Holotype: male (ISEA 6649): Bulgaria, Black Sea coast, near Tsarevo, Popska River, moss, soil and detritus, 42°10'31.7"N, 27°50'21.3"E, 16 m elev., 26.VI.2017, coll. D. Georgiev. Paratypes: 2 females (ISB A-791.1 and SMNH 90.1) same data as holotype. Other material: 10 females, 8 males, 1 preimago, 7 maturi juniores, 2 larvae II, 1 larva I, Bulgaria, St. Kirik and Yulita Monastery near Plovdiv, Carpineto-Fagetum forest, sample at decaying stump, 13.VI.1990, coll. J. Rusek; 4 females, 2 males, 1 preimago, 2 larvae II, Bulgaria, south foothills of Stara Planina, near Gurkovo town, mixed forest with Robinia pseudoacacia (L.) Gaerth., soil, 42°41'19.10"N, 24°45'09.08"E, 372 m elev., 30.VIII.2015, coll. C. Fiera; 2 females, Bulgaria, Sarnena Gora Mountains, near Kolena village, bank of stream, soil and detritus in roots of Alnus glutinosa (L.) Gaerth., 42°29'62"N, 25°41'28.61"E, 300 m elev., 15.VI.2017, coll. D. Georgiev; 1 maturus junior, 1 larva II, Bulgaria, Sarnena Gora Mountains, near Kolena village, Pinus nigra J. F. Arnold., soil and detritus, 42°24'03.1"N, 25°34'09.8"E, 296 m elev., 6.VI.2017, coll. D. Georgiev.

Setae P1a present on tergites II–VII, absent on tergite I; setae P3a present on tergite VII. Abdominal tergites VI–VII with eight anterior setae. Sternites I–III without pores, sternites IV–V with 1+1 spsm pores, sternite VI with composed spsm pores (2+2 or 2+3 pores placed adjacent to each other), sternite VII with a spm pore. Foretarsal sensilla a, b and c long, setae β1 and δ4 thick, stout and sensilliform, δ4 situated proximally to the level of sensillum c’ base.

Habitus is shown on Figure 1A. Head setae l3, sd4 and sd5 short thickened sensilla, additional seta d6 lacking (Figs 1B, C, 5A, B), length ratio of posterior setae d7:sd7 as 1.0:1.4 (Fig. 5A). Pseudoculus circular, with indistinct posterior extension, PR = 15–17 (Fig. 5B). Sensilla of maxillary palps slender, differing in length, dorsal (d) sensillum shorter than ventral (v) (Figs 1E, 5C). Labial palps with four-branched tuft of apical setae and a slender sensillum (Figs 1F, 5D). Maxillary gland with rounded calyx, long and slender posterior filament and bilobed posterior dilation (Figs 1D, 5E), CF = 4.4–5.5.

Foretarsus with t1 claviform, t3 leaf-like and the same length as t1 (Figs 1H, 5F, J). All other sensilla slender, except the broadened sensillum a’ (Figs 1H, 5J). Sensillum a long, reaching base of seta γ3; b and c long, extending past base of seta γ3, b slightly shorter than c (Figs 1G, 5F). Base of d close to c, near t2 insertion; a’ situated distal to t1 insertion (Figs 5F, J). Relative length of sensilla: (t1 = t3) < (b’= c’) < a’< g < t2 < e < (c = d) < f < b < a. Setae β1 and δ4 sensilliform and thickened, each 7 µm long (Figs 1H, I, 5J). Seta δ4 situated on the level of δ5, proximal to c’ base (Figs 1I, 5J). Single pores situated near bases of sensilla c and t3. Claw long, without inner tooth, empodial appendage short. BS = 0.3, TR = 3.7–4.1, EU = 0.1.

Formula of chaetotaxy given in Table 1. Setae on nota strongly differing in length (Fig. 6A). Length ratio of pronotal setae 1: 2 as 3.2: 1 (Figs 2A, 6A). Setae P1a and P2a on mesonotum and metanotum as small gemmate microchaetae, P4 on metanotum sensilliform, short and thick (Figs 2B, C, D, E, 6A). Seta P2a situated close to P3. Length ratio of P1: P2 on mesonotum as 1:1.2–1.4. Mesonotum with sl and al pores, metanotum with sl pores only (Figs 2B, E, 6A). Thoracic sterna without pores (Figs 3A, B, C, 6D, E). Setae A2 on sterna and M2 on prosternum short sensilliform and thickened (Figs 3A, 6D).

