Appendisotoma sibirica Stebaeva, 1985

Figs 43–59, 64

Material: Winter form. Russia, West Siberia, Kemerovo Region, 10 km SE from Mezdurechensk, left bank of Tom’ River, Kuznetsky Alatau Ridge, mountain taiga, under Abies sibirica, 18.10.1981, leg. S.S. (holotype and 6 paratypes).

Kemerovo Region, nearby Kuzedeevo, Biyskaya Griva Ridge (Altai Mts.), mixed forest (Abies, Tilia), 16– 17.09.1994, leg. O. Berezina.

Summer form: Russia, West Siberia, Kemerovo Region, Salair Ridge, Mixed forest (Abies, Pinus, Populus), litter, 10.10.2020, leg. N. Kuznetsova and М.P., N 54.2681 E 85.42581 (mixed with winter form).

Tomsk Region, ~ 10 km W from Tomsk, Timiryazev forestry, litter under Pinus sibirica, 23.08.1984, leg. N. Kuznetsova and М.P.

Description. No less than 1.3 mm (for the largest known subadult individual). Corpus stout (Fig. 55). Greyish blue, spotty, not very dark. Abd.V separated from Abd.IV and fused with Abd. VI. Cuticle with thin hexagonal primary granulation, coarser at posterior edges of body tergites. Ocelli 8+8 (Figs 58, 59), G and H smaller. Ratio PAO length: ocellus = 1.2–1.5. PAO ca 0.4 as long as width of Ant.I and much shorter than inner unguis length (0.5– 0.7). Maxillary head with unmodified lamellae. Maxillary outer lobe with 4 sublobal hairs, maxillary palp bifurcate. Labrum with 4 well visible sharp apical fields. Labral formula as 4/5,5,4. Labium with 5 usual papillae (А–Е) and labial formula A1, B4, C0, D4, E7 (16 guards at whole), guard chaetae e7 present, with 4 proximal (Fig. 45), 4 basomedian and 5 basolateral chaetae. Ventral side of head with 4+4 chaetae.Ant.I with 16–17 common chaetae (11 of basal set and 5–6 additional ones), 3–4 s-chaetae (one in more proximal position shorter) and 5 bms-chaetae, 3 dorsal and 2 ventral. Ant.II with 1 latero-distal s and 5 bms (2 dorsal set together, 2 ventral and 1 additional). Ant.III with 1 bms and 6 distal s (including 2 lateral), without additional s-chaetae (Fig. 49). S-chaetae on Ant. IV weakly differentiated. Organite small, micro s-chaeta of common shape.

Common chaetae smooth or weakly serrated. S-formula 4,4/3,3,3,4,6 (s), 1,0/0,0,1 (ms) (Fig. 43). Th. II and III with 2 and 3 accp-s-chaetae in p-row, respectively (Fig. 50). Tergal s-chaetae short (Fig. 44). All accp-s-chaetae on Th.II and III and Abd.I, II, and III situated in p-row, on Abd. IV behind p-row (Figs 50, 44). Abd. IV often with 1–2 common chaetae set between accp-s-chaetae. Four s-chaetae on Abd. V set in one posterior transversal row, two in front position (Fig. 44). Macrochaetae 2,2/3,3,3,3 in number, head, Th.II and III with some additional macrochaetae-like chaetae (Fg. 51). Macrochaetae long and straight, on Abd.V 2.1–2.3 times longer than dens and 5.0–6.3 times longer than mucro. Foil chaetae at the tip of abdomen absent. Axial chaetotaxy of Th.II–Abd.IV as 6,4/2,2,2,3. Posterior side of body with fewer microchaetae (Figs 53, 44). In comparison with oligochaetotic corpus, head appears polychaetotic (Fig. 52). Thoracic segments without ventral chaetae.

