Pseudachorutes armatus Babenko & Kuznetsova & Nakamori & Shveenkova 2021, sp. nov.

Pseudachorutes armatus Babenko & Sveenkova sp. nov.

Figs 15–24, Table 1

Type material. Russia: holotype, male, Republic of Sakha (Yakutia), Suntar-Khayata Mt. Range, upper reaches of Kyubyume River, vicinity of «Vostochnaya» weather station, 63°14.45’N 139°37.88’E, Dryas plant association, 1480 m alt., 28 July 2002. O. Makarova leg. [MSPU collection]. Paratypes, 7 females (including two immature ones) and 2 males from the same sample [MSPU collection].

Other material. 4 females and 4 males, Magadan Province, Bolshoi Annachag Mt. Range, upper reaches of Kolyma River, vicinity of former «Aborigen» field station [~ 61°56’N 149°40’E], Dryas plant association, August 2006. A. Alfimov leg.

Diagnosis. Small sized species. Ant. IV with only 5 dorsal sensilla. PAO small, rounded with 5–7 vesicles. Buccal cone elongate, 2/334 labral setae, labium with full number of ordinary setae (12), seta L and two labial organites. Dorsal chaetotaxy of the parvulus type (i.e. three ordinary setae in dorso-external group on Th. II–III and two such setae in the same position on Abd. I–III). Mucro with lateral lamella almost reaching the tip. Adult and preadult males with 6 modified (strong and spine-like) setae between furcal base and genital orifice. Each anal valve with 2 hr-setae.

Description. Length (without antennae) 0.54–0.64 mm. Colour grey-blue, not especially dark. Tegument granulation rather coarse and uniform.

Antennae slightly shorter than head, Ant. III–IV fused dorsally, ventral separation well marked. Ant. IV with simple apical vesicle, external ms, subapical or and seta i present; dorsal side of Ant. IV with clearly differentiated, curved sensilla (Fig. 17), among them three inner sensilla (S1, S3–S4) and two outer ones (S7 and S8), ventral side with few sensilliform setae of various shape, including several ones with larger sockets (Fig. 16). Antennal organ of Ant. III typical (Fig. 18), inner sensilla small, sgv slightly curved and clearly longer than sgd, ventral ms and 18–19 common setae present on Ant. III. Ant. I–II with 7 and (11)12 setae, respectively.

Head with 8+8 subequal ocelli. PAO small, rounded, consisting of 5–7(8) vesicles, slightly larger than nearest ocellus B, ratio 1.1–1.3: 1 (Fig. 21). Buccal cone elongate. Maxilla styliform with two tiny apical teeth, lamellae not clearly seen. Mandible delicate, with two thin teeth (Fig. 20). Distal edge of labrum rounded, number of labral setae as follows: 2/334. Main part of labium with four proximal ordinary setae, distal seta L on tiny papilla and two labial organites; submentum and mentum with usual set of four setae each, i.e. 4+4 (Fig. 19). Perilabial area usually with 5+5, more rarely 5+4 or 4+4 setae.

Typical pattern of dorsal chaetotaxy presented on Fig. 15, but asymmetrical abnormalities rather frequent especially in lateral parts of terga, number of sensilla as usual: 22/11111, their length 1.5–1.8 times longer than ordinary microsetae. Main characteristics: head with unpaired d0, but without a0, Th. I with 2+2 setae, Th. II with both a- and p-rows including five setae each (a1–a5, a2 usually shifted laterally, p1–p5), only lateral m6 (S) in m-row, lateral ms present. Th. III identical, but without a2 and ms. Abd. I–IV with only one seta (a3) in front of p3–p4, setae of m-row absent. Abd. V without p2 as usual.

Thoracic sterna without setae. Ventral tube with 4+4 setae, no seta on sternum of Abd. I, Abd. II with 3+3 ventral setae, Abd. III with 6–7 such setae on each side (Fig.22). Tenaculum with 3+3 teeth as usual. Furca not especially long. Manubrium with 7–8+7–8 setae on main part, 3 setae on each basolateral lobe and 2 basal setae in line (Fig. 22). Dorsal side of dens with six setae and uniform granulation, hyaline field on its ventral side rather large. Mucro with upturned tip and broad lateral lamella (Fig. 24). Adult and preadult males with 6 strong, spine-like setae between base of furca and genital orifice (Figs 22–23), in immature males these setae unmodified. Each anal valve with two tiny hr-setae.

