Pygmarrhopalites trisetosus, Vargovitsh, 2025

Vargovitsh, Robert S., 2025, Antennal troglomorphy in Arrhopalitidae (Collembola: Symphypleona) with the description of two new cave Pygmarrhopalites Vargovitsh, 2009 species from the Arabika Massif in the Caucasus, Zootaxa 5673 (4), pp. 523-556 : 536-544

Pygmarrhopalites trisetosus sp. nov.

urn:lsid:zoobank.org:act:F454BDA0-432C-40DD-8D40-D5FAE6A510AF

Figs 7–11 , Tables 1–2, 4

Diagnosis. Body length up to 1.4 mm. Diffuse reddish pigmentation. Eyes 1 + 1, indistinct, covered by granulated cuticle and with diffuse reddish pigmentation. Dorsal head setae not spine-like, 3 axial setae present. Antenna 2.8× head length; Ant IV with 6 subsegments and 13 whorls of setae; several intercalary pseudosubsegments without setae inserted between normal subsegments. Trochanters I, II, III with 4, 5, 5 setae, respectively. Femora I, II, III with 11, 13, 12 setae, respectively. Claws slender, without tunica; claw I without inner tooth (or with extremely reduced one), claws II–III with distinct inner tooth. Empodia short, without apical filaments, tips not reaching claw tip; empodia I and II with small corner tooth, empodium III without tooth. Manubrium with 6 + 6 setae. Dens with 15 (rarely 14) posterior setae, including Ie as blunt spine, IIpe as moderate spine with pointed apex, Ii weakly spine-like and setaceous. Mucro with spoon-like broadened tip; ~40–45 small teeth on posterior inner lamella, ~30 on outer lamella. Trichobothria ABC forming ~150° angle, AB <BC. Abd VI in female with regular, not broadened circumanal setae, sometimes weakly denticulate basally; dorsal valve with 10 + 2 axial + 10 setae (ms5 absent); each lateral valve with 18 setae (mi1 absent). Appendices anales palmated in distal quarter. Member of the Pygmarrhopalites principalis species group (sensu Bretfeld 1999).

Type material. Holotype: slide C-1076a-6 ( Fig. 10A ), 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 (2 males, 6 females on slides): 2 males (C-1081a-7, C-1081a-8), 6 females (C-1081a-2, C-1081a-3, C-1081a-4, C-1081a-5, C-1081a-6; one dissected and mounted in 3 slides: C-1081a-1-1: head and leg I; C-1081a-1- 2: legs and great abdomen; C-1081a-1-3: Abd VI and furca), 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.

The holotype and 8 paratypes are deposited in the collection of the Schmalhausen Institute of Zoology, National Academy of Sciences of Ukraine, Kyiv .

Description. Female: Body measurements and ratios in Tables 1 and 2. Maximum body length without appendages 1.4 mm. Pigmentation in alcohol slightly reddish ( Fig. 10A ).

Head ( Figs 7A , 10B ). Eyes 1 + 1, relatively large (~16 μm) but indistinct, poorly defined cornea covered by almost typical cuticular granulation ( Fig. 10B ). Clypeal area with six rows of setae, and median region M with 2 setae; row a: 4 +1 axial + 4 setae; rows b, c, e: 5 + 5; rows d, f: 6 + 6. Interantennal area: row α: 2 + 2; row β: 1 + 1 axial + 1. Frontal area: rows A, D: 2 + 2; row B: 1 + 1 axial + 1; row C: 2 + 1 axial + 2; all setae of frontal area slender; longest (median pair of D -row) slightly thicker ( Fig. 8D ), ~2× shorter than longest abdominal seta dI-1 ( Table 2). Head dorsum with 3 axial setae: A0 (as in Fig. 7A ) or C0 (as in Fig. 10B ) absent. Chaetotaxy of ventral side as in Fig. 7A (dashed-line setae).

