Mesoceration explanatum, Bilton & Mlambo, 2019

Bilton, David T. & Mlambo, Musa C., 2019, Two new Mesoceration Janssens, 1967 from the Piketberg, South Africa (Coleoptera, Hydraenidae), Zootaxa 4555 (2), pp. 268-274 : 269-271

Mesoceration explanatum sp. nov.

Type locality. South Africa , Western Cape Province, Piketberg, stream below Sebrakop , 32° 46 19.35S 18 45 42.09E, 809 m.

Type material. Holotype (male): " 3/x/2015 South Africa WC// Piketberg - stream below// Sebrakop 32° 46’ 19.35’’S 18° 45’ 42.09’’E 809 m // D T Bilton leg." (genitalia extracted and mounted on same card) and red holotype label (AMG).

Paratypes (30): South Africa: 8♂, 5♀ same data as holotype ; 2♀ " 3/x/2015 South Africa , WC// Piketberg - stream in bowl below// Sebrakop 32 46 12.61S // 18 45 48.59E 876 m // D T Bilton leg." ; 5♂, 3♀ " 4/x/2015 South Africa WC// Piketberg - stream flowing NW// below Sebrakop 32 45 30.28S // 18 45 50.74E 952 m // D T Bilton leg." ; 3♂, 4♀ " 4/x/2015 South Africa WC// Piketberg - stream flowing SE// below Sebrakop 32 45 53.73S 18 45// 56.01E 943 m // D T Bilton leg." ( AMG, CDTB, ISAM, NMW, SANC, TMSA) .

Description. Size: Holotype: BL 1.93 mm; EL 1.12 mm; EW 0.73 mm. Paratypes: Males BL 1.69–1.97 mm; EL 0.96–1.1 mm; EW 0.63–0.70 mm. Females BL 1.82–1.93 mm; EL 1.0 6– 1.12 mm; EW 0.68–0.72 mm.

Colour: Dorsum ( Fig. 1A ) pitchy brown to black, apex of elytra and ocelli somewhat paler. Legs orange brown; apical tarsal segments infuscated. Maxillary palpi pale orange brown, apical segment infuscated. Venter pitchy brown to black, with silvery hydrofuge vestiture. Mentum, submentum, pronotal hypomeron, epipleurs and pseudepipleurs paler.

Head: Labrum slightly transverse, broadly rounded apicolaterally with broad apicomedian emargination occupying approx. 0.5 of length. Sides of apicomedian emargination weakly raised. Surface of labrum shining, microreticulate, meshes elongate at sides, transverse and smaller basally and close to centre; with scattered, white, recumbent setae, particularly close to apicolateral angles. Frontoclypeal suture distinct, weakly arcuate, becoming obsolete laterally. Clypeus, frons and vertex somewhat dull, with rugulose microreticulation of transverse to isodiametric meshes and sparse, medium punctures bearing short, white, recumbent setae; reticulation and punctation especially strong on frons and vertex. Anteocular sulci broad, shallow, running from ocelli to frontoclypeal suture, open anterolaterally. Ocelli small and shining; head behind ocelli with sharp declivity. Compound eyes relatively small, occupying approx. 0.4 side of head, 12 ocelli in longest series.

Pronotum: Cordate, slightly transverse, broadest just before middle. Anterior margin arcuate over median 0.5, without hyaline border; posterior margin bisinuate around centre. Anterior angles obtusely rounded; posterior angles slightly obtuse, weakly rounded. Lateral margins sinuated from widest point to base, broadly marginated and weakly serrate. Surface dull, strongly microreticulate, with isodiametric to slightly transverse meshes, weakly granulose on reliefs; reliefs with dense, coarse punctures bearing white, recumbent setae. With 10 distinct foveae as follows: anterior median fovea elongate, deeper and broader posteriorly; posterior median fovea oval, approx. 0.5 length of anterior. Anterior admedian foveae short oval, open anterolaterally; posterior admedian foveae elongate, open anterolaterally. Anterior adlateral foveae deep, oval, open laterally; posterior adlateral foveae deep, elongate oval, open laterally.