Legs I–III with 1, 2, 2 setae on upper subcoxae, 0, 2, 2 setae on lower subcoxae, 3, 6, 7 setae on coxae, 6, 6, 6 on trochanters, 13, 12, (10)11 setae on femora and 19, 19, 18 setae on tibiotarsi. Unguis with tiny, often hardly visible inner tooth in mid part of inner edge, lateral teeth absent.

Etymology. The name of the new species reflects its most specific morphological feature—the presence of sexual dimorphism (armatus in Latin–armed).

Affinities. Main diagnostic characters of P. armatus sp. nov. are identical with those of P. parvulus B ӧrner, 1901. Both these species are characterized by small size of body and PAO, usually simple apical vesicle on Ant. IV, incomplete dorsal chaetotaxy, similar labium with the presence of seta L and two labial organites and only two prelabral setae. However, sexual dimorphism, which is characteristic of the new species, is not common for the genus (although not unique, see below), at least for its Holarctic representatives, and this phenomenon was never mentioned for P. parvulus. There are also several finer differences between these two species which are apparently constant, i.e. a number of dorsal sensilla on Ant. IV (5 in P. armatus sp. nov. vs 6 in P. parvulus) and labral setae (2/ 334 in P. armatus sp. nov. vs 2/ 2334 in P. parvulus), and Th. I with an unusual set of 2+2 setae (vs 3+ 3 in P. parvulus). In addition, the mandibles seem to be slightly different—only two teeth were seen in the new species, while P. parvulus usually has 1–2 extra small denticles between apical and basal teeth. This character is not easy to detect and, therefore, is unlikely to be particularly reliable.

There is also another species of the same group in the region under study, i.e. P. cf. hitakamiensis Tamura, 2001, which description is given below. It is also characterized by small size of body and PAO, usually simple apical vesicle on Ant. IV, incomplete dorsal chaetotaxy and only two prelabral setae. It differs from both P. armatus sp. nov. and P. parvulus having no seta L and labial organites on labium (see also Remarks to P. cf. hitakamiensis below).

A comparison of P. armatus sp. nov. with P. minimus sp. nov., the third similar species in the region under consideration, is given below when describing the latter.

The presence of true P. parvulus in the study region remains questionable. We have seen only one male (South Korea, Gangwon-do, Pongnaesan Mt., mixed forest, litter, 06.09.2017, leg. A. Kuprin), which is very close to P. parvulus in most diagnostic features, including dorsal and labial chaetotaxy. However, it has only five sensilla on Ant. IV (S2 is absent as in P. armatus sp. nov.) and slightly different number of labral setae (2/2324 vs 2/ 334 in P. armatus sp. nov. and 2/ 2334 in P. parvulus). The status of this form is not clear.

Of the 120 species known in the genus Pseudachorutes according to www. collembola.org we were able to find only two congeners, namely P. bobeitio Najt & Weiner, 1997 (New Caledonia) and P. polychaetosus Gao & Palacios-Vargas, 2008 (China), which have some kind of modified setae in adult males. The latter species, having two groups of such setae at the furca base in males and characterized by numerous clearly differentiated setae on the dorsal side of the body, is obviously not closely related to P. armatus sp. nov. Main diagnostic traits of P. bobeitio, which males armed by two spine-like setae located anterior to genital orifice, and P. armatus sp. nov. are more similar. Both species are characterized by small size, low number of vesicles in PAO, almost identical dorsal chaetotaxy consisting of short, undifferentiated setae, mandibles with two teeth and some others. However, additionally to the different number of modified setae in males, they differ by the number of dorsal sensilla on Ant. IV (5 in P. armatus sp. nov. vs 6 in P. bobeitio), labium (L-seta is present in P. armatus sp. nov. vs absent in P. bobeitio), and chaetotaxy of Th. I–II (Th. I with 2+2 setae and a2 is present on Th. II in P. armatus sp. nov. vs Th. I with 3+3 setae and a2 is absent on Th. II in P. bobeitio).

Distribution. The new species is known from two areas of northeastern Siberia, remote from each other by almost 1500 km.