Orthogonius species and diversity in Thailand (Coleoptera, Caraboidea, Orthogoniini), a result from the TIGER project

Abstract The carabid genus Orthogonius MacLeay is treated, based mainly on materials collected in Thailand through the TIGER project (the Thailand Inventory Group for Entomological Research). Among 290 specimens, 20 species are identified in total, 10 of them are new species: Orthogonius taghavianae sp. n. (Nakhon Nayok: Khao Yai National Park), Orthogonius coomanioides sp. n. (Phetchabun: Thung Salaeng Luang National Park), Orthogonius similaris sp. n. (Phetchabun: Thung Salaeng Luang National Park; Loei: Phu Kradueng National Park), Orthogonius setosopalpiger sp. n. (Phetchabun: Thung Salaeng Luang National Park), Orthogonius gracililamella sp. n. (Loei: Phu Kradueng National Park; Chaiyaphum: Tat Tone National Park), Orthogonius pseudochaudoiri sp. n. (Phetchabum: Thung Salaeng Luang National Park; Nakhon Nayok: Khao Yai National Park), Orthogonius constrictus sp. n. (Phetchabum: Thung Salaeng Luang National Park), Orthogonius pinophilus sp. n. (Phetchabum: Thung Salaeng Luang National Park), Orthogonius vari sp. n. (Cambodia: Siem Reap; Thailand: Ubon Ratchathani: Pha Taem National Park; Phetchabun: Thung Salaeng Luang National Park) and Orthogonius variabilis sp. n. (Thailand: Phetchabun: Thung Salaeng Luang National Park; Nakhon Nayok: Khao Yai National Park; Phetchabun: Nam Nao National Park; China: Yunnan). In addition, Orthogonius mouhoti Chaudoir, 1871 and Orthogonius kirirom Tian & Deuve, 2008 are recorded in Thailand for the first time. In total, 30 species of Orthogonius have been recorded from Thailand, indicating that Thailand holds one of the richest Orthogonius faunas in the world. A provisional key to all Thai species is provided. A majority of Thai Orthogonius species are endemic. Among the ten national parks in which orthogonine beetles were collected, Thung Salaeng Luang holds the richest fauna, including 16 species.


Introduction
Thailand has a diversity of habitat types, including various kind of forests (tropical rain, dry or semi-evergreen, montane evergreen, coniferous, swamp, including mangroves, and deciduous forests) and savanna. Thailand is a meeting place of many faunal elements including the Himalayan, east Palearctic and Oriental Regions. Faunistically, the country falls within two of the top eight biodiversity hotspots as identified by Myers et al. (2000): Indo-Burma (the majority of the country) and Sundaland (in the southern peninsula).
Since 2006, the TIGER project (the Thailand Inventory Group for Entomological Research) has been organized by Drs Michael Sharkey and Brian Broun (the University of Kentucky, Lexington, USA), by means of collaboration with the Queen Sirikit Botanic Garden in Chang Mai, Thailand. Covering 25 national parks in different regions of Thailand, the project has spanned three years and produced diverse materials available for biodiversity inventory, including 290 specimens of the termitophilous ground beetle genus Orthogonius.
Despite the fact that taxonomic research on the tribe Orthogoniini of the ground beetles in the Oriental Region has been carried out continuously since 2000 (Tian and Deuve 2000, 2001, 2003a-c, 2004, 2006a-c, 2007a-b, 2008, 2010Abhitha et al. 2009), specimens from the TIGER project represent a surprisingly and unknown diversity of species within Thailand. Among the total of 20 identified species of Orthogonius, 10 are new to science and are described and illustrated in the present paper. In addition, O. mouhoti Chaudoir, 1871 andO. kirirom Tian & are newly recorded in Thailand.