Seta P2a on tergite I of same shape as P1a and P2a on nota, P3 and P4 short and setiform; A5 a short thickened sensillum (Figs 2F, 6B). Accessory setae P1a, P2a and P4a on tergites II–VI short, sensilliform and thick, on tergite VII setae P1a, P2a, P3a and P4a thin and setiform (Figs 2G, 6C, F). Position of seta P3 on tergites II–V anterior to other P-setae, on tergites I and VI–VII P3 in the P-setae row (Figs 2F, G, 6B, C, F). Tergites II–VII each with a transverse connecting line in the anterior region (Fig. 6C, F). Pores psm on tergites I–VII, al on tergites II–VII, psl on tergites VI–VII (Figs 2F, G, 6B, C, F). Abdominal legs I with 4 setae, abdominal legs II and III with 3 setae: medial apical (ma), lateral apical (la) and subapical (sa) (Figs 3D, 6I). Accessory setae on sternites I–VI the same length (4 µm) and sensilliform as on tergites (Figs 3F, 6H, J). Accessory setae on sternite VII the same shape and length as on tergite VII (Figs 3G, 6K). Sternites II–III each with a connecting line anteriorly and with short lines in the anterolateral region (Fig. 6H); sternites IV–VI with two connecting lines, sternite VII with one connecting line (Figs 6J, K). Sternites I–III without pores. Sternites IV–V with 1+1 spsm pores (Fig. 3F), sternite VI with composed spsm pores (2+2 or 2+3) (Fig. 6J), sternite VII with single spm pore (Figs 3G, 6K).

Abdominal segment VIII with distinct striate band; tergite with a transverse row of small teeth and sternite with two rows of teeth (Figs 3J, 6G). Comb VIII with 10–12 small teeth (Fig. 6G). Pore psm without accompanying teeth. Posterior margin of sternite VIII and laterotergites smooth (Fig. 3I). Setae 1 and 1a on tergites IX and X of equal length (Fig. 3H). Dorsal lobe of Abd. XII with single median pore, ventral lobe with 1+1 sal pores. Female squama genitalis with distinct distal prolongation on stylus and long pointed acrostylus (Figs 3K, 5H). Male squama genitalis with 5+5 setae (Fig. 5I).

Body measurements (18 adults) (in µm): maximum body length 1150, head 135–138, pseudoculus 8–9, posterior part of maxillary gland 25–30; posterior cephalic setae d7 15–16, sd7 20–23, l5 7; pronotal setae 1 35–48, 2 12–16; mesonotal setae P1 35–45, P2 45–55; foretarsus 112–115, claw 28–30, empodial appendage 3.

Asymmetrical absence of seta A4 (5 specimens), and seta A2 (2 specimens) on tergite VI, asymmetrical absence of seta P1a on tergite II (1 specimen), asymmetrical presence of additional small seta P3a’ on mesonotum (1 specimen, Fig. 2B).

This species belongs to the confinis group of Acerentulus species characterized by long foretarsal sensilla a and b; only Acerentulus berruezanus Aldaba, 1983 is characterized in possessing P1a setae on tergites I and VII (setae P1a absent on tergites II–VI). Other members of the confinis group are characterized by the absence of P1a setae on tergites I–VI. Three species from the cunhai group, A. proximus Szeptycki, 1997, A. correseanus Szeptycki, 1997 and A. tuxeni Rusek, 1966, have a similar chaetotaxy (setae P1a present on tergites II to VII, eight A-setae on tergite VII) and porotaxy (notal sterna and sternites I–III without pores, sternites IV–V with 1+1 spsm pores, sternite VII with a spm pore, sternite VI with two groups of spsm pores 2+2 or 2+3 in some males, except A. tuxeni, which has 1+1 spsm pores). However, as members of the cunhai group these three species have short sensilla a and b.

In preimago specimens the A4 setae on tergite VII are absent symmetrically or asymmetrically. Setae P1a on tergites II–VII and P3a on tergite VII appear in the maturus junior stage (Fig. 4A, B). All pores on the tergites and sternites are present by the maturus junior stage, except for the posterosublateral pores (psl) on tergite VII, which appear in the preimago.