Pachysternum Motschulsky 1863
Creators: name: Fikáček, Martin
name: Jia, Fenglong
name: Prokin, Alexander
Publisher: name: Zenodo
Year: 2012
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Coleoptera
Hydrophilidae
Pachysternum

description: Asian Pachysternum
Description. Body widely oval, sexually dimorphic in shape; males widely rounded in dorsal view, without pronouncing shoulders (e.g., Fig. 52a, b, 60e); females more elongate, with pronouncing shoulders and elytra (rather) strongly narrowing posteriad (e.g., Fig. 52e, g, 60d). Body length 2.0– 4.5 mm. Body coloration often with distinct pattern of pale reddish to yellowish and dark brown or black spots (in this case beetles appearing dark with pale pattern or pale with dark pattern according to the extent of pale/dark parts); in some species body more or less uniformly pale reddish to dark brown (see P. apicatum, P. coomani and P. rugosum).
Head. Anterior margin of clypeus convex medially, strongly angulate laterally, bearing rather wide and very distinct rim. Frontoclypeal sutures angulate, lateral portions very distinct as transverse ridges arising above antennal bases and reaching submedially, median portions weakly developed or totally missing. Median portion of frons not elevated above lateral portions. Eyes moderately large, separated by 5–6× the width of one eye in dorsal view. Labrum membraneous, partly concealed by clypeus. Maxillary palpomere 2 club-like, strongly widened distally, palpomere 3–4 thin, palpomere 4 spindle-like (Fig. 2); maxilla of male with sucking disc (Figs. 1, 3). Mentum (Figs. 1, 4) transverse, ca. 2.5× as wide as long; lateral margin angulate slightly before posterolateral corner; anterior margin slightly concave medially, anterolateral corners rounded; transverse groove lying along anterior margin present medially; punctation consisting of coarse shallow punctures elongate anteriad and therefore forming low longitudinal ridges in anterior portion. Submentum densely pubescent, ridges arising from posterior margin of maxillary groove absent. Posterior tentorial pits present, minute; gular sutures weakly developed, slightly diverging posteriad; gula carinate medially in posterior portion, bearing coarse setiferous punctures (Fig. 4). Antenna with 9 antennomeres (Fig. 5); scapus strongly arcuate, bearing a distinct longitudinal ridge dorsally; pedicel ca. as long as antennomere 3, antennomeres 3–5 gradually decreasing in length; cupula small; antennal club compact, densely pubescent, widely rounded at apex, lacking any fields of special sensorial structures.
Prothorax. Pronotum arcuate on posterior margin, shallowly bisinuate on anterior margin; lateral portions deflexed and therefore seen ventrally (Fig. 8), false pronotal margin absent, lateral margin of pronotum angular in lateral view (Fig. 7). Pronotal punctation consisting of large shallow punctures intermixed among fine ground punctation (Figs. 12–14). Series of large punctures along posterior margin present. Prosternum highly elevated into a subpentagonal plate medially (Figs. 8–11), the plate sharply to indistinctly carinate medially, bearing coarse punctures; anterior margin of prosternum constricted just laterally of the plate, deep pits laterally of lateral margins of the plate present (Fig. 7). Antennal grooves very distinct, large, laterally reaching deflexed lateral margin of pronotum (Fig. 8). Profurcal arms short, reaching ca. ventral third of prothoracic height, leaf-like, strongly asymmetrical in shape, protruding of the posterior foramen of prothorax (Fig. 6). Additional ridge below posterior pronotal margin present, very distinct, reaching lateral margin of hypomeron.
Mesothorax. Mesoventrite much shorter than metaventrite, distinctly inclined and therefore not lying in the same plane as mesepimera (Fig. 15). Median portion of mesoventrite elevated into preepisternal plate, the plate triangular in shape, slightly wider than long (Figs. 15–19), bearing very distinct rim on anterolateral margins; posterior margin of the plate widely contacting metaventral process. Grooves for reception of procoxae developed, large, reaching anterior margin of mesocoxal cavities (Fig. 15). Mesepimeron with very distinct oblique ridge laterally. Mesocoxal cavities widely isolated from each other. Mesofurca not examined. Scutellar shield triangular, slightly longer than wide (Fig. 23). Elytron with 10 elytral series; all elytral intervals of the same height; lateral portions of elytron not deflexed. Epipleuron totally missing except very basally.