Materials and methods
The TIGER project has been carried out in 25 national parks in Thailand over a three year period. A total of 290 specimens of Orthogonius used for this study were collected in ten of the parks (Figure 1). Almost all specimens were caught by means of Malaise traps, except a few specimens caught in pan traps or extracted from litter samples. In addition, 98 specimens of Orthogonius variabilis sp. n. were collected from Bannahe Nature Reserve, southern Yunnan, China. Other Orthogonius specimens were borrowed from the Muséum National d'Histoire Naturelle, Paris (MNHN), from the Institut royal des Sciences naturelles de Belgique, Brussels (IRSNB), from Naturhistorisches Museum, Basel (NHMB), and the Museum of Natural History, London (MNHL) for comparative study.
All specimens were dry mounted. Dissections, drawings, and observations were made using a binocular Leica MZ75 dissecting microscope. Dissected genital pieces, including the median lobe and parameres of the aedeagus, were glued on small paper cards and then pinned under the specimen from which they were removed. Digital pictures were originally taken with Canon EOS 40D camera, and then treated by means of CombineZP and Photoshop softwares.
Abbreviations for measurements were the same as in Tian & Deuve (2006a). The specimen depository is as follow: panded margins wide, smooth and reflexed; transverse impressions well marked, basal foveae moderate.
Elytra broad and strongly convex; EL/EW=1.58; sides nearly parallel; widest at about middle, apex roundly truncate, strongly sinuate near inner angle which is pointed; base well bordered; shoulders more or less square; striae deep, punctate-striate, intervals convex; even intervals much wider than odd ones (almost twice) and with coarser punctures which extended to the subapical portion, odd intervals with a row of fine and sparse punctures; interval 3 with three discal setiferous pores, and additional two at apical portion; interval 5 with two setae near base; interval 7 narrow and carinate before middle, with seven setiferous pores.
Legs stout, fore tibia with outer angle very sharp and strongly protruded, outer margin distinctly serrate; middle tibia distinctly dilated, and strongly curved in median portion; hind tibia elongate, with tibial spurs short and more or less blunt; hind tarsomere 1 longer than 2, tarsomeres 3 and 4 subequal, tarsomere 4 bilobed; hind femur moderately dilated, with five posterior setae; all tarsal claws pectinate.
Prosternal process well bordered at apex, middle coxa with several setae in median portion; abdominal ventrite VII of male complete at apex.
Male genitalia ( Figure 16): Elongate, enlarged at about middle portion, ventral margin sinuate, dorsal opening large and long, abruptly truncate near apex; in dorsal view, apical part narrow, distinctly constricted before apical lamella, apical lamella elongate, about 2.2 times as long as wide, blunt at apex.
Remarks. This species is a member of the O. alternans species group, but with distinctive aedeagal structure.
Material examined. Holotype: male, "Thailand: Nakhon Nayok: Khao Yai NP, entrance of Hnong Pak Chee Trail, 14°27.167'N, 101°21.850'E, 758 m, 5-12.v.2007 Etymology. This new species is named in honour of Ms Azadeh Taghavian, a curator of the Coleoptera collection in MNHN, Paris, in thanks for her help in so many ways.
Distribution. Thailand. Known only from the type locality. Diagnosis. Medium sized, even elytra intervals wider than odd intervals, but less than twice as wide; similar to O. coomani Tian & Deuve, 2006, but a little larger, darker, and broader than the latter; in addition, head and eyes rather smaller and less prominent; middle tibiae strongly curved (not distinctly curved in O. coomani); aedeagus stouter, and apical lamella distinctly broader than that of O. coomani.
Head moderate, slightly longer than wide, eyes rather small, less prominent, labrum distinctly emarginate at frontal margin, sexsetose, clypeus bisetose; palpi normal; mentum and submentum each with a pair of setae; palpiger asetose; antennae extended to the shoulders of elytra, densely pubescent from antennomere 4.
Pronotum strongly transverse, PW/PL=1.70-1.72, widest a little before middle; both fore and hind angles broadly rounded; lateral expanded margins wide, striate and more or less reflexed; transverse impressions well marked, median line clear.
