Mesoceration chasmum Bilton, 2017, sp. nov.
Creators: name: Bilton, David T.
Publisher: name: Zenodo
Year: 2017
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Coleoptera
Hydraenidae
Mesoceration
Mesoceration chasmum

description: Mesoceration chasmum sp. nov. (Figs 2, 3, 5 & 6)   
 Type locality. South Africa, Northern Cape, Bokkeveld Plateau, Oorlogskloof Canyon, stream amongst boulders below Driefontein Waterfall, 640 m, 31 30 41.85S 19 0 6 58.98E (Fig. 5 C).   
 Type material. Holotype (male): “ 7/x/2015 South Africa NC// Oorlogskloof Canyon—stream // below Driefontein Waterfall // 640 m 31 30 41.85S 19 0 6 58.98E // D T Bilton leg.” (genitalia extracted and mounted on same card) and red holotype label (AMG).   
 Paratypes (2): 1 ♂ same data as holotype; 1 ♂ “ 7/x/2015 South Africa NC// Oorlogskloof Canyon— Oorlogskloof // River below Driefontein Waterfall // 492 m 31 30 50.96S 19 0 6 47.86E // D T Bilton leg.” All with red paratype labels (CDTB, AMG).   
 Description. Size: Holotype: BL 1.75 mm; EL 1.00 mm; EW 0.65 mm. Paratypes: ♂ s BL 1.70–1.75 mm; EL 1.00– 1.05 mm; EW 0.60–0.65 mm.   
Colour: Dorsum (Fig. 2 C) with head pitchy brown to black; ocelli paler. Pronotum and elytra pale brown, infuscated on pronotal disc and around elytral suture. Maxillary palpi pale yellow, apical 0.75 of terminal segment infuscated. Legs pale yellow, apex of terminal tarsal segments narrowly infuscated. Venter testaceous brown, meso and metaventrite and mentum pitchy black. Much of venter silvery due to dense hydrofuge vestiture.  
Head: Labrum quadrate, widening anteriorly. Rounded apicolaterally, with v-shaped apicomedian emargination extending 0.5 of length. Sides of apicomedian emargination narrowly raised. Surface weakly shining, with strong isodiametric microreticulation and fine punctures. Long, yellow recumbent setae along lateral and anterior margins. Clypeus weakly shining, with isodiametric microreticulation and sparse, fine punctures bearing fine decumbent setae. Frontoclypeal suture weakly arcuate. Frons weakly shining on centre, with isodiametric microreticulation and sparse, medium punctures bearing white decumbent setae. Anteocellar sulci broad, opening anteriorly, rugosely microreticulate and punctate, both in sulic and between them and compound eyes. Ocelli shining. Compound eyes moderately large, occupying 0.6 lateral margin of head, 15 ommatidia in longest series.  
Pronotum: Cordate, transverse, broadest just behind middle. Anterior margin arcuate over central 0.5, with narrow hyaline border; posterior margin weakly bisinuate around centre. Anterior angles broadly rounded; posterior angles obtusely rounded. Lateral margins sinuated from widest point to base; broadly marginated and moderately crenulated. Upper surface weakly shining, with shallow, isodiametric microreticulation and sparse, medium punctures bearing white decumbent setae. With 10 distinct fovea as follows: Anterior median fovea elongate; posterior median fovea circular. Anterior admedian foveae rounded; posterior admedian foveae elongate oval, orientated anterolaterally. Anterior and adlateral foveae small, rounded, pit-like.  
Elytra: Elongate oval, broadest at middle. Subparallel over median 0.4, then attenuated to broadly rounded apices, emarginated around suture. narrowly explanate at sides. Punctures of elytral series evident to apex; larger in anterior 0.5, smaller behind. Punctures bearing decumbent setae, each seta reaching next puncture in series. Coarsely granulated between punctures. Discal series striate impressed. Series 1 and 2 confluent 0.5 from apex; series 5 and 6 separate throughout. Intervals weakly shining, finely granulated, granules with fine, white decumbent setae. Interval 8 without evident carinae.  
Legs: Apical protarsomere with two stout ventral setae; basal three protarsomeres with suction setae.  
Venter: Mentum shining, with isodiametric to elongate microreticulation; more strongly impressed towards lateral margins and sparse, fine punctures bearing decumbent setae. Submentum shining, with transverse microreticulation and sparse, fine punctures bearing decumbent setae. Genae shining, with transverse microreticulation. Gula shining, with transverse microreticulation; meshes smaller than on genae. Prosternum with weak central ridge; surface rugulose, with dense hydrofuge vestiture. Pronotal hypomeron broad, weakly shining, with strong, slightly elongate microreticulation. Meso and metaventrite with dense hydrofuge vestiture. Mesoventral plaques weak, low, forming inverted Y, stem 0.5 length of arms. Weak admedian and adlateral ridges also present. Metaventrite with broad, shallow median fovea over posterior 0.4; fovea with hydrofuge vestiture. Elytral pseudepipleurs broad, shining, without evident microreticulation; extending to apex of abdominal ventrite 5. Elytral epipleurs shining, narrow, ridge-like. Abdominal ventrites 1–4 with dense hydrofuge vestiture throughout. Abdominal ventrite 5 with semicircular glabrous patch occupying apical 0.3; glabrous area with shallow, transverse microreticulation and irregular transverse row of fine punctures bearing long, golden decumbent setae. Abdominal ventrite 6 glabrous, shining, with medium transverse microreticulation and scattered fine punctures bearing golden decumbent setae close to apex.  
Aedeagus: Elongate, with parameres attaching close to base. Main piece somwhat arcuate in ventral view, curved ventrally in lateral view, with strong, stout tooth on left hand side in ventral view. Distal lobe short, strongly curved over apical tooth of distal lobe (Fig. 3 C).  
Female: Unknown.  
 Differential diagnosis. The entirely separated 5th and 6th elytral series and the structure of the aedeagus would place this species within the barriotum group sensu Perkins (2008). It shares with M. integrum Perkins, 2008 the complete absence of a carina on the 8th elytral interval, something which is always present, to some extent, in other members of the group. It differs from M. integrum in the more elongate habitus, paler dorsal colouration, and the presence of distinct microreticulation on the pronotal reliefs. The aedeagi of the two species are very similar in basic structure, despite these strong external differences.   
 Distribution. To date known only from the two adjacent localities (Sites 10 & 11, Figs 5 C & 6C) in Oorlogskloof Canyon on the Bokkeveld Plateau. In the type locality specimens were netted from gravel and debris between large sandstone boulders in a heavily shaded stream below a waterfall, where they were the only water beetle recorded. A single male was also taken in the Oorlogskloof River, upstream of where the type stream joins the main river. This specimen was netted from gravel below a boulder, together with abundant M. longipennis Perkins, 2008.   
 Etymology. From the latin chasma (= gorge, steep-sided depression), in reference to Oologskloof Canyon, where both known localities are situated.
description: Published as part of Bilton, David T., 2017, Water beetles from the Bokkeveld Plateau: a semi-arid hotspot of freshwater biodiversity in the Northern Cape of South Africa, pp. 191-214 in Zootaxa 4268 (2) on pages 202-206, DOI: 10.11646/zootaxa.4268.2.2, http://zenodo.org/record/580190