Pachysternum sandacanum, Fikáček & Jia & Prokin, 2012

Fikáček, Martin, Jia, Fenglong & Prokin, Alexander, 2012, A review of the Asian species of the genus Pachysternum (Coleoptera: Hydrophilidae: Sphaeridiinae), Zootaxa 3219, pp. 1-53 : 40-42

Pachysternum sandacanum sp. nov.

( Figs. 38, 41, 66)

Type locality. Malaysia, Borneo, Sabah state, 8 km S Gunung Trus Madi, 549 m a.s.l., [coordinates ca. 5°28'29''N, 116°31'6''E].

Type material. Holotype: male ( BMNH): “SABAH: 5 m. S. / Mt. Trus Madi, / 1800 ft. / 18–28.vi.1977 // rotten tapioca / skins // M. E. Bacchus / B.M. 1978–48”. Paratypes: 17 spec. ( BMNH, NMPC, NHMW, NHMB): same label data as the holotype; 1 male, 6 spec. ( BMNH, KSEM): “ SABAH: 200 ft. / R. Karamuak, 7m / SSE. Telupid / 1–7.ix.1977 // Native vegetable / garden refuse // M. E. Bacchus / B.M. 1978–48”; 1 female (BMNH): “SABAH: 250 ft. / 30mls. Sandakan / - Keningan Rd. / 19.x.1977 // leaf litter // M. E. Bacchus / B.M. 1978–48”; 1 male ( BMNH): “ SARAWAK, C. J. Brooks ”.

Differential diagnosis. Based on the general appearance and coloration in combination with continually arcuate outer margin of anterior tibiae, P. sandacanum is very similar to P. curvatum , P. coomani and typically colored specimens of P. apicatum . From P. coomani , it may be distinguished according to the shape of the median lobe (wide and nearly parallel-sided throughout in P. coomani ( Fig. 30 ), distinctly narrowed into narrow apical fourth in P. sandacanum ( Fig. 38)), clearly defined yellowish apical spot on elytra (the elytra are gradually getting paler posteriad in P. coomani , Fig. 54) and in much finer punctation of elytral intervals (punctation is rather coarse and dense in P. coomani ). Distinguishing of P. sandacanum from P. curvatum and the polymorphic P. apicatum may be rather difficult, but is clearly possible on the basis of the morphology of the median lobe of the aedeagus (distinctly narrowed in apical fourth in P. sandacanum , gradually narrowing apicad in P. curvatum and P. a p i c a t u m) and partly also on the basis of the body size (specimens of P. sandacanum are usually distinctly smaller than those of P. apicatum and P. curvatum ). However, the identification of P. sandacanum is easy when the distributional data are taken into account: the species co-occurs only with P. apicatum on the island of Borneo and may be easily distinguished from the Bornean form of P. apicatum according to the elytra coloration (with large yellow spot at apex in P. sandacanum , entirely reddish brown to brown in Bornean P. apicatum ( Figs. 50 h–j)) and according to the shape of the anterior tibia (continually arcuate on outer margin in P. sandacanum ( Fig. 41 ), but distinctly bisinuate in Bornean P. apicatum ( Fig. 39 b)). Pachysternum sandacanum does not co-occur with P. coomani and P. curvatum , which are absent from the island of Borneo.

Description. Body oval, shape of elytra slightly sexually dimorphic, female with indistinctly pronounced humeral portion of elytra. Body length 2.3–3.1 mm (holotype: 2.6 mm); body width 1.6–2.0 mm (holotype 1.8 mm).

Coloration. Frons brown with median portion indistinctly paler, clypeus reddish brown. Pronotum brown with vaguely pale reddish anterolateral corners. Elytral uniformly brown, with large vaguely limited yellow apical spot reaching apical 0.5–0.3 on intervals 2–7 and ca. apical eighth on intervals 8–10. Ventral surface of head and prothorax reddish brown, meso- and metaventrite brown, abdominal ventrites 1–4 brown with pale lateral spots, ventrite 5 entirely pale, legs pale reddish.

External morphology. Clypeus with moderately coarse and rather dense punctation consisting of uniformlysized rounded punctures separated from each other by ca. 1–2× puncture diameter. Punctation of frons similar to that on clypeus, but consisting of punctures of two slightly different sizes. Interstices on head without microsculpture. Larger punctures on pronotum rounded, shallow, much larger than small punctures; small punctures scar-like, divided from each other by ca. 1–3× puncture width. Pronotal interstices without microsculpture. Prosternum with distinct median carina. Elytral series consisting of large, shallow punctures; interval punctation with rather fine semicircular punctures much smaller than serial ones, with sparsely intermixed slightly larger setiferous punctures of the same shape; interstices without microsculpture. Elytral series weakly impressed. Preepisternal elevation of mesothorax with posterolateral pits. Femoral lines on metaventrite indistinctly angulate, median portion of metaventrite bearing moderately coarse but sparse setiferous punctation, interstices without microsculpture; lateral portions with very coarse and dense, densely pubescent punctation. Anterior tibia rather wide, outer margin continually arcuate, outer series of spines not interrupted.

Male genitalia ( Fig. 38). Tegmen 1.1 mm long, median lobe 1.1 mm long. Phallobase ca. 2× as long as parameres, bearing small, slightly asymmetrical and indistinctly detached basal manubrium. Median lobe rather wide in basal three fourths, distinctly narrowed at apical fourth and then gradually narrowing to widely triangular apical portion, blunt at very apex; lateral pubescent lobes very distinct subapically. Sternite 9 with large V-shaped apical emargination.

Variation. A very low variation was observed in the coloration of elytra (concerning the extent and distinctness of the apical elytral yellow spot, Fig. 66) and body size (see description). Otherwise the examined specimens are very constant in all characters of taxonomic importance including of the morphology of the male genitalia.

Etymology. Named after Sandakan Division in Sabah, in which two localities of this new species are located.

Biology. Specimens examined were collected from various kinds of rotting plant matter (cassava skins, vegetable garden refuse and leaf litter).

Distribution. Known from three close localities in central Sabah in northern Borneo.

Discussion. Pachysternum sandacanum seems to belong to the P. a p i c a t u m complex (based on the presence of deep pits on preepisternal elevation of mesothorax) whose taxonomy is not completely resolved in this paper, mainly because of P. apicatum is here understood as a polymorphic species but may actually consist of several species/subspecies. Following this concept, Pachysternum sandacanum may seem to be a part of the widely understood P. apicatum on the first view. However, its syntopic co-occurrence with the Bornean form of P. apicatum (uniformly reddish brown to brown in coloration, with rather deeply bisinuate anterior tibia, but with the male genitalia identical with the typical P. apicatum ), supports the separate status of P. sandacanum . Separate status is also supported by the morphology of the median lobe which clearly differs from both P. apicatum and P. c u r v a t u m.

Figures

FIGURES 29 – 32. Male genitalia of Asian Pachysternum (a: tegmen, b: median lobe, c: apical portionof median lobe, d: sternite 9). 29: P. c a rd o n i, 30: P. coomani, 31: P. haemorrhoum; 32: P. nigrovitattum.

FIGURES 39 – 50. Anterior tibia of Asian Pachysternum species. 39: P. apicatum (a: lectotype, b: specimen from Borneo, Sabah); 40: P. c u r v a t u m, 41: P. sandacanum; 42: P. c a rd o n i; 43: P. coomani, 44: P. haemorrhoum, 45: P. kubani, 46: P. nigrovittatum, 47: P. rugosum, 48: P. stevensi, 49: P. s u l a w e s i c u m.