Elytra broad, strongly convex; EL/EW=1.63-1.64; sides parallel; apex roundly truncate; even intervals well bordered at base; striae deep, punctate-striate; intervals convex, even intervals much wider than odd ones (but less than twice as wide) and with coarser punctures extended to apical 1/3 of elytra; interval 3 with only basal and apical setiferous pores, middle pore absent; interval 5 with two setae near base; interval 7 narrow, distinctly carinate, with eight to nine setiferous pores.
Legs moderate, fore tibia with outer angle very sharp and strongly protruded, outer margin hardly serrate; middle tibia distinctly dilated at apex, and strongly curved in median portion; hind tibia elongate, with tibial spurs moderately long, sword-like, sharp; hind femur moderately dilated, with four posterior setae; hind tarsomere 3 (1.2 times) longer than 4, tarsomere 4 deeply emarginate (a little more than half of the joint); all tarsal claws weakly pectinate.
Prosternal process bordered at apex, middle coxa with several setae; ventrite VII of male complete at apex.
Male genitalia ( Figure 17): Short, and stout, ventral margin expanded strongly at middle portion, apex distinctly bent ventrally; dorsal opening very wide and long; apical lamella broad, but much longer than wide.
Remarks. This species is closely allied to O. coomani, with differences as mentioned above.
Upper surface densely punctate, pronotum with transverse striae, elytra with dense and very short, transverse and granular wrinkles (esp. near base); underside surface smooth and glabrous, polished.
Pronotum transverse, widest at about basal 1/3, PW/PL=1.74, disc slightly and evenly convex, both fore and hind angles broadly rounded, both basal and fore margins beaded, lateral expanded margins rather wide, even and hardly reflexed; fore and hind transverse impressions faint, basal foveae not well marked.
Elytra elongate ovate, EL/EW=1.64; moderately convex, basal border complete, shoulders broadly square; sides more or less parallel at middle, widest at about middle; striae deep, punctate-striate, intervals distinctly convex; intervals subequal in width in middle; apex quite broadly truncate, inner angle nearly rectangular; interval 3 without setiferous pore, interval 7 simple, without pore.
Prosternal process well bordered at apex. Middle and hind coxae smooth and glabrous. Apical margin of abdominal ventrite VII of male deeply and widely emarginate at apical margin, then strongly sinuate at sides behind paramedial setae.
Legs stout. Fore tibia with apical outer angle nearly rectangular, not protruded or pointed; outer margin distinctly serrate; middle tibia evenly curved, gradually dilated towards apex, in lateral view, while slender in dorsal view; hind tibia slender, apical spurs moderate long, sword-like, tarsomere 1 as long as 2, tarsomere 3 longer than 4, tarsomere 4 bilobed; fore tarsi much wider than middle and hind tarsi (which are slender); all tarsal claws strongly pectinate.
Male genitalia ( Figure 18): Median lobe of aedeagus quite stout, less expanded at middle portion; apex broadly blunt; dorsal opening wide and long; in dorsal view apical lamella small and sharp.
Pronotum moderately transverse, PW/PL=1.52, widest at about middle; both fore and hind angles broadly rounded; front and hind margins well beaded; lateral expanded margins wide, almost evenly wide throughout, and slightly reflexed; transverse impressions well marked at base, faint at subapex; basal foveae small.
Elytra broad and strongly convex; EL/EW=1.67; sides nearly parallel; widest at about middle, apex roundly truncate, not sinuate before inner angles; base well bordered; shoulders more or less square; striae deep, punctate-striate, intervals convex; even intervals much wider than odd intervals (almost twice as wide, except interval 4, which is less twice as wide as 3) and with coarser punctures extended to apical 1/4 of elytra, odd intervals with a few fine punctures more or less arranged in a row; interval 3 with three setiferous pores; interval 5 with one seta near base; interval 7 narrow but not carinate throughout, with eleven setiferous pores.
Prosternal process well bordered at apex, middle coxa with three or four setae; ventrite VII of male complete at apex.
Wrinkles and punctures: surface impunctate except elytral intervals 3, 5 and 7 with tiny, and sparse punctures arranged as a row, head wrinkled, pronotum faintly striate.
