Holotype ♂ (MRAC 20150), Democratic Republic of the Congo, Parque National Albert, secteur Sud, Berlese extraction, 1956, leg. R.P. Celis. Paratypes: 3 ♂ (MRAC 20150), same data, together with holotype; 5 ♀ (MRAC 20151), same data; 1 ♂, 1 ♀ (ZMUM p2441), same data; 2 ♀ (MRAC 20171), same data; 4 ♂, 2 ♀ (MRAC 20149), same data; 1 ♂, 1 subadult ♀ (MRAC 20274), same data; 1 ♂ (MRAC 20201), same data; 5 ♂, 16 subadult & earlier instar ♂, 1 ♀ (MRAC 20294), same data.

To emphasize the provenance of the new species from Congo.

Minute polydesmidans (length 1.5–2.1 mm, width 0.6–0.8 mm) with 18 or 19 body segments in both sexes, missing ozopores, simple biramous gonopods, and evident sexual dimorphism in tergal structure: metaterga in ♂ very densely pilose all over, with only few longer setae, devoid of a median transverse gutter, whereas ♀ metaterga with a deep transverse gutter in anterior third supporting a single row of ca 10–18 long setae positioned at gutter’s bottom.

Length of adults 1. 5–2.1 mm, width 0.6–0.9 mm, ♂ usually a little smaller than ♀. Adult body with 18 or 19 segments (17+1+T) (♂, ♀). Entire dorsal surface covered with a thin layer of secretions (= cerategument) borne by microvilli, often also a dirt crust, under which the body integument is uniformly yellowish. Body shape as in Figs 1A, 3A, with caudal body end tapering towards a relatively small telson, the latter not being concealed by last paraterga (Figs 1C, 3F). Head small, only partly concealed under front edge of collum (Figs 1B, 3B); upper half of head densely granulate, lower part smooth and densely setose. Interantennal isthmus about as wide as antennomere 1 or antennal socket diameter (Fig. 1B). Antennae very strongly clavate due to subequal antennomeres 5 and 6 (Fig. 1B). Collum subquadrate, slightly impressed along longitudinal axis (Fig. 3B); tergum 2 as usual, hypertrophied, with strongly enlarged, spatuliform paraterga concealing the head in lateral view (Figs 1A, 3A, C). Limbus nearly smooth, at most with sparse, well separated, short spicules. Metaterga either regularly convex and uniformly very densely pilose throughout, with only few scattered longer setae (♂, including juveniles) (Fig. 1E, F, G) or nearly smooth, showing a deep transverse gutter in anterior 1/3, the latter supplied with ca 10–18 especially long, microspiculate setae at bottom and bearing a small pit marking ventral end of gutter about midheight of paraterga (♀, including juveniles) (Fig. 3A, D). Prozonae and stricture thereafter beset with round scales with peripheral hair-like cuticular outgrowths (Fig. 1F, G). Paraterga vertical, continuing the highly convex outline of dorsum, their ends rounded, projecting far below venter/coxae (Figs 1E, 3E), increasingly angular towards telson (Fig. 3E). Anteroventral edges of paraterga 3 to 15 each with a notch forming a groove for paratergum 2 to hinge into during volvation. Ozopores absent. Telson small (Figs 1C, 3F). Hypoproct triangular (Figs 1C, 3F).

Legs rather slender, but short, barely reaching tips of paraterga (Fig. 1C, I); femora and tarsi subequal in length, longer than other podomeres; claw normal, simple, very slightly curved ventrad; last pair of ♂ and ♀ legs modified, typical of Ammodesmus, with a long distodorsal flagellum borne on a stump on tibia (Figs 1H, 3G).

Figure 1. 

Ammodesmus congoensis sp. n., a 18-segmented ♂ paratype. A habitus of male, lateral view B head, ventral view C posterior part of body, caudal view D caudal view of a midbody segment, caudal view E midbody paraterga, lateral view F, G tegument texture, dorsal view H last left leg, lateral view; I both gonopods in situ, ventral view J left gonopod, submesal view. Scale bars: A = 200 µm; B, D = 100 µm; C, I = 50 µm; E, F = 20 µm; G, J = 10 µm. (l lateral finger-shaped branch, m mesal finger-shaped branch, pu pulvillus).

Gonopods (Figs 1I, J, 2) relatively simple. Coxae small, globulose, micropapillate and microsetose laterally. Telopodite long and well-exposed beyond gonocoxae, divided into two finger-shaped branches: a shorter mesal (m) and a longer lateral (l); a small hairy pulvillus (pu) marking the orifice of a short seminal groove at base between m and l. A solenomere absent.

Figure 2. 

Gonopods of a 18- (A, B) and a 19-segmented ♂ paratype (C). A, B both gonopods in situ, frontal and caudal views, respectively C left gonopod, frontal view. Scale bar: 1.0 mm (cx coxae, l lateral finger-shaped branch, m mesal finger-shaped branch, pu pulvillus, tl telopodite).

Figure 3. 

Ammodesmus congoensis sp. n., a 19-segmented ♀ paratype. A habitus, lateral view B head and collum, dorsal view C, D, E anterior, middle and caudal parts of body, respectively, lateral view F posterior part of body, caudal view G last left leg, lateral view H tergal setae, dorsal view I tegument texture, dorsal view. Scale bars: A, B = 200 µm; C, D = 100 µm; E, F = 50 µm; G, H = 20 µm; I = 5 µm.

The total absence of ozopores in A. congoensis sp. n. is well documented by Fig. 1A which shows an adult ♂ with 18 segments clean enough to be certain. In both other Ammodesmus species, in addition to the pits near the base of the paraterga, which are also observed in the new species, ozopores are clearly visible closer to the ventral ends of paraterga 5, 7, 9, 10, 12, 13, 15–17(18). A similar pore formula is observed in Elassystremma species: 5, 7, 9, 12, 13, 15–17(18). As variation in ozopore distribution in Ammodesmidae is a proven fact, we are reluctant to create a separate, new genus for A. congoensis sp. n. only because it totally lacks ozopores. The most important character that brings the new species together with A. granum and A. nimba is the strangely modified last leg-pair.

Variation in adults of both sexes showing either 18 or 19 segments is another piece of evidence bringing A. congoensis sp. n. especially close to A. granum. The latter has hitherto remained the only species of Polydesmida where the number of segments varies regardless of sex. As in the ♂ the size of the body likewise correlates positively with gonopod size (Fig. 1I), an additional molt of the 18-segmented adult turning it into the 19-segmented one is obviously implied.