Prionobelum naevium Donworth & Wesener, new species

Figs 2C, 6B, 7, 8

Material examined. Holotype: Male, MHNG 4B-1, Thailand (Province Yala): Distr. Than To, Bang Lang National Park ( Than To Waterfall), 150 m, 1.II.1991, leg. P. Schwendinger.

Paratypes: 1 male, NHMD 1184671, Thailand, Yala Province, Bang Lang NP, lowland rainforest, <400 m, 6°04’N, 101°11’E, coll. M. Andersen, O. Martin & N. Scharff, 20.X.1991 ; 1 female, NHMD 1184672, Thailand, Yala Province, Bang Lang NP, lowland rainforest, <400 m, 6°04’N, 101°11’E, coll. M. Andersen, O. Martin & N. Scharff, 20.X.1991 ; 1 male, NHMD 1184673, Thailand, Yala Province, Bang Lang NP, lowland rainforest, 270 m, 6°11’47.5”N; 101°9’50.9”E, coll. ATOL Expedition 2003, 15.X.2003 .

Derivatio nominis. Latin adjective meaning ‘having moles or spots’. Named after the irregular black spots covering the tergites (Fig. 2C), first thought to be artifacts of bleaching in ethanol, but also visible in living specimens (Fig. 6B).

Diagnosis. Prionobelum naevium n. sp. is a large (> 35 mm length,> 20 mm width) giant pill-millipede species with a shiny, glabrous surface and a reddish-brown-black coloration pattern. P. naevium n. sp. is far larger than all other known Prionobelum species, none of which are larger than 30 mm in body length. The axe-shaped antennomere 6 with>100 apical cones (Fig. 8B) is unique for this species among Prionobelum, with only P. durum showing a non-cylindrical antenna (which is swollen but not axe-shaped). The posterior telopod with telopoditomere 4 strongly projecting above the immovable finger (Fig. 7E, F) is shared with P. durum, P. joliveti, P. leve and P. multidentata, but P. naevium n. sp. differs from all of the latter species in the shape and dentition of the anterior telopods (Fig. 7B–D).

Description (based on holotype).

Size: Length: 36.8 mm. Width of thoracic shield 21 mm, of widest segment (9th) 21.3 mm. Height of thoracic shield 11.4 mm, of highest segment (9th) 12.1 mm.

Colour (Fig. 2C): Faded after 20 years in alcohol. Head black. Antennae olive green with dark brown spots. Legs medium brown, tarsi dark brown and olive green. Tarsal claws apically dark brown. Collum black. Thoracic shield black, in some areas faded to dark brown. Tergites light brown with chestnut spots, posterior margin black. Anal shield light brown with extensive chestnut spotting (Fig. 2C).

Head: Number of ommatidia ca. 70. Tömösváry organ located in antennal groove.

Antennae (Fig. 8A–C): Antennomere lengths: 6>1>2>3>4>5 (Fig. 8A, B). Antennae reaches leg pair 3. Antennomere 6 strongly apically swollen, axe shaped, number of apical cones 106/123 (Fig. 8B).

Gnathochilarium: Lamellae linguales with numerous long setae, medially glabrous. Sensory cones on gnathochilarium palps in pads.

Mandible: Not examined.

Collum: Largely glabrous with one row of setae on posterior margin and 2–3 rows near the anterior margin.

Thoracic shield: Thoracic shield grooves anteriorly narrow and steep, laterally wide (Fig. 2C), with 9 crests (sclerotized ledges?) on each side.

First stigmatic plate: rounded, with a sub-triangular apex.

Tergites: Tergites glabrous, shiny, at (100x magnification) sparse tiny pores become visible. Paratergite tips on posterior half straight (Fig. 2C).

Pleurites: Pleurite 1 with a slightly elongated and very rounded tip. Pleurite 2 projecting less than 1 and with a very rounded apex. Pleurite 3 and following rectangular with no tip.

Endotergum (Fig. 8D, E): with a regular flat margin. Outer zone with two or three dense rows of irregular marginal setae extending up to two thirds of the distance from setal base to posterior margin (Fig. 8D, E). A single row of rounded cuticular impressions 30 µm in diameter present next to marginal ridge, distance between impressions 30 to 45 µm (Fig. 8D). Intersegmental membrane smooth, without cones and with very few setae (Fig. 8D). Setae with tiny scales (Fig. 8E).

Legs: Ventral spines on leg pair 1 3/4, on 2 7/8, on 3 10/11. Apical spine on leg 3 present. A single apical spine and 12 to 14 ventral spines on midbody legs (Fig. 7A, arrow). Femur regularly shaped, 1.9 times longer than wide. (Fig. 7A). Tarsus 4.5 times longer than wide (Fig. 7A).

Anal shield: Glabrous. One medium-sized locking carina 3 times the length of those on tergites, located close to margin.

Male sexual characteristics: Gonopore inconspicuous, consisting of large membranous opening located directly at mesal margin.

Anterior telopods (Fig. 7B–D): Syncoxite without any peculiarities (Fig. 7D). Podomere 1 rectangular, as long as wide (Fig. 7D). Long setae covering both medial and lateral sides of the first 2 podomeres, as well as much of the posterior side of podomere 2 (Fig. 7B, D). Podomere 2 with a large, well-rounded immovable process protruding inward towards podomere 3 (Fig. 7B, D, arrow), which does not extend to podomere 4 (Fig 7B). Podomere 3 and 4 irregularly fused, suture visible in lateral but not in anterior view (Fig. 7C, D). Podomere 3 and 4 strongly curved backwards, with a pointed, sharp tip towards process of podomere 2. Podomere 3 with sparse setation, bending towards immovable finger. Podomere 4 very short, with one laterally protruding spine and three nodules near tip on mesal side (Fig. 7B, C).

Posterior telopods (Fig. 7E, F): Inner horns projecting into white triangular membranous tip (not drawn). Podomere 1 slightly longer than wide, clusters of setae on lateral and mesal margins (Fig. 7E). Podomere 2 setose, with especially dense setation on immovable finger. Immovable finger straight and long, apically tapering in a well-rounded projection, reaching but shorter than podomere 4. Membranous lobe extending from immovable finger bearing two elongated processes, fused at base. Podomere 3 posteriorly glabrous, with sparse setation on anterior side and 20 or 21 small crenulated teeth on posterior side (Fig. 7F, arrow). Membranous ledges on podomeres 3 and 4. Podomere 4 glabrous, curved and overlapping tip of immovable finger and bearing a posteriorly visible spine (Fig. 7F).

Distribution. Currently, this large and conspicuous species is only known from the lowland rainforests of the Bang Lang National Park where it appears to be relatively widespread (Fig. 6A). The smaller species Sphaerobelum meridionalis Bhansali & Wesener, 2022 occurs in direct sympatry.