Pygmarrhopalites arbaicus sp. nov.

urn:lsid:zoobank.org:act: B690DBEF-51BF-449A-9A95-F4F91CA27CD6

Figs 2–6, Tables 1–3

Diagnosis. Body length up to 1.5 mm. Diffuse-spotted reddish pigmentation or unpigmented. Eyes 1 + 1, distinct. Dorsum of head with 4 axial setae and, posteriorly, 4 + 1 axial + 4 slightly spiny setae. Antenna 2.5× head length; Ant IV with 11 subsegments, bearing 17–18 whorls of setae. Trochanters I, II, III with 4, 5, 5 setae, respectively. Femora I, II, III with 14, 15, 14 setae, respectively. Claws I–III slender, without tunica; each with an inner tooth. Empodia I–II with corner tooth and long apical filament extending beyond claw tip; empodium III without tooth, apical filament reaching claw tip. Manubrium with 7 + 7 setae. Dens with 16 setae on posterior side, including 2 massive blunt external spines and 3 moderate internal spines with sharply pointed apices. Mucro with rounded, non-broadened tip; posterior inner lamella with ~40 small teeth; outer lamella mostly smooth. Trichobothria ABC forming ~160° angle, AB <BC. Abd VI in female with 5 broadened circumanal setae per side, finely denticulate basally, sometimes weakly lamellate; dorsal valve with 11 + 2 axial + 11 setae; each lateral valve with 19 setae. Appendices anales curved, deeply bifurcate, palmate, with secondary branching. Member of the Pygmarrhopalites principalis species group (sensu Bretfeld 1999).

Type material. Holotype: slide C-1076a-1 (Fig. 5A), female, W Caucasus, Abkhazia, Gagra Mt. R., Arabika Massif, 2,266 m a.s.l., Sirota Cave, - 22 m, 6.ix.2017, R.S. Vargovitsh leg.

Paratypes (3 males and 10 females on slides):

2 males (C-1076-5; one male dissected and mounted on 4 slides: C-1076a-4-1: head; C-1076a-4-2: furca; C-1076a-4-3: Abd VI; C-1076a-4-4: body and legs), 2 females (C-1076a-2; one female dissected and mounted in 4 slides: C-1076a-3-1: head; C-1076a-3-2: furca; C-1076a-3-3: Abd VI; C-1076a-3-4: body and legs), collected together with holotype, same cave and collector.

Male (C-1027a-1), adult female (C-1027a-3), juv. female (C-1027a-2), W Caucasus, Abkhazia, Gagra Mt. R., Arabika Massif, 2,289 m a.s.l., Tochka Zh Cave, - 10 m, 12.vii.2016, R.S. Vargovitsh leg.

3 females (C-1081a-9, C-1081a-10, C-1081a-11), W Caucasus, Abkhazia, Gagra Mt. R., Arabika Massif, 2,354 m a.s.l., Nadezhda Cave, -20 to - 25 m, on liquid surface of Barber traps, exposed 13.vii.2016 – 08.ix.2017, R.S. Vargovitsh leg.

3 females on slides (C-1050a-1: 1ex., C-1050a-2: 2 ex.) W Caucasus, Abkhazia, Gagra Mt. R., Arabika Massif, 2,266 m a.s.l., Moskva Cave, - 10 m, on liquid surface of Barber traps, exposed 06.ix.2017 – 07.ix.2018, A.G. Koval and R.S. Vargovitsh leg.; numerous specimens preserved in alcohol.

The holotype, 13 slide-mounted paratypes, and numerous alcohol-preserved specimens are deposited in the collection of the Schmalhausen Institute of Zoology, National Academy of Sciences of Ukraine, Kyiv.

Description. Female: Body measurements and ratios as in Tables 1 and 2. Maximum body length without appendages 1.5–1.6 mm. Pigmentation in alcohol-preserved specimens absent (Fig. 5A) or faintly reddish. In adults, reddish pigmentation as diffuse spots, more intense dorsally on head and thorax.