Metathorax. Metaventrite (Fig. 15) without elevate median portion; metaventral process wide, widely attaching posterior margin of preepisternal elevation of mesothorax. Anterior margin of metaventrite with distinct anterior ridge lying subparallel to posterior margin of mesocoxal cavity, defining rather wide marginal bead, but disappearing submedially. Femoral lines developed, complete, reaching anterolateral corners of metaventrite. Anepisternum extremely narrow anteriorly, slightly widening posteriad (Fig. 15). Metafurca Y-shaped, basal stalk ca. as long as arms, nearly parallel-sided, bearing high median carina; lateral arms long, narrow, lateral extensions of furcal arm rather large.
Legs. Procoxa without ventral ridge; ventral surface of anterior femur bearing very coarse and rather shallow punctures (Fig. 24); anterior tibia variable in shape, in some species bisinuate on outer margin (Figs. 39–49). Mesoand metacoxa deeply excised on posterior margin, with dense pubescence developed only basally; ventral surface of meso- and metafemur with sparse semierect setae; meso- and metatibiae widened apically, bearing longitudinal rows of stout spines and some intermixed fine setae on ventral surface, distal margin with a series of stout spines and two long inner spurs; metatarsus shorter than metatibia, bearing dense brushes of thin setae ventrally, metatarsomere 1 ca as long as metatarsomere 2. Claws arcuate.
Abdomen with five exposed ventrites, ventrite 1 carinate medially, bearing dense and coarse setiferous punctation, lacking sublateral longitudinal ridges (Fig. 25). Punctation on ventrites 2–5 much finer than on ventrite 1.
Male genitalia. Phallobase as long as parameres or distinctly longer, with or without distinct basal manubrium (Figs. 29–38,a). Median lobe ca. as long as phallobase and parameres combined, not attached to bases of parameres and therefore freely movable within tegmen; gonopore subapical, rather indistinct. Sternite 9 with tongue-like median process of variable shape. Sternite 8 without median projection.
Female genitalia corresponding to those of the genus Kanala Balfour-Browne 1939 described by Fikáček (2010).
Recognition. The Asian species of Pachysternum may be easily distinguished from remaining Megasternini genera by the combination of the following characters:
(1) Pronotum with two sizes of punctures (Figs. 12–14): small ground punctation is intermixed with sparser but much larger and more apparent punctures. [A similar kind of punctation also occurs in Pseudocercyon Orchymont, 1926, Australocyon pilocnemoides group, Cercyon diversipunctus Hebauer, 2002 and some Oriental Cryptopleurum Mulsant, 1844; all of them may be distinguished by the characters below].
(2) Antennal grooves large, reaching to lateral margin of hypomeron (Fig. 8). [This character distinguish Asian Pachysternum from all taxa mentioned under (1) except of Cryptopleurum]
(3) Preepisternal elevation of mesothorax triangular, only slightly wider than long (Figs. 15–19). [The shape of the elevation distinguishes Pachysternum from all taxa mentioned under (1). Wide preepisternal plate is also developed in Cryptopleurum in which it is much wider than long and pentagonal rather than triangular, and in Australocyon pilocnemoides group, in which it is bluntly arcuate rather than sharply pointed apically.]
(4) Median portion of the prosternum elevated into prosternal plate, which is finely but distinctly carinate medially (Figs. 8–11). [There is no prosternal plate in Pseudocercyon and Cercyon, the plate is developed but not carinate medially in Cryptopleurum, in Australocyon pilocnemoides the median portion of prosternum is rather similar to Pachysternum on the first view].
