikokuobius altaicus sp. n., male paratype (14–17) and female paratypes (18, 19). 14, 15 labrum, ventral views 16, 17 apical claw of leg 15, ventrolateral views 18, 19 apical part of forcipular telopodite, ventral views. Scale bar: 0.05 mm.

Figures 20–24. 

Shikokuobius altaicus sp. n., male holotype (20–22) and male paratype (23, 24). 20 left part of second maxilla, ventral view 21 ventral spine on prefemur 15, lateral view 22 ventral spine on trochanter 15, lateral view 23 leg 11, lateral view 24 leg 10, lateral view. Scale bars: 0.05 mm (20–22), 0.1 mm (23, 24).

Paratype ♂. Length 4.0 mm, width 0.4 mm. All other characters as in holotype, but coxal process on leg 15 broken off on both legs.

Non-type material ♂. Length 4.9 mm, width 0.5 mm. All other characters as in holotype (Figs 39–40, 42), but antennae with 17+17 articles, first maxillae with at least six plumose bristles (Fig. 37); second maxillae with four plumose bristles; 14 C ventrally with a tiny denticle, as in Fig. 38; 15 P with a bifurcate spine at distodorsal end (Figs 41, 43); 15 leg with a single well-developed accessory spine; 3^rd gonopodal segment with two long setae on the external face.

Paratype ♀♀. All characters as in ♂♂. The number of antennomeres in females unknown: one ♀ with antennae completely broken off, while another ♀ with damaged antennae, having 12+7 antennal articles. Coxal pores as in holotype, formula 1,1,1,2 (Figs 34–35). The number of accessory spines unknown: both females had no 15 leg-pairs. Gonopods without setae on internal face, with 2+2 conical spurs and simple claw (Fig. 27). All segments of gonopods with long setae (broken off as in Fig. 27): 1^st segment with eight setae, 2^nd with four ones, while 3^rd with a single seta on the external face of gonopod.

Figures 25–29. 

Shikokuobius altaicus sp. n., male holotype (25, 26, 28, 29) and female paratype (27). 25 left gonopod, ventral view 26 left mesodistal process on 15 coxa, ventral view 27 left gonopod, ventral view 28 left part of first maxilla 29 distodorsal process on tibia 3, lateral view. Scale bar: 0.05 mm.

The new species was collected in the lowland Altais in small-leaved and mixed taiga forests at 450 to 920 m a.s.l. (Fig. 45), mainly in soil samples, frequently in deep layers down to 40 cm.

The new species belongs to the genus Shikokuobius Shinohara, 1982 that shows the following synapomorphies: antenna with up to 18 articles, 3+3 coxosternal teeth; spiracles on leg-bearing segments 3, 5, 8, 10, 12 and 14; coxal pores on 12–15 legs; 15 C with a prominent, acute, mesodistal process; 15 t and 15 P with spines, ventrally bifurcated at their tips; at least 15 P with a bifurcate spine at distodorsal end (as some specimens with spines apparently broken off, so these are not visible).

S. altaicus sp. n. is similar to S. japonicus (Murakami, 1967), so far the single species in the genus Shikokuobius, with the above characters. The main differences between them are given in Table 1. Besides this, the new species differs from S. japonicus by: (1) a small distodorsal process on tarsus 2 of legs 13–15 (absent from S. japonicus); (2) the number of coxal pores (1,1,1,1(2) in S. altaicus sp. n. vs. 2(1),2,2,2 sensu Murakami 1967 and 2,2,2,2(3) sensu Shinohara 1982 in S. japonicus).

Finally, S. altaicus sp. n. is also rather similar to Ghilaroviella valiachmedovi Zalesskaja, 1975, from the Tajikistan in showing the same body length, simple and plumose bristles on the second maxillae; the number of antennomeres, 1–2 coxal pores, 2+2 spurs and a simple ♀ gonopodal claw. However, S. altaicus sp. n. is well-distinguished from the latter species by: (1) 3+3 coxosternal teeth (vs. 2+2 in G. valiachmedovi); (2) coxal process well-developed only on leg 15 (vs. on legs 14 and 15 in G. valiachmedovi) and (3) the absence of small warts at the base of the ♀ gonopodal claw (vs. 2 small warts in G. valiachmedovi).

Figures 30–35. 

Shikokuobius altaicus sp. n., female paratype. 30, 31 front body part, dorsal view 32, 33 forcipular and 1–3 leg-bearing segments, dorsal view 34, 35 rear body part, ventral view. Scale bar: 0.1 mm.

Figures 36–43. 

Shikokuobius altaicus sp. n., male paratype (36) and not-type male (37–43). 36 left part of first maxilla, ventral view 37 right part of first maxilla, ventral view 38 distal part of coxa 15, ventral view 39 spine on trochanter 15, lateral view 40 spine on prefemur 15, lateral view 41 trochanter and prefemur 15, lateral view 42 right gonopod, ventral view 43 distodorsal spine on prefemur 15, lateral view. Scale bars: 0.05 mm (36–40, 42, 43), 0.1 mm (41).

Figure 44. 

Shikokuobius japonicus (Murakami, 1967), DNA voucher specimen of Edgecombe and Giribet (2003, 2004), labrum, ventral view. Scale bar: 0.01 mm (courtesy of G.D. Edgecombe).

Figure 45. 

Habitat of Shikokuobius altaicus sp. n. (♀, PSU-612) in the Tigirek State Nature Reserve (courtesy of T.M. Krugova).

Figure 46. 

Distribution of Shikokuobius species: altaicus sp. n. (red triangle) and japonicus (Murakami, 1967) (yellow triangle).

Table 1.

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The main differences between S. japonicus (Murakami, 1967) and S. altaicus sp. n.

	S. japonicus (Murakami, 1967)		S. altaicus sp. n.
	sensu Murakami 1967	sensu Shinohara 1982
front body part			cephalic plate slightly elongate, width/length ratio 0.8; posterior margin of T1 slightly sinuate
	cephalic plate equal in width and length; posterior margin of T1 straight
number of antennomeres	18	up to 18	15*
labrum	sides are smooth; pair of setae projecting across the labral midpiece absent**	no data	sides with poorly-expressed fringes of bristles; pair of setae projecting across the labral midpiece present
forcipular coxosternite			narrower, width/length ratio 1.2–1.3:1
	approximately broad, width/length ratio 1.6–1.8:1
dental margin of forcipulae coxosternite			teeth relatively large, separated from each other by distances less than width at the base of a tooth
	teeth very small, separated from each other by distances more than width at the base of a tooth
leg 15			P, F & T shorter and thicker
	P, F & T elongate and thin

* antennae in a single male (non-type material) with 17+17 articles

** the apparent absence of a pair of setae projecting across the labral midpiece is likely due to them not being noticed by the author of the original description, but sensu G.D. Edgecombe (previously unpublished SEM image of the DNA voucher specimen of Edgecombe and Giribet (2003, 2004) as in Fig. 44), this pair of setae present.