Cybaeus eremospelyngus, Matsushita & Ihara & Nakano, 2025

Cybaeus eremospelyngus sp. nov. [New Japanese name: Gosen-namihagumo] ( Figs 2–5 View Fig View Fig View Fig View Fig )

Diagnosis. Body length reaching ~ 5mm [= “small-” to “medium-sized” Japanese Cybaeus, sensu Ihara (2004) ; Fig. 2A View Fig ]. The female of C. eremospelyngus sp. nov. is most similar to that of C. inagakii in the shapes of their spermathecae with SH, SS, and SB, all of which are bulbous, and their arrangement. However, the female of this new species can be clearly distinguished from the latter by the atrium, which is wider than long, whereas that of C. inagakii is longer than wide (see fig. 14 in Ono 2008). The male of C. eremospelyngus sp. nov. is distinguishable by the shape of its PA, which is directed anteriorly, while that of C. inagakii expands retrolaterally (see fig. 12 in Ono 2008). Furthermore, C. eremospelyngus sp. nov. is clearly distinguishable from C. inagakii by its possession of eight eyes (originally six reduced eyes in C. inagakii ; fig. 8 in Ono 2008).

Material examined. Holotype: male, KUZ Z5306 View Materials , from Osawa-shonyudo Cave, Gosen , Niigata, Japan (cave entrance: 37.68254°N, 139.1011°E), collected by Yoh Ihara on 16 October 2023. GoogleMaps

Paratype: one female, KUZ Z5307 View Materials , same collection data as for holotype GoogleMaps .

Additional material: one subadult female, KUZ Z5088 View Materials , same locality data as for holotype, collected by Takafumi Nakano on 20 September 2023 .

Description. Male ( holotype, KUZ Z5306: Figs 2A View Fig , 3 View Fig ). Measurements (mm): CL 2.29, CW 1.57; head 0.93 wide; abdomen 2.50 long, 2.25 wide; ocular area 0.19 long, 0.53 wide; sternum 1.17 long, 1.05 wide; CW/CL 0.69, TibIL/CL 0.92. Leg formula IV> I> II> III; length of legs (femur+ patella + tibia+ metatarsus+ tarsus): leg I 8.58 (2.17 + 0.75 + 2.10+2.03 +1.53); leg II 8.23 (2.06+0.77 +1.86+2.00+1.54); leg III 7.38 (1.93+0.72 +1.53+1.92+1.28); leg IV 8.98 (2.24+ 0.74 + 2.12 +2.34 +1.54).

Carapace ( Fig. 2A View Fig ). Head narrow, ~0.59× as wide as thoracic region; thoracic region almost as high as head. Eyes 8 in number; AER slightly recurved in frontal view; PER slightly recurved in dorsal view; AME smallest, <1/2 diameter of other eyes; ocular area ~2.8× wider than long. Clypeus as long as median ocular area.

Mouthpart. Chelicerae geniculate, promargin of fang furrow with 3 teeth (median one largest), retromargin with 4 teeth and 4 denticles, and basally with lateral condyle. Labium wider than long.

Leg macrosetae (left legs). Leg I: tibia p3, r2, v2-2-2-2; metatarsus p3, r1, v2-2-3. Leg II: tibia p4, r2, v2-2-1-2; metatarsus p4, r2, v2-2-3.

Abdomen ( Fig. 2A View Fig ). Colulus two groups of 1 or 2 setae.

Palp ( Fig. 3 View Fig ). PA directed anteriorly, short digitiform in retrolateral view, retrolateral surface with 4 peg setae. Tibia as long as patella; RTA plate-like, occupying 1/2 of length of tibia. Cymbium ~2.6× longer than wide, slightly expanded prolaterally. Genital bulb almost circular in ventral view. Conductor: distal part short; PCO conical and curved prolaterally. EM simple, originating and terminating, respectively, at ~10 o’clock and ~5 o’clock.

Color ( Fig. 2A View Fig ). Carapace: head pale brown, with whitish weak reticulate markings; thoracic region pale beige, with pale brown weak radiating bands. Chelicerae, maxillary lobe, and labium orangish brown. Sternum pale beige, dark- er towards margins. Legs pale beige, darker distally, without annulations. Abdomen: pale gray, dorsally without chevron-like markings.