Taxonomic status. The Asian species of the genus Pachysternum were placed into the formal Pachysternum nigrovittatum group defined by Fikáček (2006), who considered all Asian species as highly similar in the morphology of male genitalia and the presence of the sexual dimorphism in the body shape. The detailed examination of all Asian species revealed that this is valid for all species from the Oriental region, but not for the Palaearctic P. haemorrhoum which differs from the Oriental species in several aspects. The monophyly of the Asian Pachysternum needs therefore to be tested, which was currently impossible for us as the taxonomy of the Afrotropical species assigned to Pachysternum is unsolved. No formal phylogenetic analysis is performed here and we therefore refrain from a redefinition of P. nigrovittatum group in this paper. Below, the characters of possible phylogenetic importance are only briefly discussed.
1. Phallobase of the aedeagus: (0) with large, asymmetrical and very distinctly detached manubrium; (1) with small basal manubrium or manubrium absent.
The basal portion of the phallobase is only slightly symmetrical and does not bear any distinctly detached manubrium in all Asian Pachysternum species (Figs. 29, 30–38, a) except P. haemorrhoum, in which the manubrium is large and very distinct (Fig. 31 a). The Afrotropical P. capense Mulsant, 1844 as well as most other taxa of the Megasternum group of genera (i.e. those having large antennal grooves reaching lateral margin of hypomeron, see Hansen (1991) and Fikáček (2008)) bear a very distinct asymmetrical manubrium, which suggests that the character state (1) might be a synapomorphy of all Oriental species of Pachysternum.
2. Apex of the paramere: (0) with two minute setae; (1) with one long and thick seta, and one very minute one.
Two apical setae are present on the paramere apex of all Pachysternum examined. In all Asian species except P. haemorrhoum, the apical seta is very long and rather stout (Fig. 28). In P. haemorrhoum, both setae are minute only (Fig. 31 a). Pachysternum haemorhoum shares the character state (0) with P. capense as well as with many other taxa of the Megasternum group of genera (see character 1 for the definition of the group), and the large apical setae might therefore be a synapomorphy of the Oriental Pachysternum species.
3. Basal part of struts of the median lobe: (0) expanded but not divided into a separate sclerite; (1) hook-like, divided into a separate sclerite.
Basal part of the struts of the median lobe is divided into a separate sclerite of the hook-like shape in all Oriental Pachysternum (Fig. 27 b), whereas the basal part is only expanded but not divided in P. haemorrhoum (Fig. 26 b). The character state (1) is unique for the Oriental Pachysternum and was not observed in any other taxon of the Megasternini examined so far (basal portion is never divided into a separate sclerite in other Megasternini examined, even though they may be hook-shaped (see, e.g., the median lobe of Cercyon dieganus Régimbart, 1903 in Fikáček (2005)).
4. Apical portion of the median lobe: (0) at most with very narrow membranous lobes; (1) with large membranous lobes forming a wide rim of the apex.
Only narrow membranous rim are present in Asian species of Pachysternum (Fig. 27 a) except P. haemorrhoum, in which the membranous parts are wide and the apex of the median lobe is therefore widely rounded (Fig. 26 a). Only small membranous lobes are usually developed in the Megasternini, but this character is highly variable on the species level in some cases (e.g., Fikáček et al. 2009, Fikáček & Hebauer 2009). Relevance of this character is therefore unclear at present.
5. Lateral portion of the metaventrite (lying laterad of the femoral lines): (0) with moderately coarse punctation and shiny interstices, similar to median portion of the metavetrnite; (1) with coarse punctation and dense pubescence, therefore of different appearance than the median portion of the metaventrite.
Lateral portions of the metaventrite bears the coarse setiferous sculpture in all Oriental species of Pachysternum (Fig. 20), but fine punctation and shiny interstices in P. haemorrhoum (Fig. 21). The state present in P. haemorrhoum corresponds with the condition found in P. c a pe n s e. However, the appearance is rather variable in various genera of the Megasternum group (see character 1 for the definition of the group) and its relevance is therefore unclear at present.
