Pachysternum coomani Orchymont, 1926

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 : 23-25

Pachysternum coomani Orchymont, 1926

( Figs. 30 , 54)

Pachysternum coomani Orchymont, 1926a: 221 .

Pachysternum coomani: Orchymont (1928: 83, catalogue) ; Mukhopadhyay & Sengupta (2004: 453, faunistics); Hansen (1999: 307, catalogue).

Type locality. Vietnam, Hoa Binh province [coordinates ca. 20°50'N, 105°20'E].

Type material examined. Holotype: 1 female ( IRSNB): “ Hoa Binh / Tonkin / de Cooman // Coll. d’Orchym. / / Pachysternum / coomani / Type”. Paratypes: 2 females, 4 spec. ( IRSNB): same data as the holotype.

Additional material examined. MALAYSIA: Kedah: 1 male, 4 spec. ( NMPC, ASKC): Pulau Langkawi , environs of hotel Berjaya [= Pantai Kok village, coordinates ca. 6°22'N, 99°40.5'E], 7–22.xi.2009, lgt. U. Schmidt. MYANMAR: Shan: 1 male, 1 spec. ( FHGC): Kinda dam, Palaung river mouth [coordinates ca. 21°5'N, 96°19'E], 23–25.xi.1989, lgt. Tobias . THAILAND: Phetchabun: 1 male ( FHGC): 120 km N Phetchabun , Lom Sak , elev. 120 m [coordinates ca. 16°47'N 101°14'30''E], viii.1987, lgt. W. Thielen; VIETNAM: Ho Chi Minh: 1 spec. ( IRSNB): Saigon [= Ho Chi Minh city, coordinates ca. 10°46.5'N 106°41.5'E], without date and collector.

Published records. INDIA: Manipur: Imphal district, Ucharthal, 5 km E of Jiribam ( Mukhopadhyay & Sengupta 2004).

Differential diagnosis. Rather easily distinguishable from all remaining species occurring in continental SE Asia by its pale reddish general coloration ( Fig. 54), small body size and continually arcuate outer margin of anterior tibiae ( Fig. 43 ); the very wide and nearly parallel-sided median lobe of the aedeagus ( Fig. 30 ) is also very characteristic and cannot be confused with any other species. Darker specimens with stronger interstitial microsculpture on elytra may resemble P. stevensi , but can be easily distinguished from it by continually arcuate outer margin of anterior tibiae (distinctly sinuate in P. stevensi ) and wide, nearly parallel-sided median lobe. By general coloration, Pachysternum coomani also slightly resembles Bornean specimens of P. a p i c a t u m, from whose it also differs by continually arcuate outer margin of anterior tibiae (rather deeply sinuate in Bornean P. apicatum ).

Description. Body widely oval, shape of elytra not sexually dimorphic, female without distinctly pronounced humeral portion of elytra. Body length 2.2–2.9 mm (holotype: 2.4 mm); body width 1.5–1.8 mm (holotype 1.6 mm).

Coloration ( Fig. 54). Whole dorsal surface more or less uniformly reddish brown, with slightly darker lateral portions of frons, posterolateral portions of pronotum, base of elytra and elytra at midlength. Ventral surface of head reddish brown, thoracic sclerites brown, abdominal ventrites reddish brown, legs pale reddish.

External morphology. Clypeus with moderately coarse but sparse punctation consisting of uniformly-sized rasp-like punctures separated from each other by ca. 1–3× puncture diameter. Punctation of frons similar to that on clypeus. 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. 3× puncture width. Pronotal interstices without microsculpture. Prosternum with very 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 setiferous punctures of the same shape and size; interstices with weak mesh-like microsculpture. Elytral series 1 not impressed, lateral series only very weakly impressed. Preepisternal elevation of mesothorax without posterolateral pits. Femoral lines on metaventrite indistinctly angulate, median portion of metaventrite bearing rather fine, moderately dense setiferous punctation, interstices without microsculpture medially, with weak microsculpture consisting of partially joint ridges laterally; 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. 30 ). Tegmen 1.0– 1.1 mm long, median lobe 1.0– 1.1 mm long. Phallobase 2.5× as long as parameres, bearing small, slightly asymmetrical, distinctly detached basal manubrium. Median lobe rather wide, nearly parallel-sided at midlength, strongly narrowing apicad, apex widely rounded; gonopore subapical, rather indistinct; lateral pubescent lobes narrowly developed subapically. Sternite 9 with moderately large V-shaped apical emargination.

Variation. Dorsal coloration varies slightly in the extent of the darker areas on the head, pronotum and elytra: type specimens are paler (uniformly reddish) than some specimens from Malaysia, in which some parts of the elytra are distinctly darker, brown ( Fig. 54d). Microsculpture of elytral interstices varies from weakly developed to very distinct.

Biology. The specimens from Malaysia were collected in cow excrements; most probably a coprophilous species.

Distribution. Based on the examined specimens, the species seems to be widely distributed in the mainland SE Asia, reaching from northeastern India (Manipur state) to southern Vietnam and Malayan peninsula, but the specimens are rather rare in collections. The species is recorded here for the first time from Malaysia, Myanmar and Thailand.

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.