The millipede genus Stemmiulus Gervais, 1844 in Cameroon, with descriptions of three new species (Diplopoda, Stemmiulida, Stemmiulidae)

Abstract The large pantropical millipede genus Stemmiulus, which currently encompasses more than 150 species, i.e. the bulk of the species diversity of the family Stemmiulidae and entire order Stemmiulida, is shown to comprise seven species in Cameroon, including three new ones: S. ongot Nzoko Fiemapong & VandenSpiegel, sp. n., S. uncus Nzoko Fiemapong & VandenSpiegel, sp. n., and S. mbalmayoensis Nzoko Fiemapong & VandenSpiegel, sp. n. In addition, S. beroni Mauriès, 1989, previously known only from the type locality in Nigeria, is recorded from Cameroon for the first time, also being redescribed based on new samples. A key is given to all species of the genus encountered in the country, based on male gonopodal conformation, except for S. camerunensis (Silvestri, 1916), which was described only from female and juvenile material.


Introduction
The Stemmiulida is a small pantropical order of Diplopoda which contains only three genera in a single family, Stemmiulidae. According to the latest classification (Enghoff et al. 2015), apart from two monobasic genera, one each in the Caribbean and Vietnam, the family is largely represented by the likewise pantropical genus Stemmiulus Gervais, 1844. Its 150+ species in comparable shares range from Central (one species introduced to Florida, USA) to northern South America (south to the Brazilian states of Amazônas and Bahia, as well as northern Peru), on the one hand, and Central Africa, on the other. Several Stemmiulus species occur in southern India and Sri Lanka, while only a few marginally also in New Guinea and the neighbouring island of Halmahera, Indonesia (Mauriès et al. 2010; Shelley and Golovatch 2011).
At present, Stemmiulus in Africa is comprised of 51 species or subspecies (Table 1) which range from Senegal to Tanzania and cover most of tropical Africa with the exception of southern Africa and Madagascar (Shelley and Golovatch 2011). Of them, only four species have been reported from Cameroon. The present paper puts on record three new species of Stemmiulus from Cameroon. In addition, S. beroni is found in Cameroon for the first time, also being redescribed from new samples, the first outside its type locality in Nigeria.

Materials and methods
The material underlying the present contribution was collected in Cameroon in 2014-2016. All type specimens are housed in the collection of the Royal Museum for Central Africa, Tervuren, Belgium (MRAC). The samples are stored in 70% ethanol. Specimens for scanning electron microscopy (SEM) were air-dried, mounted on aluminium stubs, coated with gold, and studied using a JEOL JSM-6480LV scanning electron microscope. Photographs were taken with a Leica DFC 500 mounted on a Leica MZ16A stereomicroscope. Images were processed with Leica Application Suite. After examination, SEM material was removed from stubs and returned to alcohol, all such samples being kept in MRAC. Diagnosis. Small to medium-sized stemmiulid millipedes, reaching up 50 mm in length. Body compressed laterally, tapering gradually towards telson, metaterga striated, eyes consisting of one or two large ommatidia on each side of head.  Paratype: 1 ♂ (SEM, lost). Etymology. The species is named after Ongot, the type locality. Diagnosis. Stemmiulus ongot sp. n. is characterized by the first six pairs of male legs being densely setose, the lateral projection of the subterminal lobe of the gonopodal angiocoxites relatively short (Fig. 1H, I), the apical parts of the angiocoxite densely setose (Fig. 1H, I) and, especially, by the peculiar second pair of male legs ( Fig. 1E-G), the telopodites of which are 2-segmented, the proximal segment being expanded apicolaterally and bearing a lateral fringe of setae.