6. Deep pits in posterolateral margins of the preepisternal plate of mesothorax: (0) absent; (1) present.
The appearance of the preepisternal plate of the mesothorax is very uniform in all Asian Pachysternum (Figs. 15–19), but the species differ in the presence/absence of deep pits in its posterolateral corners: pits are missing in all Asian Pachysternum except of those assigned here to the P. apicatum species group (i.e., P. apicatum, P. c ur v a - tum and P. sandacanum; Fig. 18). No pits are developed in the Afrotropical P. capense, and the presence of the pits may therefore be a possible synapomorphy of the P. apicatum species group.
7. Sexual dimorphism in body shape (males widely rounded, females more elongate with distinctly pronounced shoulder): (0) absent; (1) present.
Sexual dimorphism in the body shape is not known in any other Megasternine taxa and seems to be unique for the Asian species of Pachysternum; at the same time it seems to be the only character supporting the monophyly of all Asian Pachysternum including P. haemorrhoum. On the other hand, the extent of the difference between the shapes of male and female varies both intra- and interspecifically.
List of species
P. apicatum Motschulsky, 1863 China (Hainan), Indonesia (Sumatra, Mentawai Isl., Java, Lombok, Sulawesi), Laos, Malaysia (peninsular part, Sarawak, Sabah), Singapore, Thailand, Vietnam
P. cardoni d’Orchymont, 1926 Nepal, northeastern India, southern China, Laos, Thailand, Vietnam
P. coomani d’Orchymont, 1926 India (Manipur), Myanmar, Thailand, Vietnam, Malaysia (peninsular part)
P. curvatum d’Orchymont, 1925 Philippines (Luzon, Leyte, Mindanao)
P. haemorrhoum Motschulsky, 1866 Kazakhstan, Mongolia, Russia (Far East, southern Siberia, Sakhalin, Kuril Isl.), northern China, Korea, Japan
P. kubani sp. nov. northern Laos, China (Sichuan)
P. nigrovittatum Motschulsky, 1863 Nepal, India, Sri Lanka, southern China, Laos, Singapore, Thailand,
Vietnam, Malaysia (peninsular part), Indonesia (Sumatra, Sumba) P. rugosum sp. nov. China (Gansu, Shaanxi)
P. sandacanum sp. nov. Malaysia (Sabah, Sarawak)
P. stevensi d’Orchymont, 1926 Nepal, southern and northeastern India, southern China, Laos, Vietnam P. sulawesicum Fikáček, 2006 Indonesia (Sulawesi)
Diagnostic characters.
The Asian species of Pachysternum are rather uniform in most external characters. The characters varying between the species and therefore useful for identification are often quite variable even intraspecifically, which makes the identification rather difficult in many cases. Therefore, diagnostic characters are briefly commented here to make the understanding of their importance and pitfalls easier to the reader.
General habitus and body coloration. General habitus and the coloration of dorsal side are rather reliable for the identification, but their using usually requires some experience and/or comparison with correctly identified specimens. Shape of the elytra varies to variable extent between male and female of the same species in most taxa, which may be confusing when body shape is used as a diagnostic criterion. Shape of the elytra in females may moreover vary even within a single species, with shoulders being more or less pronounced (the highest variation in this aspect was observed in P. apicatum and P. nigrovittatum). Coloration of dorsal surface is a very useful diagnostic character, but may be extremely variable intraspecifically. The intraspecific variation is described under each species below, and we also tried to cover it by the habitus photographs published within this paper (Figs. 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70).
Shape of anterior tibia. The shape the anterior tibia is a very useful character (Figs. 39–49). The outer margin of the tibia may be either continually arcuate, or slightly angulate at midlength (the median portion is more or less straight instead of being arcuate in such cases), or bears a distinct but usually shallow emargination (the outer margin is bisinuate in such cases). The shape is invariable in most species except P. apicatum. In addition, the appearance of the series of spines on the outer margin of protibia may be used for the identification – it is either continuous (e.g. in P. cardoni, P. coomani and P. sulawesicum) or interrupted by a wider gap without any spines (e.g. in P. kubani, P. nigrovittatum, P. stevensi). In few species (e.g., P. apicatum), this character is however variable and both continuous and interrupted series of spines may be present in various specimens of the same species. Teratologic malformation of the protibial spines was observed in one specimen of P. kubani (the spines are extremely reduced in this specimen).
