Small (< 10 mm long) moniliform polydesmoids with pale body and 19 segments in both sexes (Figs 1, 5). Metaterga mostly with 2–3 rows of setae; exceptions are M. saetosus (Strasser, 1960) and M. bosniensis (Verhoeff, 1901) (see below). Pleurosternal carinae poorly developed, present only on ring 2 or on rings 2–4. Antennae long, clavate; antennomeres 5 and 6 each with a group of bacilliform sensilla. In males, femora of pregonopodal legs enlarged, each usually with a small adenostyle. Gonopods directed anteriad, with a somewhat elongated and slender coxite (cx). Prefemorite (pf) densely setose, mostly elongated, at least equal in length to acropodite, the only exception is M. albus with a shorter prefemorite. Acropodite consisting of two or three, more or less well-developed branches. The two dominant branches are mostly solenomere (sl) and solenophore (sph); the exception is M. petrovi sp. n. with three acropodital branches, but with a completely reduced solenophore.

Holotype male (NMNHS), Bulgaria, Western Rhodopi Mts., Satovcha District, Cave Stapalkata, 650 m a.s.l., clay-guano, 17.VI.2006, B. Petrov & P. Stoev leg.

2 males, 5 females (NMNHS), same data as holotype.

2 females, 1 juvenile (NMNHS), Bulgaria, Western Rhodopi Mts., Pazardzhik District, Peshtera town, Cave Snezhanka, 19.IV.2009, P. Beron leg.; 1 male, 1 female (NMNHS), Bulgaria, Plovdiv, Bunardzhik Hill, under decaying wood, 10.IV.2018, P. Mitov leg.

The species is named after Boyan Petrov, a renowned Bulgarian mountaineer, a dear friend and colleague zoologist from the National Museum of Natural History, Sofia, who disappeared in Tibet in May 2018 during the ascent of his 11^th eight-thousander, Shishapangma. Boyan was one of the collectors of this new species. Noun in genitive case.

The new species belongs to the Metonomastus group of species with three acropodital branches, but clearly differs from both previously described species in this group, M. strasseri Hoffman & Lohmander, 1968, and M. pomak Golovatch & Stoev, 2004, by the presence of a strongly developed, beak-shaped solenomere, by the completely reduced solenophore and by the development of a strongly arched, microspiculate, ventromesal, acropodital process.

Length 4.4–4.8 mm (males), 4.2–5.5 mm (females). Width of midbody rings 0.31–0.37 and 0.33–0.42 mm (males), 0.35–0.42 and 0.42–0.47 mm (females) on pro- and metazonae, respectively. Holotype male 4.8 mm long, 0.37 and 0.42 mm wide on midbody pro- and metazonae, respectively.

Body moniliform (Figure 1A), with 19 segments in both sexes. Colouration entirely pallid. Texture microreticulate throughout (Figure 2A, B).

Figure 1. 

Metonomastus petrovi sp. n., holotype ♂ A habitus, lateral view B anterior part of the body, lateral view C posterior part of the body, lateral view.

Head densely pubescent throughout, clypeolabral region densely setose. In males, width of head (0.41–0.5 mm broad) >> collum ≤ ring 2 ≤ 3 ≥ 4 < 5 ≥ 6 = 17; thereafter body gradually, but significantly tapering. In females, width of head (0.42–0.5 mm broad) >> collum = ring 2 < 3 = 4 < 5 = 17; thereafter body rather rapidly tapering; in larger females, ring 5 = 14 < 15 = 17.

Antennae (Figure 2C, D) long, clavate, in situ reaching behind ring 2 when stretched dorsally. Antennomere length 2 = 3 = 6 > 4 = 5 = 7 > 1, 6^th being the thickest, both 6^th and 5^th each with a compact group of bacilliform sensilla dorso-apically. Interantennal isthmus ca 0.7 times as broad as diameter of antennal socket.

Collum with 3 rows of setae; two rows of similar setae per postcollum metatergum: one frontally, the other caudally, setae long and simple. Metaterga (Figure 2A, B) with a weak, mid-dorsal, transverse sulcus on rings 5 to 17; sulcus absent from 18^th. Paraterga (Figure 2A, B) laterally extremely poorly developed, being very faintly delimited by a shallow sulcus only dorsally. Certain midbody rings occasionally with visibly more or less strongly developed paranota compared to neighboring rings. Ozopores indistinct, located near posterior margin of paraterga; pore formula normal. Strictures between pro- and metazonae very faintly striolate, deep and narrow. Pleurosternal carinae present on ring 2 as small lobes, thereafter missing. Limbus faintly microcrenulate. Epiproct long, slender, nearly half as long as telson height. Hypoproct semi-circular, 1+1 strongly separated caudal setae borne on minute knobs. Sterna broad and weakly impressed.

Figure 2. 

Metonomastus petrovi sp. n. A paratype ♀, ring 9, dorsal view B paratype ♀, ring 10, lateral view C paratype ♂, left antenna D paratype ♂, tip of left antenna. Scale bars: 0.1 mm (A, B, C), 0.05 mm (D).

Legs (Figure 2B) about 1.1–1.2 times as long as midbody height in males, 0.8–0.9 times in females, without modifications, only pregonopodal male prefemora slightly bulged dorsally (Figure 3A, B).

Gonopods (Figs 3C, 4): Coxite (cx) massive, moderately long, dorsolaterally very sparsely setose. Prefemorite (pf) somewhat longer than acropodite, densely setose ventrally. Postfemoral sulcus distinct, short, traceable on median and, partly, ventral sides. Acropodite consisting of three branches: mesal process (ss) curved and saddle-shaped, ventromesal process (vm) more strongly arched, its outer surface microspiculate (Figure 4F), solenomere (sl) robust, blunt, somewhat beak-shaped, ventrolaterally with a deep and broad transverse groove. Seminal groove running on mesal side all along prefemorite, then shifting dorsally on acropodite, terminating ventromesally on solenomere.