Stemmiulus ongot
Description. Holotype: adult male, ca 15 mm in length, 1.7 mm in maximum diameter, body with 43 rings. Head and collum dark brown, other body rings brown with a light axial dorsal stripe, legs and antennae yellowish.
Collum without any ornamentation. Body rings ovoid in transverse section, height/ width ratio of midbody rings ca 0.41; no legless body rings in front of telson. Prozonites smooth, metazonites with oblique transverse striae.
First six pairs of legs covered with numerous plumose setae. First pair unmodified, tarsi with a fringe of ventral setae in basal 2/3, but forming no true brush, coxae, femora, postfemora and tibiae each with an apical cluster of prominently enlarged spatulate setae (Fig. 1B, C).
Second pair of legs with coxa enlarged and elongated, anterior face with traces of segmentation, setose over entire anterior surface, glabrous on posterior surface; laterally each produced into a prominent, elongated, conical projection (Fig. 1F, G) and with an apicomesal cluster of elongated setae. Telopodite 2-segmented, proximal segment with an apicolateral projection bearing a lateral fringe of setae, an apicomedial cluster of setae and a ventromedial cluster of long setae (Fig. 1F, G); distal segment long and slender, curved mesad, with a basal cluster of setae and plumose distally (Fig. 1G).
Pair 7 similar to following ones, without specialized setae. Gonopod structure (Fig. 1H, I) typical of the genus, angiocoxite with a small, projecting, subapicolateral process. Apex of colpocoxite simple, with neither a lobe nor a projection surrounding the flagella (Fig. 1H).
Paragonopods small and 3-segmented, median segment carrying a short series of long setae on medial side, distal segment minute, conical, with a few apical setae (Fig. 1D).
Relationships. By the relative complexity of the gonopodal structure S. ongot sp. n is closely related to S. albicephalus from Tanzania, but the striations of the lateral sides of prozonae remind of those observed in S. infuscatus from Cameroon. Nevertheless, the males of these species can easily be distinguished by the structure of the lateral projection of the colpocoxite which is small and apically setose in S. ongot sp. n., and relatively elongate without setae in S. albicephalus and S. infuscatus. On the other hand, the conformation of the second pair of legs of S. ongot sp. n. is unique in the entire genus Stemmiulus.
Distribution. Known only from the type locality.  Etymology. The species name emphasizes the characteristic apical part of the colpocoxite which is unciform and pointed at the apex.

Stemmiulus uncus
Diagnosis. A species of Stemmiulus characterized by the first six ambulatory legs being especially robust and covered with peculiar, spatulate setae, also showing a field of numerous simple setae on the inner side of the tarsus (Fig. 2B-E). The gonopod has a relatively simple angiocoxite which forms a densely setose apical corolla. The tip of the colpocoxite forms a characteristic apical hook.
Description. Holotype: adult male, ca 20 mm in length, 1.8 mm in maximum diameter, body with 46 rings. Head and collum dark brown, other body rings brown with a light axial dorsal stripe, legs and antennae yellowish.
Head typical in shape, beset with numerous simple macrosetae; ommatidia 2+2, posterior ommatidia larger than anterior ones; antennae long and setose, apices reaching third body ring. Gnathochilarium concave, stipes densely and uniformly porose, pores surrounded by a field of minute setae.
Collum with a single fold at anterior edge, this being better expressed at lateral margin.
Body rings ovoid (height/width ratio of midbody rings ca 0.31), telson short and upcurved. Both pro-and metazonites with transverse oblique striae better pronounced at pleurotergal margin.
First six pairs of legs as in S. ongot sp. n., but mostly with filiform and plumose setae (Fig. 2B-E). First pair of legs relatively simple and unmodified. Second pair of legs with enlarged coxae (Fig. 2F, G), their anterior surface with a few setae, posterior surface glabrous. Telopodite 2-segmented, proximal segment longer, about twice as long as distal segment, curved caudad, with a ventromedial cluster of long setae (Fig. 3G). Distal segment more slender, with an apical row of short setae (Fig. 3G).
Gonopods (Fig. 2H, I) with a large and relatively simple angiocoxite forming an apical corolla and covered with a dense field of numerous setae. Colpocoxite with its tip forming a characteristically strong and curved hook (Fig. 2H-J).
Paragonopods small and 3-segmented, each of medial and distal segments carrying a small series of short setae.
Female unknown.
Relationships. The peripheral characteristics and simple gonopods bring S. uncus sp. n. close to S. beroni, from Nigeria, and S. pullulus, from Mount Nimba. All these species share the simplicity of their second pairs of male legs, despite the fact that the basal segment of the telopodite in the new species is about twice as large and broad as the distal segment. Nevertheless, the males of this trio can easily be distinguished by the structure of the apical part of the colpocoxite. The latter ends up in a pointed curved hook in S. uncus, versus a pointed straight tip in S. beroni or a rounded tip in S. pullulus.
Distribution. Known only from the type locality.  Etymology. The species is named after the Mbalmayo Reserve Forest, the type locality.