Chaetotaxy of mouth region. Labrum ( Fig. 7A, C ): labral / prelabral chaetotaxy 4, 5, 5 / 6. Labium: basomedian field with 4 setae, basolateral field with 5 setae. Labial palp ( Fig. 7B ): 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 and Ƨshaped; 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. 10G . Maxillary outer lobe ( Fig. 7C ) with 2 setae, including long apical seta with short and thin basal branch, sublobal plate with 3 sublobal hairs. Oral fold with 2 setae.

Antenna ( Figs 7D , 10C–F ) ~2.8× head length, 0.92× total body length. Mean segment length ratio I / II / III / IV = 1 / 3.0 / 5.2 / 12.4. Total number of setae: 166. Ant I with 7 setae, p as microseta (8 μm). Ant II: 15 setae, two anterior setae longer (mean 75 μm). Ant III: 15 simple setae, 5 special setae in apical region ( Fig. 10E–F ): Api and Ape thinner and shorter than simple setae; 2 sense rods (Ant III organ) (mean 10.2 μm) in shallow pit; Aai as small (7 μm) blunt sensillum. Ant IV subdivided into 6 subsegments. Basal subsegment separated from median by intercalary pseudosubsegment lacking setae as well as 3 median subsegments from each other ( Figs 7D , 10C ). Three apical subsegments separated from each other by simple suture ( Figs 7D , 10C ). Subsegmental formula: 1 + 4 + 1. Ant IV with 124 setae in 13 whorls: 4 on apical subsegment (AI–AIII + M1: 33 setae), 4 on median subsegments (M2–M5: 60 setae), 5 on basal subsegment (BA + BM1–BM3 + BB: 31 setae). Each middle subsegment with 12 setae (7 ordinary + 5 thin H -setae or 8 ordinary + 4 H -setae). Chaetotaxy of whorl BA resembles that of M -whorls, with 8 ordinary setae and 4 H -setae. Apical subsegment, besides regular setae, with 4 H -setae, hooked AIIpe seta, knobbed and deeply embedded subapical organite, mossy microseta AIp, and 11 ordinary microsetae at tip ( Fig. 7D ).

Legs. Foreleg ( Figs 8A , 10I ): precoxae 1, 2 and coxa with 1, 0, 1 setae, respectively ( Fig. 9A ). Trochanter with 4 setae; i2 strongly curved. Femur with 11 setae; a4 slightly thickened ( Fig. 8F ), oriented perpendicularly to segment axis; p1 and p3 much thinner than other setae; p1 as microseta about twice shorter than p3. Tibiotarsus with 44 setae; tibiotarsal formula: 9, 8, 8, 8, 7, 4 (from apex to base); Ja of distal whorl I curved, not spine-like; region F with secondary FSa seta present. Pretarsus with single anterior microseta lager than posterior one. Foot complex ( Figs 8A , 10I ). Claw: slender (max. 80 μm), without tunica, without or with vestigial inner tooth, with two pairs of barely visible lateral teeth; ~5.3× (mean) shorter than tibiotarsus. Empodium: thin, with small corner tooth; ~1.7× shorter than claw (max. 52 μm); tip of empodium far not reaching claw tip.

Midleg ( Figs 8B , 10J ): precoxae 1 and 2 with 1, 1 setae, respectively; precoxal process present; coxa with 3 setae and spine-like microsensillum. Trochanter with 5 setae, a1 as trochanteral organ ( Fig. 10H ). Femur with 13 setae; p1 and p3 as thin, short microsetae. Tibiotarsus with 44 setae; tibiotarsal formula: 9, 8, 8, 8, 7, 4 (from apex to base); Ja of distal whorl I not modified. Pretarsus with single anterior microseta somewhat larger than posterior one. Foot complex ( Figs 8B , 10J ). Claw: slightly broader and subequal in length to foreclaw (max. 80 μm), without tunica, with distinct inner tooth and two pairs of lateral teeth; ~5.4× (mean) shorter than tibiotarsus. Empodium: broader and shorter than in foreleg, with corner tooth; ~1.7× shorter than claw (max. 48 μm); empodial filament not developed, empodial tip far from reaching claw tip.