Elytra: Elongate oval, broadest just behind middle. Subparallel over median 0.4, then weakly attenuated to broadly truncate, cojointly emarginate hind angles. Dorsally flattened with shallow posterior declivity. Lateral margins broadly explanate; weakly serrate, at least anteriorly. Elytral series of coarse, moderately shallow punctures, evident to apex; larger in anterior 0.5, becoming smaller posteriorly. Serial punctures with fine decumbent setae, not usually reaching next puncture in row. Discal series weakly striate impressed. Series 1 and 2 confluent behind posterior declivity; series 5 and 6 confluent in anterior 0.4. Interval 8 strongly carinate from just behind shoulder to approx. 0.1 from apex. Intervals shining, granulate, bearing white decumbent setae.

Legs: Apical protarsomere with two stout ventral setae; basal three protarsomeres with suction setae.

Wings: Full.

Venter: Mentum and submentum shining, microreticulate with weakly impressed, slightly transverse meshes; mentum with short, stout yellow recumbent to erect setae around anterior margins. Genae shining, weakly microreticulate; meshes tranverse. Gula shining in centre, with weak transvere furrows; laterally with fine, rugulose microreticulation. Prosternum with narrow, shining, glabrous central ridge; remainder rugulose, with dense hydrofuge vestiture. Pronotal hypomeron broad, shining, with fine, elongate microreticulation and small foveae situated anterior to anterior margin of procoxae. Mesoventrite and metaventrite with dense hydrofuge vestiture. Mesoventral plaques weak, forming an inverted Y, stem 0.5 length of arms. Traces of admedian and adlateral ridges visible. Metaventrite with oval median fovea over posterior 0.6 and shallow depressions anterior, below mesocoxae. Elytral pseudepipleurs shining, with fine, sparse punctures and no microreticulation; epipleurs narrow, shining, ridge-like to apex. Abdominal ventrites 1-4 with dense hydrofuge vestiture. Abdominal ventrite 5 with hydrofuge vestiture anterolaterally; broad, arcuate posterior patch glabrous, with fine, transverse microreticulation; irregular transverse row of fine punctures bearing long, fine, decumbent setae evident laterally, close to posterior margin. Abdominal ventrite 6 glabrous, with fine, transverse microreticulation; irregular transverse row of fine punctures bearing long, fine, decumbent setae towards posterior margin; apex weakly emarginate at centre.

Aedeagus: Elongate, relatively broad in ventral view, with parameres attaching close to base and extending just beyond apex of main piece. Main piece with characteristically shaped, asymmetrical apex. Distal lobe elongate, curving strongly to the left, then apically in ventral view ( Fig. 2A ).

Female: Slightly broader than male, particularly towards the apex of the elytra; explanate elytral margins broader. Abdominal ventrite 7 produced to bluntly rounded apex.

Variation: Paratypes show some variation in extent of pale colouration at lateral margins of the elytra, yellowish to shoulder in some specimens.

Differential diagnosis. A member of the truncatum group sensu Perkins & Balfour-Browne (1994), the elytra having well developed posterolateral angles, and truncate apices. In this work M. explanatum sp. nov. would key to M. brevigranum Perkins & Balfour-Brown, 1994, on the basis of its granulose pronotal reliefs. The new species differs from M. brevigranum in the much broader pronotum, and the more strongly truncate, cojointly emarginate, elytral apices. These characters also readily distinguish it from M. granulovestum Perkins, 2008, M. umbrosum Perkins, 2008. M. rugulosum Bilton, 2015 and M. castaneum Bilton, 2017 also have relatively broad pronota, but have much less truncate elytral apices. The aedeagus of M. explanatum sp. nov. is characteristic, with a uniquely shaped main piece.

Distribution. To date only known from the Piketberg massif in the Western Cape Province, where it was, however, abundant in all four streams sampled for water beetles, all of which were above 800 m a.s.l. Here it occurred with a relatively diverse water beetle assemblage, including M. hirsutum Bilton, 2015, a species previously only known from the holotype, collected at close to 1,000 m a.s.l. in the Cederberg range.

Etymology. Named in reference to the broadly explanate elytra.

Figures

FIGURE 1. Mesoceration species, holotype habitus. A) Mesoceration explanatum sp. nov.; B) Mesoceration piketbergense sp. nov. Scale bar = 1 mm.

FIGURE 2. Mesoceration species, paratype aedeagi. A) Mesoceration explanatum sp. nov.; B) Mesoceration piketbergense sp. nov. Scale bar = 0.1 mm.