Morphology of male genitalia. Male genitalia (Figs. 29–38) are very uniform in most species, and only those of P. haemorrhoum may be easily distinguished from the remaining species even without any detailed examination. In spite of that, the shape of the median lobe provides useful characters in some cases, with the differences found usually in its general shape and/or the shape of its apical portion. The reliable examination of these characters, however, requires mounting the median lobe separately from the tegmen. The shape of the sternite 9 differs considerably between the species (it may be either entire or bear a more or less deep V-shaped emargination apically). Although the shape is rather constant in most species, a wide variation was observed in P. stevensi (Figs. 34d–e) and P. kubani (Figs. 33f–h), and may possibly concern other species as well (although we did not observe any variation in specimens examined for this study).
Meso- and metaventrite. The shape of the preepisternal elevation of the mesothorax is very similar in all species treated in this paper, but varies considerably in the presence/absence of slightly depressed areas and/or deep pits in posterolateral corner of the elevation (Figs. 15–19). Reliable examination of this character is, however, very difficult using an usual binocular microscope and it is therefore useless for identification. The appearance of the lateral portions of the metaventrite (Figs. 20–21) provides a good character for distinguishing P. haemorrhoum from all remaining species, but is otherwise uniform in all Oriental species (see under P. haemorrhoum for details).
Microsculpture of dorsal body surface. The presence/absence of the mesh-like microsculpture was used in the key by Orchymont (1926a). Although it may be helpful in some cases (e.g., P. cardoni usually bears distinct microsculpture, whereas P. kubani usually lacks it), the character is very variable in most species examined (i.e., the microsculpture may be completely absent or very strongly developed in various specimens of the same species). The only exception seems to be P. stevensi in which a strong microsculpture is always present and no variation was observed.
Identification key
Because of the scarcity of useful diagnostic characters and their considerable variability within species, the identification may be rather problematic in some cases. We recommend to use the following sequence for the identification: (1) compare your specimen with the photographs of dorsal habitus; (2) check the shape of anterior tibia for the species similar to your specimens by general habitus and coloration; (3) use the following key to confirm the preliminary identification and separate between similarly looking species (the key counts with the intraspecific variability and some species are therefore keyed several times); (4) check carefully the differential diagnosis and variation of the species you decided for in steps 1–3.
1. Pronotal disc uniformly dark (without pale submedian spots in posterior half), only lateral margins of the pronotum narrowly to widely pale (Figs. 50, 54, 56, 58, 60d–i, 64, 66, 68, 70). Outer margin of anterior tibia continually arcuate to deeply excised.................................................................................................... 4
- Pronotal disc usually pale, with dark median M-shaped spot of variable extent (Figs. 52a–e, 62a–e); in dark specimens pronotal disc is more or less dark, but with clearly defined pair of submedian spots on the disc (Figs. 52f–g, 60a–c, 62f–g). Outer margin of anterior tibia angulate (Fig. 42) to deeply excised (Figs. 45–46), never continually arcuate....................... 2
2. Large, widely oval species, body length 2.9–4.3 mm. Elytra pale, darkened along striae, dark spots often extending to the intervals and joining together, elytral interval 4 never completely dark; in extremely dark specimens the elytra may be nearly entirely black (Figs. 52, 60). Median lobe of the aedeagus gradually narrowing apicad (Figs. 29 b–c, 33b–e).............. 3
- Medium-sized to small species (2.2–3.5 mm). Elytra with some intervals completely pale, and at least intervals 1 and 4 largely dark. In darker specimens, dark spots are more extending and joining together, but at least interval 2 always pale throughout (Fig. 62). Median lobe very narrow and nearly parallel-sided in apical fourth (Figs. 32 b–c).............. P. nigrovittatum