Hindleg ( Figs 8C , 10K ): precoxae 1 and 2 each with 1 seta; process on precoxa 1 present, coxa with 3 setae and spine-like microsensillum. Trochanter with 5 setae; a1 as trochanteral organ. Femur with 12 setae; p1 and p3 very thin, short microsetae. Tibiotarsus with 45 setae; tibiotarsal formula: 9, 8, 8, 8, 8, 4 (from apex to base); Ja of whorl I slightly thickened. Pretarsus with 1 anterior microseta somewhat larger than posterior one. Foot complex ( Figs 8C , 10K ). Claw: shorter and broader than in fore- and midlegs (max. 69 μm), without tunica, with distinct inner tooth and two pairs of lateral teeth; ~7.4× (mean) shorter than tibiotarsus. Empodium: broad, untoothed, shaped as in Fig. 10K ; 1.5–1.6 × shorter than claw (max. 44 μm); lacking apical filament, with tip far from reaching claw tip.

Length ratio of tibiotarsi I / II / III = 1 / 1/ 1.17. Length of tibiotarsus I subequal to head.

Ventral tube ( Fig. 9A ) with 1 + 1 subapical microsetae (9 μm) on lateral flaps.

Tenaculum ( Figs 9C , 11I ): each ramus with 3 teeth and basal process; anterior lobe with 2 apical setae.

Furca ( Figs 9B , 11C–H ). Manubrium with 6 + 6 posterior setae. Dens with 22(21) setae: 15(14) posterior (IIIpi variable); 7 anterior, formula (apex to base): 3, 2, 1,…1. Posterior side: Ie as blunt spine on projection ( Figs 8M , 11G ); IIpe as moderate spine with sharply pointed setaceous apex ( Fig. 8N ); Ii basally thickened ( Figs 8O , 11H ); others unmodified ( Fig. 9B ); IIIpi variable, absent in 50% of individual dentes; IVpi absent. Mucro ( Figs 9B , 11C– F ): inner posterior lamella with 40–45 teeth, outer with 30–35 teeth; anterior lamella smooth, moderately developed; tip spoon-like, distinctly broadened. Dens 1.9–2× mucro length, ~1.3× (rarely 1.2×) manubrium length.

Great abdomen ( Figs 9A , 11A ). Th II with single small sensillum (7 μm) in row a; 3 setae in row m, with m1 unmodified ( Fig. 8E ), ~3× shorter than most posterior setae of posterior dorsal complex ( Fig. 8G ). Th III with sensillum in row a (17.5 μm) and 3 setae in row m. Anterior part of abdomen: row a with 5 short setae followed by row m with 4 short setae anterior to trichobothrial complex. Trichobothrial complex ( Figs 9A , 11A ): ABC forming very obtuse angle (~150°); AB ~1.5× shorter than BC; seta p located below level of B; b1 on line BC, midway between B and C; c1 as ordinary seta (~28 μm, not microseta), in front of C; c2 below C. Posterior lateral complex with 5 setae in two rows (2 + 3). Furca base complex with 9 setae in two rows (5 + 4); neosminthuroid seta absent. Central dorsal complex with seta 2 little longer than 1 and 3. Posterior dorsal complex with ~20 long setae in 3 longitudinal rows (dI: 8, dII: 7, dIII: 5); longest (posteriormost in rows dI–dIII) = 87–105 μm ( Fig. 8G ), ~1.5× longer than hind claw ( Table 2). Ventral complex with 2 or 3 setae.

Small abdomen. Abd V ( Fig. 9A ) with 2 setae and trichobothrium D in row a (a2 distinctly thinner and shorter than a1), and 2 setae in row p. Genital field with 4 + 4 to 6 + 6 microsetae along anterior margin of genital opening.