Head. (Figs 2A, 5D–E). Eyes 1 + 1, relatively large (~16 μm), unpigmented or reddish, corresponding to general head pigmentation (Fig. 5E). Clypeal area with six rows of setae: row a: 4 + 1 axial + 4 setae; rows b, c, e: 5 + 5; rows d, f: 6 + 6; median region M with asymmetrical 3 setae. Interantennal area: row α: 2 + 2; row β: 1 + 1 axial + 1. Frontal area: rows A, C: 2 + 1 axial + 2; row B: 1 + 1 axial + 1; row D: 2 + 2. Setae of rows A, B slender; 4 + 1 axial + 4 setae of rows C and D thickened, slightly spine-like (Figs 2A, 3D, 5D). Row D setae longest in frontal area, ~3× shorter than longest abdominal seta dI-1 (Table 2). Head dorsum with 4 axial setae (Figs 2A, 5D).

Chaetotaxy of mouth region. Labrum (Figs 2A, C; 5I): labral / prelabral chaetotaxy 4, 5, 5 / 6. Labium: basomedian field (submentum) with 4 setae; basolateral field (mentum) with 5 setae. Labial palp (Fig. 2B): 5 proximal setae; 5 papillae (A, B, C, D, E), each with long deeply embedded terminal sensillum; 13 guard setae (a1, b1–b4, d1–d4, e1–e4), a1 blunt, Ƨ-shaped; lateral process (l. p.) in papilla E short, pointed; 3 hypostomal setae (H, h1, h2); papilla B with accessory papilla. Maxillary head as in Fig. 5J. Maxillary outer lobe (Figs 2C, 5I) with 2 setae, including long apical seta with short, thin subparallel basal branch; sublobal plate with 3 sublobal hairs. Oral fold with 2 setae.

Antenna (Figs 2D, 5F–H): ~2.4× head length, 0.85× total body length. Mean length ratio of antennal segments I / II / III / IV = 1 / 2.6 / 4.4 / 11.6. Total number of setae: 218–220.Ant I with 7 setae, p as microseta (10 μm). Ant II: 15 setae, including two longer interior setae (mean 76 μm). Ant III: 15 simple setae, 5 special setae in apical region (Figs 2D, 5H): Api and Ape thin, shorter than simple setae; 2 sense rods (Ant III organ) (mean 9 μm) in shallow pit; Aai as small (7 μm) blunt sensillum. Ant IV subdivided into 11 subsegments, distinctly separated from each other by simple suture (Fig. 2D). Subsegmental formula: 1 + 9 + 1. Ant IV with 176 setae in 17 whorls: 4 on apical subsegment (AI–AIII + M1: 33 setae), 9 on median subsegments (M2–M10: 108 setae), 4 on basal subsegment (BA + BM1–BM2 + BB: 36 setae). Each median subsegment with 12 setae (8 ordinary + 4 H -setae or 7 ordinary + 5 H -setae). Whorls M10 and BA (apical whorl of basal subsegment) likely incomplete, with 11 setae (7 ordinary + 4 H -setae). Besides ordinary setae, apical subsegment with 4 H -setae, hooked AIIpe seta (Fig. 2D), knobbed and deeply embedded subapical organite (Figs 2D, 5F), mossy microseta AIp (Figs 2D, 5G), and 11 ordinary microsetae at tip (Fig. 2D).

Legs. Foreleg (Figs 3A, 5K): precoxae 1, 2 and coxa with 1, 0, 1 setae, respectively (Fig. 4A). Trochanter with 4 setae; i2 strongly curved. Femur with 14 setae; a4 spine-like, positioned perpendicularly to segment axis (Fig. 3F); p1 and p3 much thinner and shorter than other setae. Tibiotarsus with 45 setae; tibiotarsal formula: 9, 8, 8, 8, 8, 4 (from apex to base); seta Ja (distal whorl I) curved, not spine-like; region F with primary setae e, ae, pe and secondary seta FSa. Pretarsus with single anterior microseta larger than single posterior one. Foot complex (Figs 3A, 5K). Claw: slender (max. length 89 μm), without tunica, with small inner tooth and two pairs of lateral teeth; ~5× shorter than tibiotarsus. Empodium: thin, with corner tooth; subequal with claw (max. 86 μm); empodial filament extending beyond claw tip.