Stemmiulus mbalmayoensis
Diagnosis. A species close to the previous new one and to S. beroni by its external characters, but is easily distinguished by the structure of the colpocoxite whose apical part is axe-shaped.
Description. Holotype: adult male, ca 20 mm in length, 1.8 mm in maximum diameter, body with 46 rings. Head and collum dark brown, other body rings brown with a light axial dorsal stripe, legs and antennae yellowish.
Head typical in shape, beset with numerous simple macrosetae as in previous species; ommatidia 2+2, posterior ommatidia slightly larger than anterior ones. Antennae reaching the fourth body ring, and covered with minute setae.
Gnathochilarium concave, without special modification, stipes densely and uniformly porose, pores surrounded by a field of setae. Collum with a single fringe at anterior edge, this being best visible laterally. Body rings ovoid (height/width ratio of midbody rings ca 0.38), metazonites with transverse oblique striae better visible at pleurotergal margin. Striations on prozonites more weakly developed than on metazonites. Annal valves beset with numerous setae.
First pair of legs and legs 3 to 6 as in S. uncus (Fig. 3A). Second pair of legs with enlarged and subquadrate coxae (Fig. 3B, C), anterior surface with traces of segmentation, a few setae on entire anterior surface, posterior surface glabrous. Telopodite 2-segmented, proximal segment longer, about twice as long as distal one, curved ventrad, with a ventromedial cluster of long setae (Fig. 3B, C). Distal segment more slender, curved mesad, with an apical row of short setae.
Gonopods (Fig. 3D-F) relatively simple in structure, angiocoxite with a well prominent constriction in subapical part, apical part forming a setose corolla. Colpocoxite ending up in an axe-shaped structure slightly protruding from angiocoxite.
Paragonopods small, 3-segmented, quite similar to those in most of the African congeners.
Female unknown.
Relationships. Most of the peripheral characters and especially the simple gonopods seem to bring S. mbalmayoensis sp. n. close to S. uncus sp. n., S. beroni and S. pullulus. Nevertheless, the males of all these species can easily be distinguished by the structure of the colpocoxite, in which the apical part is axe-shaped in S. mbalmayoyensis sp. n., pointed and unciform in S. uncus sp. n., pointed and straight in S. beroni, but with a rounded tip in S. pullulus.
Distribution. Known only from the type locality. Figure 4 New material.  1.15). Body light brown with 2-3 marbled spots lying symmetrical to mid-dorsal region which is covered by a large yellowish band all along its extent (Fig. 4A). Metazonites and dorsal margins of antennomeres darkish; legs and ventral parts of body yellowish.
First pair of male legs with short and globular coxae, telopoditomeres clothed with numerous plumose setae. First article of telopodite long and voluminous, nearly equal in length to all three other telopoditomeres combined, tarsal segment with a brush of setae on basal two-thirds of ventral surface.
Second pair of male legs relatively simple, with rounded coxae and 2-segmented telopodites, distal segment of the latter being relatively slender. Anterior side of proximal part of telopodite covered with long plumose setae (Fig. 4G, H).
Ventral surface of first six pairs 3 to 7 of male legs clothed with numerous plumose setae, tarsal segment with a fringe of setae in basal two-thirds of ventral surface but no true brush formed (Fig. 4D, E).
Legs 8 and following unmodified (Fig. 4F). Gonopods (Fig. 4J-M) relatively simple; angiocoxite subconical, forming distally a corolla covered with a field of numerous setae. Colpocoxite shorter than angiocoxite, folded leaf-shaped, encompassing the flagellum tip and ending in a finger-like apical structure.
Remark. This species is new to the fauna of Cameroon and is illustrated, based on new material taken from outside the type locality (Jos, Plateau State, Nigeria) for the first time. The fresh males from Cameroon are peculiar in the apical part of the colpocoxite being slightly curved (Fig. 4K-M), versus straight in the holotype.

Taxonomic comments on S. nigricollis
Among the Stemmiulus species known to occur in Cameroon, S. nigricollis was the first to be described (Porat 1894). According to Mauriès (1967), who revised the type material of S. nigricollis, Porat based the description on one adult and one subadult female, both labelled "Types" and actually representing syntypes. Regrettably, there was no other geographical label given other than "Kamerun". Working on a diplopod collection from Gabon, Mauriès (1967) discovered a species he identified as S. nigricollis in view of marked external similarities and the proximity of Gabon to Cameroon.
He designated a male neotype from Gabon, erroneously thinking that could stabilize nomenclature. However, the act of neotype designation is only warranted when true type material is lost. Therefore, since the syntypes are still available and kept at the Stockholm Museum, the species from Gabon described by Mauriès is to be referred to as S. nigricollis (Porat, 1894) sensu Mauriès, 1967. Since the key below is based on male characters alone, the female-based S. camerunensis is excluded from treatment. Silvestri (1916) described his S. camerunensis from a series of syntypes which included an adult female and two juveniles, all taken at Victoria, Cameroon. Only recollecting fresh topotypes, including male material, would finally allow us to clarify the identity of S. camerunensis and to incorporate this species into a key.