Abd VI ( Figs 9E–F ; 11B, J ): dorsal valve with 10 + 2 axial + 10 setae, ms5 absent; each lateral valve with 18 setae, mi1 absent. Circumanal setae not broadened ( Fig. 11B ), some of them with few denticles in subbasal part ( Fig. 8J–K ), rarely with long outgrowth as in seta mps 3 in Fig. 8I ; longest circumanal seta (48–80 μm) ~1.3× shorter than longest posterior seta of great abdomen ( Fig. 8 G–H ). Appendices anales ( Figs 8L , 11J ): moderately palmate in distal quarter; length ~35 μm, width ~3.6 μm (basal part) and ~11 μm (palmated area in dorso-ventral view); sitting on subglobular somewhat elongated basal papilla. In total, normally 58 setae on Abd VI present.

Male. Body length ~ 1 mm, smaller than female. Chaetotaxy the same as in female apart sexually dimorphic small abdomen ( Fig. 9D ). Abd V with ~10 + 10 short setae associated with genital opening. Abd VI: dorsal anal valve with 7 + 2 axial + 7 setae; each lateral valve with 14 setae. Separations between both normal subsegments and intercalary pseudosubsegments of Ant IV strongly expressed, much more distinct than in female ( Fig. 10D ); basal subsegment often secondarily subdivided into up to 5 pseudosubsegments by 3 groups of annulations (5–8 annuli in each) ( Fig. 10D ). Differences in size of body parts and proportions provided in Tables 1 and 2 respectively.

Variability. Head dorsum with 3 axial setae, though their positions may vary slightly. Inner tooth in claw of foreleg extremely small or absent. Number of setae in ventral complex of great abdomen varies from 3 to 0. In one female seta mps2 of Abd VI absent (marked as variable seta in Fig. 9E ). Some circumanal setae with or without few small teeth or long outgrow ( Figs 8I , 11B ).

Ecology and distribution. Pygmarrhopalites trisetosus sp. nov. has only been found in two of the 17 caves sampled within the Arabika Massif, where it co-occurs with P. arbaicus sp. nov. This limited presence suggests a restricted distribution, likely confined to the ‘Central Plateau’ of the Arabika Massif, located in the Gagra Mountain Range, Western Caucasus. Eight specimens were captured using Barber traps at depths of 20 and 25 m in Nadezhda Cave, where the temperature was 2 °C. Unlike some other troglobiont Collembola species, no specimen of P. trisetosus sp. nov. was observed on the water surface, wet walls, or in other traps set deeper within the cave, down to - 200 m. The only manually collected specimen was found in Sirota Cave, near seeping water amongst pebbles, together with a few specimens of P. arbaicus sp. nov. Based on its morphology and preferred habitat, P. trisetosus sp. nov. can be classified as belonging to the atmobiont troglomorph life form ( Vargovitsh 2022).

Etymology. The species name trisetosus refers to the presence of only three (not four or five as usual) axial setae on the head dorsum, within the interantennal and frontal areas, a rare characteristic within the Arrhopalitidae .

Taxonomic remarks. The new species belongs to the principalis species group ( Bretfeld 1999), characterized by palmate appendices anales.

Among the known Caucasian species, P. trisetosus sp. nov. most closely resembles P. dbari from Psyrtskha Cave in Novy Afon, sharing several diagnostic features: elongated antennae (up to 3× head length) with subsegments separated by intercalary bare pseudosubsegments on Ant IV (males also possessing annulating zones on the basal subsegment); shape of head dorsum setae (only slightly spiny in P. dbari ); chaetotaxy of the great abdomen; shape of female circumanal setae; shape of the foot complex; presence of 2 external and 1 internal spine-like setae on the dens. Key differences between P. trisetosus sp. nov. and P. dbari include: larger body size (1.3 mm vs. 1 mm); reduced number of axial setae on the head dorsum (3 vs. 4); number of subsegments on Ant IV (6 vs. 7); number of setae on the dorsal valve of Abd VI (10 + 2 + 10 vs. 11 + 2 + 11; ms5 absent in P. trisetosus sp. nov.); setae on lateral valves of Abd VI (18 vs. 20); dorsal setae on manubrium (6 + 6 vs. 7 + 7); number of posterior setae on the dens (15 vs. 16) with IVpi absent in P. trisetosus sp. nov.