3. Dark spot on pronotum M-shaped or with a pair of pale spots in posteromesally (Fig. 52). Darkening of elytral series 2–4 reaching subapically (Fig. 52). Elytral interstices usually with distinct microsculpture. Median lobe of the aedeagus subtriangular apically (Fig. 29 c)................................................................ .. P. c a r d o n i (in part)
- Dark spot on pronotum wide posteriorly, trilobate anteriorly (Figs. 60a–c). Darkening of elytral series 2–4 reaching at most elytral midlength (Figs. 60a–e). Elytral interstices shiny, without microsculpture. Median lobe of the aedeagus widely rounded apically (Fig. 33c, e).............................................................. P. kubani sp. nov. (in part)
4. Outer margin of anterior tibiae continually arcuate, without any trace of straight or concave portion in distal third (Figs. 39 a, 41, 43, 44, 47, 49)..................................................................................... 5
- Outer margin of anterior tibiae not continually arcuate, bearing a distinct straight or concave portion in distal third (39b, 42, 45, 46, 48).............................................................................................. 11
5. Lateral portions of metaventrite (lying laterad of the femoral lines) of the same appearance as the median portion of the metaventrite, lacking rugose sculpture and pubescence (Fig. 21). Median lobe abruptly narrowing just before the apex (Fig. 31 c), phallobase with large and well detached basal manubrium (Fig. 31 a). Elytra generally dark, with pale apex (Fig. 58). East-Palaearctic species................................................................... P. haemorrhoum
- Lateral portions of metaventrite bearing rugose sculpture and dense pubescence, hence of the different appearance than the median portion of the metaventrite (as in Fig. 20). Apex of the median lobe gradually narrowing to apex (Figs. 30 c, 35c, 36c, 37c, 38c). Phallobase without large and distinctly delimited manubrium (Figs. 30 a, 35a, 36a, 37a, 38a). Elytra dark, with pale apical spot or longitudinal stripes (Figs. 50, 54, 56, 64, 66, 70). Species from Oriental Region and islands of SE Asia....... 6
6. Large and widely oval species (body length 3.3–3.8 mm). Dorsal surface bears a rugose sculpture (Fig. 14), therefore the beetle is very opaque in general appearance (Fig. 64). Body uniformly dark brown....................... P. rugosum sp. nov.
- Small to large species (2.2–3.6 mm). Punctation of the dorsal surface never as coarse and dense to form rugose sculpture (Figs. 12–13, 22), the beetle is therefore rather shiny in general appearance. Body uniformly dark brown, with pale elytral apex or pale spots in humeral area and elytral apex (Figs. 50, 54, 56, 66, 70)............................................. 7
7. Elytron dark, with sharply defined yellowish horseshoe-shaped spot at apex and large yellowish spot in humeral area; in pale specimens additionally with pale elytral interval 2 and base of interval 3 (Fig. 70). Median lobe nearly parallel-sided, very narrow throughout (Fig. 35b). Sulawesi........................................................... P. sulawesicum
- Elytron uniformly pale or dark, or with small to large yellowish spot on elytral apex, never with sharply defined spot in humeral area (Figs. 50, 54, 56, 66). Median lobe either parallel-sided and wide (Fig. 30 b), or distinctly widened at midlength (Figs. 36b, 37b, 38b).................................................................................. 8
8. Dorsal surface either uniformly reddish brown, or brown with elytra slightly paler subbasally and apically (Fig. 54). Median lobe parallel-sided and rather wide throughout (Fig. 30 b).............................................. P. coomani
- Dorsal surface usually dark brown to black, elytra uniformly dark or with small to large apical yellowish spot (Figs. 50 a–g). Median lobe distinctly widened at midlength (Figs. 36–38, b). Pachysternum apicatum complex...................... 9
9. Small species (2.3–3.1 mm). Median lobe slightly constricted subapically, its apex rather wide (Figs. 38b–c). Sternite 9 always with V-shaped apical emargination (Fig. 38d). Elytra always with yellowish spot on apex (Fig. 66). Northern Borneo (Sabah, Sarawak)......................................................................... P. sa nd aca nu m sp. nov.
