Bolbochromus Boucomont, 1909

Krikken, Jan & Li, Chun-Lin, 2013, Taxonomy of the Oriental genus Bolbochromus: a generic overview and descriptions of four new species (Coleoptera: Geotrupidae: Bolboceratinae), Zootaxa 3731 (4), pp. 495-519 : 497-499

Genus Bolbochromus Boucomont, 1909

Generic description. Head. Outline of mandibles (sub)symmetrical. Outer margin of right mandible sinuate, poorly lobed, or simply arcuate. Labrum with at most shallowly concave anterior side (surface proper may be transversely elevated, but lacks discrete ridge). General outline of clypeus in dorsal view in both sexes (extra marginal projections aside) approximately semicircular to nearly semielliptic, anterior section of perimarginal ridge bordering clypeal surface arcuate or straight (sides always rounded, not angularly trapeziform as in many Bolboceras and related genera, see Krikken 2013); clypeal margin in some species with apicomedian protrusion (slight, in some species longer, some with long horn), clypeogenal angles on either side of clypeal base may also be protuberant (distinctly so in. B. sulcicollis , see below). Frontovertex usually (major individuals) with single median projection (development and position different according to species, in some species shifted forward, just behind clypeus, in some with pair of slight paramedian tubercles on low transverse ridge). Vertex immediately below pronotal edge not steeply declivous. Anterior margin of eye canthus simply arcuate (not angular or protuberant). Eye canthus and temporal lobe (sub)contiguous, dividing eye in upper and lower part. Size of dorsally visible area of eye (in foramen) varying according to species. Paraocular ridge delimiting eye canthus from frontal disc distinct, posteriorly fine, absent, or obsolescent.

Pronotum and scutellum. Pronotum at most with (up to 4) very slight (short, more-or-less transverse) antediscal protrusion(s), anterior declivity generally convex, in some species slightly deplanate, or more strongly depressed; disc in many species with punctate median longitudinal impression; some species have deeper (punctate) median longitudinal sulcus. Pronotal base medially immarginate, posterolateral border widely rounded. Pronotal apex medially marginate or immarginate, border evenly arcuate (axial view), margin may be deplanate (as distinct in lateral view), but lacking isolated protrusion(s). Scutellum deltoid, anterior half of lateral edge depressed, on either side leaving slight slit against elytral border; scutellum wide, l/w ratio 1.0–1.5, apex (sub)angular; general surface of scutellum even, base more or less abruptly delimited.

Elytra. Elytral base immarginate (margin not elevated-ridged). Elytral epipleuron reaching apicosutural angle. Elytron with up to 13+1 punctate striae, including either 7 or 5 between suture and humeral umbone (i.e., in species with 7 striae 2 and 5 are shortened in front, not reaching base; these striae 2 and 5 completely obsolete in some species). Stria 1 proceeding to elytral base alongside scutellum. Most discal striae distinctly impressed (and interstrial surface more or less convex); juxtasutural interstria (= 1) more convex than 2. Humeral angle of elytron unmodified. Elytral epipleuron as well as adjacent marginal gutter with numerous long setae. Alae normally developed.

Antenna. Antennal club large, not remarkably thickened (lamellae at most indistinctly telescoped, their plane slightly, evenly curved), outline of club generally oval; proximal surface of club antennomere 1 with distinct elliptic, glossy brown “plate”, surrounded by narrow marginal pubescent surface; distal side of club antennomere 3 entirely pubescent and unmodified (lacking the long grooves of Bolbotrypes Olsoufieff, 1907 ).

Pectus (ventral thoracic elements). Postocular surface slightly concave. Preprosternum slightly tricostate (low paramedian ridges, and one median); postprosternum lacking any median protrusion. Mesocoxae subcontiguous, anteromedian process of metasternum indistinct. Metasternal disc more or less rhombic in outline.

Legs. Distal fossorial elevations of mesotibiae and metatibiae with arcuate spiniferous crest, usually with 2–3 complete anteapical elevations, proximal elevations smaller, obsolescent or noticeable as pairs of slight denticles. Protibia normally dilated, usually with 7–11 external denticles, gradually decreasing in size proximad. Femora unmodified (not dentate or otherwise modified). Terminal protibial spur long, tapering. Protarsomere 1 short, mesotarsi and metatarsi unmodified, slender.

Genitalia. Aedeagal structure shows considerable diversity, usually with externally distinct parameres, and with or without other more or less complex elements, including median apparatus (male genital structure used for infrageneric classification, see further below; note new subgenus Metabolbochromus). Distal abdominal segments not particularly modified.

Sexual dimorphism. Externally limited, if distinct at all. Protrusions and impressions on head and pronotum (if present at all) similar. Punctation-rugulation may be different (e.g., stronger punctation on head of females).