The presence of only three axial setae on the head dorsum is a particularly striking feature of P. trisetosus sp. nov., as most Arrhopalitidae have four or five.A similar reduction has only been documented in P. maestrazgoensis Jordana, Fadrique & Baquero, 2012 ( Jordana et al. 2012b) and in a P. principalis population from Fennoscandia ( Fjellberg 2007: 190), though this character remains unknown or unclarified in many incompletely described species.

The subdivision of Ant IV into 6 subsegments in P. trisetosus sp. nov. is a trait shared with several other species of the principalis group. However, those species typically possess spine-like setae on the head dorsum (except P. ashcraftensis Zeppelini, Taylor & Slay, 2009 ) and a 2 + 3 spine arrangement on the dens, whereas the new species has only 2 + 1.

Pygmarrhopalites trisetosus sp. nov. differs from North American P. ashcraftensis in several aspects, including longer antennae (Ant/head ratio 2.8 vs. 1.8), fewer axial setae on head dorsum (3 vs. 5), absence of a corner tooth on empodium III (present in P. ashcraftensis ), and fewer spines on dens (2 + 1 vs. 2 + 3). Further comparative morphological data for other members of the principalis group with elongate antennae and thin dorsal head setae are provided in Table 4

Figures

FIGURE 1. Type locality of Pygmarrhopalites arbaicus sp. nov. and P. trisetosus sp. nov.: A, location of the Arabika Massif in the Caucasus; B, location of ‘Central Plateau’ on the Arabika Massif; C, ‘Central Plateau’ of the Arabika Massif with marked entrances to caves: Nadezhda (1), Sirota (2), Moskva (3), Tochka Zh (4); D–E, landscape of ‘Central Plateau’ of the Arabika Massif: D, view from Arabika Mt.; E, view from locality of Nadezhda Cave; F–G, Nadezhda Cave; H, Moskva Cave; I, Sirota Cave; J–K, Tochka Zh Cave.

FIGURE 7. Pygmarrhopalites trisetosus sp. nov.: A, chaetotaxy of head; B, chaetotaxy of labial palp; C, labrum and maxillary outer lobe; D, chaetotaxy of antenna and details of the apical subsegment.

FIGURE 8. Pygmarrhopalites trisetosus sp. nov.: A, chaetotaxy of foreleg; B, chaetotaxy of midleg; C, chaetotaxy of hindleg; D–O, shape of setae: D, D2 of head dorsum; E, m1 of mesothorax; F, a4 of fore tibiotarsus; G, dI-1 of great abdomen; H, ms1 of Abd VI; I, mps3 of Abd VI; J, mpi1 of Abd VI; K, mpi2 of Abd VI; L, mi5 of Abd VI (appendices anales); M, Ie of dens; N, IIpe of dens; O, Ipi of dens.

FIGURE 9. Pygmarrhopalites trisetosus sp. nov.: A, chaetotaxy of great abdomen; B, chaetotaxy of furca; C, tenaculum; D, chaetotaxy of male Abd VI; E–F, chaetotaxy of female Abd VI: E, lateral view; F, posterior view, schematic.

FIGURE 10. Pygmarrhopalites trisetosus sp. nov.: A, habitus of mounted and alcohol specimen; B, chaetotaxy of head dorsum; C–D, Ant IV: C, female; D, male; E–F, Ant III organ; G, head of maxilla; H, trochanteral organ of midleg; I–K, foot complex: I, foreleg; J, midleg; K, hindleg.

FIGURE 11. Pygmarrhopalites trisetosus sp. nov. A, chaetotaxy of trichobothrial complex; B,Abd VI of female with circumanal setae; C–F, mucro; G–H, spine-like setae of dens: G, Ie; H, Ipi; I, tenaculum; J, appendices anales.