- Small to large species (2.2–3.6 mm). Median lobe gradually narrowing apicad, subapical constriction absent (Figs. 36b–c, 37b–c). Sternite 9 usually entire to very shallowly emarginate apically (Figs. 36d, f, 37d). Elytra uniformly dark or with small to large yellowish apical spot (Figs. 50 a–g, 56)............................................................. 10
10. Outer margin of anterior tibia at least with slight hint of straight or angular shape at midlength (Fig. 39 a; see also from slightly different angles than only dorsally); the series of spines on outer margin usually interrupted at least by a small gap at midlength (Fig 39 a). Median portion of the head very distinctly paler than lateral portions, or whole head pale reddish brown (Fig. 50). Median lobe rounded at extreme apex (Fig. 36c). SE Asia and islands of Sundaland, but absent from Philippines ............................................................................................... P. apicatum (in part)
- Outer margin of anterior tibia continually arcuate, the series of spines on the margin continual, not interrupted by a gap (Fig. 40). Median portion of frons not apparently paler than lateral portions (Fig. 56). Median lobe acute at extreme apex (Fig. 37c). Philippines ................................................................................. P. cu rva tum
11 Elytra uniformly colored or only with sharply or vaguely defined pale apical spot (Figs. 50, 52g, 60h–i, 68g –h)......... 13
- Elytra with pale areas basally as well as apically (Figs. 60e–g, 68a–f).......................................... 12
12 Elytral interstices without microsculpture, shiny. General coloration of elytra yellowish, with sharply defined dark transverse stripe at anterior third which is extending along elytral series both anteriad and posteriad and is therefore lobate in shape (Fig. 60). Median portion of frons pale.................................................... P. kubani sp. nov. (in part)
- Elytral interstices with strong, mesh-like microsculpture. General coloration of elytra reddish, usually with rather vaguely defined dark reddish to black area at anterior third (Figs. 68a–f), margins of darker area never sharply lobate. Median portion of frons dark, not paler than lateral portions and clypeus........................................ P. stevensi (in part)
13. Whole head (in pale specimens) or at least median portion (in dark specimens) of frons pale reddish. Elytra with or without distinct mesh-like microsculpture on interstices................................................................ 14
- Whole head dark brown to black, without any trace of paler median portion of frons (Fig. 68g –h). Elytra always with strong mesh-like microsculpture on interstices..................................................... P. stevensi (in part)
14. Large and robust species (body length 2.9–4.3 mm). Pronotum posteriorly at least with rather indistinct paler areas submedially (Fig. 52g). Series of spines on outer margin of anterior tibia always not interrupted on the emargination (Fig. 42). Elytra usually with distinct mesh-like microsculpture at least laterally, rarely totally without microsculpture..... P. cardoni (in part)
- Smaller species (body size 2.2–3.6 mm). Pronotum never with paler submedian spots along posterior margin. Series of spines on outer margin of anterior tibia interrupted on the emargination (Figs. 39 b, 45) or forming continual series. Elytra without any trace of mesh-like microsculpture on interstices............................................................. 15
15. Elytron dark with very distinct pale reddish apical spot (Figs. 60h–i). Apex of median lobe rounded (Figs. 33c, e). Anterior tibia always deeply emarginate, outer series of spines always interrupted (Fig. 45)............. P. kubani sp. nov. (in part)
- Elytron either uniformly colored, reddish to dark brown (Figs. 50 f–j), or black with vaguely to rather sharply defined reddish apical spot (Figs. 50 a–e). Apex of median lobe triangular (Figs. 36b–c, e). Outer margin of anterior tibia deeply to very shallowly emarginate (Figs. 39 a–b), outer series of spines usually not interrupted (specimens with interrupted series rarely occur in continental Asia only).................................................................. P. apicatum (in part)
description: Published as part of Fikáček, Martin, Jia, Fenglong & Prokin, Alexander, 2012, A review of the Asian species of the genus Pachysternum (Coleoptera: Hydrophilidae: Sphaeridiinae), pp. 1-53 in Zootaxa 3219 on pages 4-15, DOI: 10.11646/zootaxa.3219.1.1, http://zenodo.org/record/280223