Midleg (Figs 3B, 5L): precoxae 1 and 2 with 1, 1 setae, respectively; precoxal process present; coxa with 3 setae and spine-like microsensillum (Figs 4A, 6K). Trochanter: 5 setae; a1 modified as trochanteral organ (Fig. 6J). Femur: 15 setae; p1 thin and short, p3 thin but subequal with others. Tibiotarsus: 44 setae; tibiotarsal formula: 9, 8, 8, 8, 7, 4 (from apex to base); Ja (distal whorl I) unmodified. Pretarsus with single anterior microseta larger than single posterior one. Foot complex (Figs 3B, 5L). Claw: slightly broader and subequal in length to claw I (max. 87 μm), without tunica, with distinct inner tooth and two pairs of lateral teeth; ~5.2× (mean) shorter than tibiotarsus. Empodium: slightly broader than in foreleg; with distinct corner tooth; slightly shorter than in foreleg (~1.1–1.2× shorter than claw, max. 75 μm); tip of empodium typically extending beyond claw tip.

Hindleg (Figs 3C, 5M): precoxae 1 and 2 with 1, 1 setae, respectively; precoxa 1 with process; coxa with 3 setae and spine-like microsensillum (Fig. 4A). Trochanter similar to midleg: 5 setae, a1 as trochanteral organ. Femur: 14 setae; p1 and p3 as very thin and short microsetae. Tibiotarsus: 44 setae; tibiotarsal formula (apex to base): 9, 8, 8, 8, 7, 4; Ja of whorl I somewhat thickened. Pretarsus with 1 anterior microseta larger than posterior one. Foot complex (Figs 3C, 5M). Claw: slightly broader and subequal in length to claw II (max. 85 μm), without tunica, with distinct inner tooth and two pairs of lateral teeth; ~6.3× (mean) shorter than tibiotarsus. Empodium: broad, untoothed, smoothly shaped at its widest part; 1.4–1.5× shorter than claw (max. 62 μm); apical filament very short, tip approximately reaching claw tip.

Lengths ratio of tibiotarsi I / II / III = 1 / 1 / 1.2 (Table 2); tibiotarsus of foreleg 1.2–1.3× shorter than head.

Ventral tube (Fig. 4A) with 1 + 1 subapical microsetae on lateral flaps.

Tenaculum (Fig. 4C): each ramus with 3 teeth and basal process; anterior lobe with 2 small apical setae.

Furca (Figs 4B, 6C–I). Manubrium with 7 + 7 posterior setae. Dens with 23 setae in total: 16 posterior; 7 anterior, formula (apex to base): 3, 2, 1,…1. Posterior side with Ie (Figs 3H, 6G) and IIpe (Figs 3I, 6H) as subequal, massive spines with blunt apices; Ii, IIIpi, and IVpi (Figs 3J, 6I) as moderate spines with sharply pointed, setaceous apices. Mucro (Figs 4B; 6E–F): inner posterior lamella with ~40 teeth, outer—with mostly untoothed margin; anterior lamella smooth and moderately developed; tip rounded, not broadened. Dens 1.7–1.8× mucro length, ~1.1× (rarely 1.2×) manubrium length.

Great abdomen (Figs 4A, 6A). Th II with single small sensillum (7–8 μm) in row a; 3 setae in row m, with m1 slightly spine-like (Fig. 3E), longer than other Th II setae and ~3× shorter than longest setae of posterior dorsal complex (Fig. 3G). Th III with sensillum in row a (12–13 μm) and 3 setae in row m. Anterior part of abdomen: row a with 5 short setae; row m with 4 short setae anterior to trichobothrial complex. Trichobothrial complex (Figs 4A, 6A): trichobothria ABC forming very obtuse angle (~160°); AB ~1.2× shorter than BC; seta p located below level of trichobothrium B; seta b1 almost on line BC, slightly closer to C; c1 as ordinary seta (~30 μm, not microseta), positioned in front of C and seta c2 —below C. Posterior lateral complex with 6 setae in two rows (3 + 3). Furca base complex with 9 setae in two rows (5 + 4); neosminthuroid seta absent. Central dorsal complex with seta 2 slightly longer than 1 and 3 (Fig. 4A). Posterior dorsal complex with ~25 long setae in 3 longitudinal rows (dI: 9, dII: 9, dIII: 7); longest setae (most posterior in rows dI–dIII) = 95–113 μm (Fig. 3G), ~1.4× longer than hind claw (Table 2). Ventral complex with 3 (rarely 2) setae (Fig. 4A).