Female ( paratype, KUZ Z5307: Figs 2B, C View Fig , 4 View Fig , 5 View Fig ). Measurements (mm): CL 2.29, CW 1.47, head 0.88 wide; abdomen 2.71 long, 2.06 wide; ocular area 0.18 long, 0.54 wide; sternum 1.11 long, 1.04 wide; CW/CL 0.64, TibIL/CL 0.75. Leg formula IV>I> II>III; length of legs (femur +patella +tibia +metatarsus +tarsus): leg I 7.00 (1.89 +0.73 +1.72 +1.56 +1.10); leg II 6.68 (1.75+ 0.71+ 1.54 +1.59 +1.09); leg III 6.06 (1.64+ 0.64 + 1.28 + 1.52 + 0.98); leg IV 7.44 (1.99 + 0.64 + 1.75 + 1.89 +1.17).

Carapace ( Fig. 2B View Fig ). Head narrow, ~0.60× as wide as thoracic region; thoracic region almost as high as head. Eyes 8 in number; AER slightly recurved in frontal view; PER slightly recurved in dorsal view; AME smallest, <1/2 diameter of other eyes; ocular area ~3.1× wider than long. Clypeus as long as median ocular area.

Mouthparts. Chelicerae geniculate, promargin of fang furrow with 3 teeth (median one largest), basally with lateral condyle, and retromargin with 5 teeth and 4 denticles on left side, 4 teeth and 6 denticles on right side. Labium wider than long.

Leg macrosetae (left leg). Leg I: tibia p3, r1, v2-2-2-2; metatarsus p5, r2, v2-2-1. Leg II: tibia p4, r1, v2-2-1-2; metatarsus p5, r3, v2-2-1.

Abdomen ( Fig. 2C View Fig ). Colulus two groups of 3 setae.

Genitalia ( Figs 4 View Fig , 5 View Fig ). EPPM curved. Atrium located posteromedially on epigyne. CPs separated on anterior side of atrium; CD thick and short, running anteriorly and curved dorsally. SH developed, SH with PP located dorsally; SS bulbous, located retrolaterally to SH; SB bulbous, directed posteriorly; BG located on SS posteromedially around connection between SS and SB; FD running from SB, descending posteromedially, and then turned anterodorsally.

Color ( Fig. 2B, C View Fig ). Carapace: head pale brown, with whitish weak reticulate markings; thoracic region pale beige, with pale brown weak radiating bands. Chelicerae, maxillary lobe, and labium orangish brown. Sternum pale beige, darker towards margins. Legs pale beige, darker distally, without annulations. Abdomen: pale gray, with whitish reticulate markings, dorsally without chevron-like markings.

Subadult (KUZ Z5088). Measurements (mm): CL 2.22; abdomen 2.41 long.

Mouthparts. Chelicerae geniculated, promargin of 3 teeth (median one largest), with basally lateral condyle, and retromargin 4 teeth and 4 denticles on left side, 5 teeth and 4 denticles on right side. Labium wider than long.

Leg macrosetae (left leg). Leg I: tibia p3, r2, v2-2-2-2; metatarsus p4, r1, v2-2-2. Leg II: tibia p4, r0, v2-1-2-2; metatarsus p5, r2, v2-2-1.

Abdomen. Colulus two groups of 2 or 3 setae.

Color. Carapace: head pale brown, with whitish weak reticulate markings; thoracic region pale beige, with pale brown weak radiating bands. Chelicerae, maxillary lobe, and labium light orangish brown. Sternum pale beige, darker towards margins. Legs uniformly pale beige or slightly darker distally, without annulations. Abdomen: pale gray, dorsally without chevron-like markings.

Distribution. This species inhabits the subterranean environment in Osawa-shonyudo Cave, in Niigata Prefecture, Japan.

Retreat. Cybaeus eremospelyngus sp. nov. constructs a Y-shaped retreat ( Fig. 6 View Fig ); ‘V-shaped’ retreats were not found in the cave.

Etymology. The specific name is a compound noun in the genitive case derived from the Ancient Greek “eremos” (solitary) and “spelynx” (cave). It refers to the type locality of this new species, which is isolated from large karst regions.

DNA sequences. In total, two sequences were obtained from the subadult specimen (KUZ Z5088), two sequences, COI (202 bp; INSD accession number, LC833823 View Materials ) and 28S (285 bp; LC833824 View Materials ). Due to the insufficient preservation of the specimen, short fragments only could be determined.

Remarks. Cybaeus eremospelyngus sp. nov. has pale body coloration and relatively small eyes. Therefore, this species is considered to be somewhat adapted to hypogean environments. Female morphological characteristics of the