Skleroprotopus membranipedalis Zhang, 1985

Vagalinski, Boyan, Meng, Kaibaryer, Bachvarova, Darina & Stoev, Pavel, 2018, A redescription of the poorly known cave millipede Skleroprotopusmembranipedalis Zhang, 1985 (Diplopoda, Julida, Mongoliulidae), with an overview of the genus Skleroprotopus Attems, 1901, Subterranean Biology 26, pp. 55-66 : 55

Skleroprotopus membranipedalis Zhang, 1985

Skleroprotopus membranipedalis Zhang, 1985: 154-156, figs 1-8.

Material.

3 males, 7 females & 16 juv., China, Beijing, Fangshan Distr., Shi-Hua (Stone Flower/ Stone Buddha) Cave (type locality), 39°47'36.7"N, 115°56'32.1"E, 12.XI.2013, P. Stoev, Ch. Delchev & S. Li leg. (NMNHS); 7 females & 14 juv., same Distr., Mt. Shangfang, Yun-Shui (Cloud Water) Cave, the touristic part, 39°40'29"N, 115°48'35"E, 3.XI.2013, under stones, creeping on walls, humid clay, some 100 m from the entrance, same collectors (NMNHS).

Diagnosis.

Differs from congeners mostly by the flattened, blade-like, basolateral process on the caudal face of the anterior gonopod, and by the completely pigmentless ocelli.

Redescription.

Measurements: males 49-51 mm in length, 2.5-2.6 mm in height at mid-body, body ring formula 62+(1-2)+T; females 49-60 mm, 2.4-2.8 mm, and (61 –67)+(1– 2)+T, respectively.

Colouration in life (Fig. 2): light brown-beige, head whitish; metazonites with darker, narrow, concentric bands, passing just behind the ozopores; repugnatorial glands reddish-brown in living specimens, becoming brown-greyish in ethanol.

General morphology: Adults with 25-40 very small and completely pigmentless ocelli arranged in a narrow triangular field; eye rows unclear. Vertigial setae and pits absent; 4 supralabral and 22-28 labral setae. Antennae (Fig. 3) slender, 1.4-1.5 times as long as head; antennomere 3 longest, ca. twice as long as 6; 2, 4 and 5 subequal in length; 5 and 6 insignificantly thicker than the previous three, their distal margins with a dense whorl of minute sensilla basiconica. Mandibles with 7 pectinate lamellae. Gnathochilarium (Figs 4, 5) with only three apical setae on each stipes and with 7-8 setae on each lamella lingualis arranged in a L-shaped row; promentum of an elongated pentagonal shape, just slightly longer than lamellae linguales, completely separating them.

Collum with ca. 10 shallow striae on each side. Body rings considerably vaulted, this becoming increasingly pronounced towards telson. Prozonites with several shallow, somewhat undulating, transverse striae encircling them (striae being more pronounced dorsally), and dense, short and shallow, longitudinal striation on dorsum near the pro-metazonital suture. Metazonites with rather sparse and shallow (deeper ventrally) longitudinal striae, these disappearing above the ozopore level; without setae on hind margins. Ozopores relatively small, set far behind the pro-metazonital suture, at ca. metazonital mid-length.

Epiproct very short and blunt in both sexes, marginally with one to several setae. Hypoproct broad and short, nearly semi-elliptic, edentate, tightly fitting under the paraprocts; its margin slightly more strongly vaulted in males. Paraprocts with only 2-3 setae each situated near the caudal margins. Walking legs slender; tarsi of mid-body legs 2-2.3 times longer than tibiae, and 3.3-3.7 times longer than the apical claw.

Male sexual characters: Male mandibular stipites enlarged, ventrally incised, forming two nearly equal, subconical lobes. Promentum (pr in Fig. 5) of gnathochilarium distally swollen. Leg-pair 1 (Fig. 6) massive, six-segmented, strongly compressed dorso-ventrally, reaching up to distal margin of gnathochilarium, without any remnant of an apical claw; podomere 5 very large, strongly arched dorsad, approx. as long as the basal one; podomeres 2-4 each with several long setae in a transverse row, 5 and 6 medially with numerous minute setae. Leg-pair 2 (Fig. 7) with massive, robust coxae, fused to each other, basally forming a deep sinus for the penis. Penis (Fig. 8 & pn in Fig. 7) elongated, membranous, dorso-medially with an oval groove, and with two long apical setae. All walking legs (Fig. 9) without adhesive pads or other modifications. Pleurotergite 7 ventrally forming small, rounded protrusions originating entirely from the metazonite, directed mesad. Leg-pair 7 (Fig. 10) modified to elongated, non-segmented, leaf-like coxites, proximally with rather small, non-segmented telopodites (Fig. 11 & te in Fig. 10), the latter with a minute subapical remnant of a tarsus (ta) bearing a claw.

Gonopods (Figs 12-16): in situ jutting out of the gonopodal sinus, distal parts of posterior gonopods partly concealed by the anterior gonopods laterally.

Anterior gonopod (Figs 12, 13) with an elongated coxite, gradually narrowing towards a rounded apex, with several setae medially on caudal face; coxite bearing a large, flattened axe blade-like process (b), emerging subbasally from its lateral margin, directed caudo-distad, and a well-developed telopodite (te), almost half as long as the coxite, non-segmented, apically setose, laterobasally with a minute remnant of a second podomere (r). Flagellum (f) ca. as long as the coxite, distally densely microsetose, giving it a brush-like appearance; tip not branched.

Posterior gonopod (Figs 14-16) stout, with a massive base, distally divided in two parts: a broad, shield-like posterior process (p), marginally and disto-frontally bearing long setae; and a simple, unipartite anterior process (a), gradually narrowing towards a fine, somewhat bent, filiform apex; mesobasal hump bearing a short spine (sp).

Female sexual characters: Leg-pair 1 somewhat thicker and shorter, leg-pair 2 also shorter, but not thicker than the following legs. Vulva (Fig. 17) elongated, subconical; operculum (op) slightly higher than bursa, both rather sparsely setose; receptaculum seminis (rs) rather small compared to overall size of vulva, in shape of a simple tube, with some very short, pocket-like branches.

Remarks.

With its light coloration, slender legs and antennae, and pigmentless ocelli S. membranipedalis seems to be the most troglobiomorphic species within the genus (see e.g. Liu et al. 2017). Of the remaining six species occurring in caves, namely S. ikedai , S. inferus , S. osedoensis , S. platypodus , S. sidegatakedensis , and S. toriii , only S. sidegatakedensis and S. ikedai display similarly light-colored bodies, and all possess blackish ocelli, except for S. ikedai and S. platypodus , for which this feature is unknown.

However, the presence of ocelli in S. membranipedalis suggests a still ongoing adaptation towards troglobiism, meaning that the species has entered the underground relatively recently, possibly in the Pleistocene, in response to the increasingly cooler and drier climate and the gradual replacement of forests by grasslands in the temperate zones - the so called "climatic relict hypothesis" as a model of subterranean colonization ( Culver and Pipan 2009). This makes sense also in the light of the rather young geological age of the caves Shi-Hua and Yun-Shui (see introduction part).

Apart from its troglobiomorphic alterations, S. membranipedalis is morphologically most similar to S. coreanus and S. serratus , with whom it shares the lack of claws in the male first legs, the elongated coxites of male 7th legs, the apically non-branched flagellum, and the well-divided posterior gonopod. On the other hand, its six-segmented male first legs suggest a proximity to S. hakui , but this is in contradiction with the apically bifurcated flagellum in the latter species.