Small abdomen. Abd V (Fig. 4A) with 2 setae and trichobothrium D in row a (a2 very thin and short), and 2 longer setae in row p. Genital field with 5(4) + 5(4) microsetae along anterior margin of genital opening.

Abd VI (Figs 4D, 6B): dorsal valve with 11 + 2 axial + 11 setae, ms5 present; each lateral valve with 19 setae. Five setae of circumanal row (mps1–mps3 and mpi1–mpi2) broadened and finely denticulated in basal half, some of them weakly lamellate; relatively long (longest = 80–97 μm, ~1.2× shorter than longest posterior seta of great abdomen) (Figs 3K, 6B). Appendices anales (Figs 3L, 5B): long, deeply bifurcate and palmate in distal half with secondary branches; length ~50 μm, width ~3.6 μm (basal part) and ~20 μm (palmated area in dorso-ventral view); sitting on subglobular somewhat elongated and bilobed basal papilla. In total, normally 62 setae on Abd VI present.

Male. Body length ~ 1.2 mm, slightly smaller than female. Chaetotaxy identical to female, except for sexually dimorphic small abdomen (Fig. 4E). Abd V with ~12 + 12 short setae associated with genital opening. Abd VI: dorsal anal valve of with 7 + 2 axial + 7 setae; each lateral valve with 14 setae. Setae of Abd VI unmodified, shorter than in female. Ant IV: subsegment separations strongly pronounced, distinctly more visible than in female; basal subsegment with numerous annulations (Fig. 5C). Differences in body part sizes and proportions shown in Tables 1 and 2.

Variability. Chaetotaxy of posterior surface of dens generally consistent, with 16 setae per dens. However, some variations observed: in one male 16 + 15 setae (Vpe missing); in one female and one juvenile female 16 + 17 setae, both with 4 interior spines instead of usual 3. In the same aberrant female, separations between subsegments of Ant IV indistinct or absent, and one seta on trochanter III bifurcate. In single juvenile female, all spines sharply acuminated rather than blunt.

Ecology and distribution. Pygmarrhopalites arbaicus sp. nov. has so far been recorded in only four of the 17 caves sampled on the Arabika Massif. Its distribution appears to be restricted to the ‘Central Plateau’ of the Arabika Massif, situated in the Gagra Mountain Range of the Western Caucasus. The species has not been found in other areas of the massif, including the deepest caves such as Krubera and Veryovkina. Most specimens were collected using Barber traps in Nadezhda Cave (at depths of 20–25 m; t = 2.1 °C) and Moskva Cave (at - 10 m; cave temperature ranges from 2.0 to 5.8 °C). Unlike other troglobiont Collembola species, P. arbaicus sp. nov. was not observed on water surfaces, wet walls, or in other traps in deeper cave sections (down to - 200 m in Nadezhda Cave). A few specimens in Sirota Cave were captured near seeping water among pebbles on the cave floor. Based on its morphology and predominantly terrestrial habitat, the species is classified as an atmobiont troglomorph (Vargovitsh 2022).

Etymology. The species name arbaicus is derived from Arbaika, the historical name for the Arabika Massif. This name itself is a distorted form of the original Abkhaz toponym Arbaika, which translates to “ridge (of the mountain) Arbaa ”, where aika means “ridge” (Kvarchia 2019).

Taxonomic remarks. Pygmarrhopalites arbaicus sp. nov. is clearly placed within the principalis species group sensu Bretfeld (1999), characterized by distinctly palmate appendices anales. Within this group, eight species are known to possess elongated antennae (~2 times or more the head length), and five of these also exhibit thickened (spine-like) setae on the head dorsum. A key distinguishing feature of P. arbaicus sp. nov. is the exceptionally high number of subsegments (11) on Ant IV, which sets it apart not only from the aforementioned five species (as outlined in Table 3), but also from all other known members of the genus Pygmarrhopalites. The new species shares the closest resemblance to the Caucasian P. dbari and P. kovali, but can be differentiated by its larger body size, Ant IV divided into more subsegments, and a foot complex in which empodia I and II surpass the tips of their respective claws (see Table 3).