Unguis with large lateral and weak or missing inner teeth (Fig. 46). Empodial appendage 0.6–0.7 as long as unguis. Tibiotarsi with 4 T-chaetae (at whole with 11 chaetae in apical whorl, Fig. 46). Tibiotarsi with many additional chaetae (>28 chaetae at whole). Tibiotarsal tenent hairs pointed or with blunt tips, 1.0–1.2 as long as length of inner edge of unguis (probably affected by cyclomorphosis). Ventral tube with 4+4 (more rarely 3+3, as in first description) laterodistal chaetae, with uncommon arrangement: 2 anterior chaetae set together (Fig. 47). Posterior side of ventral tube with 5 chaetae (2 in distal transversal row and 3 proximal), anteriorly without chaetae. Tenaculum with 3+3 teeth (4+ 4 in two individuals of "summer" form) and 2 (rarely 1) chaeta. Anterior furcal subcoxa with 17–20, posterior one with 10–11 chaetae. Anterior side of manubrium with 3+3, 1+1 or 2+2, 1+1 chaetae (often asymmetrically 2+3, 1+1) (Figs 48, 56, 57), posterior side with many chaetae, with 2+2 or 1+1 latero-central chaetae. Dens smooth or with weak notches on posterior side, with 6 anterior chaetae. Posterior side of dens with 5 chaetae (Figs 48, 56, 57). Outer external lobe near apex of dens present or absent (cyclomorphosis). Mucro with 3 teeth. Ratio manubrium: dens: mucro = 4.4–5.4: 2.3–3.0: 1.

Males present, but adult stage not found, therefore presence of differentiated spurs on Leg III and antennae is unknown.

Cyclomorphosis. In October the population from Salair Ridge had specimens with and without external lobe on dens, which appeared to be winter and summer forms, respectively (Figs 57, 56). Winter individuals showed enlarged cornea of ocelli (Fig. 59 vs 58). Tenaculum is possibly affected by cyclomorphosis (3+3 or 4+4 teeth). To confirm the cyclomorphosis of A. sibirica and to find more detail differences between the seasonal forms, additional material is necessary.

Remarks. In Palearctic region, A. sibirica resembles A. absoloni Rusek, 1966 (Europe) in chaetotaxy of furca and long macrochaetae on body. To supplement the description of the latter species, we studied one subadult specimen of A. absoloni from Czechia (Suchý žleb, Moravian karst, 3.VIII.1960, det. J. Rusek) and several adults from south-eastern part of European Russia (Middle Volga, Samarskaya Luka NP, Zhiguli Mts, at quarry, Verblyud Mt., 02.05.2010, coll. Y. Shveenkova), corresponding to original description. The Czech and Russian individuals belonged to summer form (apical lobe absent). Appendisotoma sibirica and A. absoloni share several unique characters for the genus: s- and ms-formulas (4,4/3,3,3,4,6; 1,0/0,0,1), increased number of s- and bms-chaetae on Ant.I, bifurcate maxillary palp, 4 chaetae on proximal field of labium, long body macrochaetae, and other characters (Table 1). Appendisotoma sibirica differs from A. absoloni by fewer chaetae on anterior side of manubrium (3–4+3–4 vs. 5+5) and fewer chaetae in axial group on Abd.I and II (2+2 vs 3+3: Figs 53 vs 54). A. absoloni has variable axial set (2+2–3+3) on Abd. III (additional pair encircled in Fig. 54). Appendisotoma sibirica has longer macrochaetae, ratio Mac: dens = 2.1–2.2 (vs. 1.4–1.7), Mac: mucro = 5.0–6.7 (vs. 3.5–4.1). Summer specimens of A. absoloni lack apical lobe on dens, like in A. sibirica. After Fjellberg (2007), A. abiskoensis (Agrell, 1939) has similar s-formula (4,4/3,3,3,3,5) and 4 chaetae on proximal field of labium (based on our data). However, Appendisotoma sibirica and A. abiskoensis are well separated considering other characters. Appendisotoma sibirica has possibly more closely related congeners, but the characters mentioned above are poorly studied in the genus.

Distribution and ecology. The species is distributed in the taiga forests of southwestern mountain ridges and nearby lowland of Siberia (Salair Ridge, Kuznetsky Alatau Ridge, north of Altai Mts: Biyskaya Griva Ridge, Tomsk). It is a rare, local endemic species (Fig. 64).