Colours, pilosity, body shape. Head black or dark (rufous) brown. Dorsal side of body largely glossy, colour in majority of (sub)species not uniformly black or brown, but with simple, variably delimited symmetric pronotal and elytral pattern (black or brown versus yellow to red); few (sub)species more uniform on pronotum, scutellum and elytra (e.g., entirely black or pale yellow). Pilosity on dorsal side virtually absent (frons may have some long, erect setae, juxtepipleural elytral margin with row of setae); pilosity elsewhere abundant. Body generally strongly convex, length usually 4.5–13.0 mm.

Immature stages. Unknown.

Type species. See under nominotypical subgenus.

Generic distribution. Sri Lanka and India, Himalayas, Myanmar, Thailand, China, Indochina, Taiwan and S Ryukyu Islands, Philippines, Sulawesi, Sundaland, east to the Lesser Sundas (! Sumbawa: unidentifiable, worn female seen). Undoubtedly also in some missing adjacent countries. Total 23 named species, additional species seen from continental Oriental Region.

Related genera. Bolbochromus is an Oriental branch of the bolboceratine section defined by the presence of (a) a generally rounded, curved anterior side of the clypeus and (b) (sub)contiguous mesocoxae. Most of these bolboceratines occur in the Holarctic, or at least north of the equator— Bolbocerosoma (Krikken 1979) , Bolbelasmus Boucomont, 1911 (Krikken 1977) , Bolbotrypes Olsoufieff, 1907 , etc. Bolbelasmus enters the tropics with some species in both Central America and southeast Asia (east to Java); in Palaearctic Asia, Bolbocerosoma reaches the southern side of the Himalayas. The two character states just mentioned are also shared with Odonteus Samouelle, 1819 (see Carpaneto et al. 2005), but this genus appears morphologically very different, for instance by the unusual ornamentation of the forebody, including (in the male sex) a very long, slender, backward-curved clypeal horn (note the recent proposal of the group Odonteini Shokhin, 2008 ). We hypothesise that all these genera have a long history in the northern hemisphere, and that their interrelationships will prove to be paraphyletic; although they are similar, there is at this moment no obvious correlation of morphological apomorphies between Bolbochromus and any of the other genera (see also comments in Introduction).

The distribution of the morphological character states (including potential apomorphies) over the genera as hitherto analysed appears chequered (clypeal outline, configuration of elytral striae and interstriae, absence/ presence of broader metasternal process between mesocoxae, shape and extension of eye canthi, etc.). The genital structure is very diverse, but not yet critically analysed over the various genera; the structure of the genitalia is unique in some species clusters (subgenera) within Bolbochromus , and very probably autapomorphic. Note the remarkable Afrotropical genus Somalobolbus Carpaneto, Mignani , & Piattella, 1992, which, although not closely related, superficially looks like Bolbochromus —if bolboceratine supraspecific character states indeed have a chequered distribution, Somalobolbus would constitute a prime example.

Generic subdivision. The species with a median frontal protrusion only and an unmodified (and an at most apicomedially slightly angular) clypeus are all kept in the nominotypical subgenus Bolbochromus (see introduction, key, and subgeneric treatments below). Their genitalia show distinct parameres and various median elements (median lobe), in some species characteristically shaped and complex; this diversity needs further study (homologies!) and may, apart from species identification, have consequences for the supraspecific taxonomy of the nominotypical subgenus (note that there is the question of the type species identity). The species with a distinct (usually long) projection on their clypeal apex are placed in two new subgenera, Metabolbochromus and Bolbochromops, which may then be separated on their very different male genitalia, Metabolbochromus having a curious aedeagal capsule, with apparently modified parameres and/or ditto median lobe (Figs. 58–59). These two subgenera seem to be limited to the Sundaland-Philippine region. A full phylogenetic discussion of the infrageneric classification (and a decision as to which hierarchic rank is then plausible) must be postponed due to the limited overall scope of information currently available, particularly on the continental Bolbochromus fauna, which also includes forms with odd aedeagi (like B. sinensis new species, Figs. 60–61). As is obvious from Table 1 further grouping is possible, the B. laetus- like forms, for instance, taking a somewhat separate position, for instance, in their pronotal antediscal shape and the forward position of their frontal tubercle. One further point must be made: the reduction of the number of elytral striae from 7 to 5 between the suture and the humeral umbone in Indian and Sulawesi species, respectively, is likely a homoplasy, judged from the sum of their differences; the onset of a reduction of striae 2 and 5 of the original seven is already noticeable in other species, like B. lineatus (Westwood, 1848) (other Bolboceratinae , like the odd Somalobolbus mentioned above, also have a reduced set of striae). Further information on intrageneric diversity in the next sections (including Table 1 ).