Figure 3. 

Metonomastus petrovi sp. n., paratype ♂ A leg 4 (claw fallen off) B leg 6 C right gonopod, mesal view. Abbreviations: cx coxite, pf prefemorite, sl solenomere, ss saddle-shaped process, vm ventromesal process. Scale bars: 0.1 mm (A, B), 0.05 mm (C).

This species is known both from caves and from an epigean environment. Like some other representatives of the genus, this new species can be considered a troglophile.

Figure 4. 

Metonomastus petrovi sp. n. A paratype ♂, left gonopod, ventromesal view B paratype ♂, distal part of left gonopod, ventromesal view C paratype ♂, right gonopod, ventral view D non-type ♂, right gonopod, mesal view E non-type ♂, right gonopod, ventral, slightly lateral view, F non-type ♂, right gonopod, ventromesal process, same view, enlarged. Abbreviations: cx coxite, pf prefemorite, sl solenomere, ss saddle-shaped process, vm ventromesal process. Scale bars: 0.2 mm (B), 0.05 mm (A, C, D, E), 0.005 mm (F).

Holotype male (NHMSC), Croatia, Dalmatia, island of Mljet, Blato, near Kozarica, under stones, 23.XII.2015, T. Rađa leg.

1 male (NHMSC), 2 males (IZB), 4 females (NHMSC), 7 females (IZB), 4 juveniles (IZB), same data as holotype.

Figure 5. 

Metonomastus radjai sp. n., paratype ♀ (IZB) A habitus, lateral view B anterior part of the body, lateral view C posterior part of the body, lateral view. Scale bars: 1 mm (A), 0.5 mm (B, C).

The new species is named after the collector, Tonći Rađa, a renowned Croatian biospeleologist who discovered numerous new or interesting arthropods. Noun in genitive case.

The new species belongs to the Metonomastus group of species with two postfemoral branches, but clearly differs from all of these by the presence of a well-developed, broad, lamellar solenophore (= tibiotarsus) directed strongly mesad, and proximally curved ventrad. The solenomere is without additional processes and is also directed strongly mesad.

Figure 6. 

Metonomastus radjai sp. n., paratype ♀ (IZB) A anterior part of the body, lateral view B head, frontal view C pleurosternal carinae of the body rings 2 and 3, lateral view D tip of left antenna (with artefacts) E left antenna. Arrows indicate pleurosternal carinae (A, C) or artefacts on antenna (D). Scale bars: 0.2 mm (A), 0.1 mm (B, E), 0.05 mm (C, D).

Length 3.7–3.9 mm (males), 4.3–5 mm (females). Width of midbody rings 0.30–0.33 and 0.37–0.4 mm (males), 0.42–0.44 and 0.47–0.5 mm (females) on pro- and metazonae, respectively. Holotype male 3.7 mm long, 0.3 and 0.37 mm wide on midbody pro- and metazonae, respectively.

All other characters as in M. petrovi sp. n., except as follows.

Microreticulated texure more obvious (Figs 6A, 8A, B). Width of head 0.42–0.44 mm (males), 0.5–0.55 mm (females). Antennomere length 2 = 3 = 6 > 4 = 5 > 7 > 1 (Figure 6E). Postcollum metaterga each with a barely visible, mid-dorsal, transverse sulcus (Figure 8A, B). Pleurosternal carinae present on rings 2–4 in the form of small denticles, more strongly developed on 2^nd ring (Figure 6A, C). Legs about 1.5 times as long as midbody height in both sexes. In males, pregonopodal legs each with a distinct dorsobasal hump on prefemur; leg-pairs 5–7 each with a ventrobasal femoral adenostyle (Figure 7B, C).

Figure 7. 

Metonomastus radjai sp. n., paratype ♂ (IZB) A leg 4 B leg 5 C leg 6. Scale bar: 0.1 mm.

Gonopods (Figs 8E-G, 9): Coxite (cx) massive, slightly elongated, laterally with three setae, one being particularly long. Prefemorite (pf) subquadrate in ventral and dorsal views, somewhat narrower in lateral and mesal views, about as long as acropodite, densely setose ventromesally. Acropodite represented by two processes: solenomere (sl) and solenophore (sph). Both solenomere and solenophore simple and directed mesad. Solenomere ventral in position, slender, acuminate, distally terminating in two small teeth. Solenophore strongly developed, dorsal in position, wide, lamellar, proximally curved ventrad. Seminal groove running through dorsomesal side of prefemorite, then shifting laterodistally through solenomere.

This species was found under stones next to a dry stone wall close to the seashore.

Figure 8. 

Metonomastus radjai sp. n., A, B, C, D paratype ♀ (IZB) A body rings 9 and 10, dorsal view B body rings 9 and 10, lateral view C body ring 10, ozopore lateral view D body ring 11, caudal view E, F, G paratype ♂ (IZB), right gonopod E oral (distal) view F ventral view G ventromesal view. Abbreviations: cx coxite, pf prefemorite, sl solenomere, sph solenophore. Scale bars: 0.1 mm (A, B, D), 0.05 mm (E, F, G), 0.01 mm (C).

Figure 9. 

Metonomastus radjai sp. n. paratype ♂ (IZB), right gonopod A dorsal view B mesal view C ventral view D lateral view. Abbreviations: cx coxite, pf prefemorite, sl solenomere, sph solenophore. Scale bar: 0.1 mm.