Pronotum strongly transverse, PW/PL=1.56, moderately convex; sides evenly rounded, widest at about middle, both basal and fore margins beaded, lateral expanded margins well defined, uneven, slightly reflexed; fore and hind angles rounded; fore transverse impression unclear, hind one distinct, basal foveae small, but well marked, middle line clear.
Elytra ovate, EL/EW=1.60, convex, basal border complete, sides slightly expanded, not parallel at middle, striae deep, intervals convex, intervals subequal in width in middle; apex roundly truncate, but shortly and obliquely truncate at inner margin of the tip to form an obvious sutural angle between elytra; interval 3 with three setae, all are well marked; interval 7 not carinate, without seta.
Middle coxa glabrous in median portion; hind coxa with two setae. Legs moderate, fore tiba with outer angle rectangular, serrate on outer margins, apical margin oblique; middle tibia straight in middle, abruptly dilated at apex; hind tibiae slender, slightly dilated only at apex; hind tibial spurs very long and sharp; tarsomere 4 much longer than tarsomere 3 (almost 1.25 times as long), tarsomere 4 very shallowly emarginate at apex; hind femora rather slender, with two setae posteriorly; all tarsal claws strongly pectinate.
Prosternal process well bordered at apex. Apical margin of abdominal ventrite VII very shallowly and slightly emarginate.
Male genitalia ( Figure 20): Moderately elongate, ventral margin more or less sinuate ventrally, apex pointed in lateral view; in dorsal view, apical lamella very long and nearly parallel-sided.
Female. Unknown. Remarks. The apical portion of the aedeagus is very elongate, a little more twisted in the holotype than in the paratype, and the apical lamella is slender and parallelsided, with the apex broadly rounded.
Material examined. Diagnosis. Small to medium sized, labrum straight at frontal margin; prosternal process well bordered at apex, abdominal ventrite VII slightly emarginate at apical margin; very similar to O. mouhoti Chaudoir, 1871, but apical lamella of the aedeagus much longer than that of the latter species. Length: 12.5-16.0 mm; width: 5.5-7.0 mm. Habitus as in Figures 7-9. Description. Dark brown to black, lateral expanded margin of pronotum, antennae, mouthparts palpi, legs and underside surface reddish brown.
Microsculptural meshes densely isodiametric, clear on elytra, but faint on pronotum and head.
Pronotum strongly transverse, PW/PL=1.88-1.90, sides evenly rounded, widest at about middle, both basal and fore margins beaded, lateral expanded margins well defined, wide and even, flat and smooth; both fore and hind angles rounded; disc strongly convex, fore transverse impression faint, basal one moderate, basal foveae small.
Legs moderate, fore tibia with outer angle nearly rectangular, blunt, outer margin not serrate; middle and hind coxae smooth and glabrous; middle and hind tibia slender, apex slightly dilated, hind apical tibial spurs very long and sharp; tarsomere 3 much longer than 4, tarsomere 4 deeply emarginate at apex (about 1/3 deep as the joint); all tarsal claws strongly pectinate; hind femur with 2 posterior setae on ventral.
Prosternal process well bordered at apex; apical margin of abdominal ventrite VII narrowly and shallowly emarginate in male.
Male genitalia ( Figure 21): Very similar to that of O. chaudoiri, straight, and arrowhead-shaped at apex in dorsal view, but more distinctly so than in O. chaudoiri, upper margin less sinuate, and apical lamella in dorsal view much longer and more elongate.
Remarks. This species is very similar to O. chaudoiri, but the apex of its aedeagus is more distinctly arrowhead-shaped than that of O. chaudoiri, less sinuate, and apical lamella much longer; labrum slightly emarginate (straight in latter); body a little more slender; and ventrite VII of male with a small emargination at apical margin.
Variability. Shape of the arrow-headed apex of the aedeagus is variable, wider in some specimens, but narrower in others; however, in all specimens of this species examined, the apical lamella is much longer than that of O. chaudoiri.
Material Diagnosis. Medium sized, labrum sexsetose, nearly straight at apical margin, palpiger asetose, even and odd intervals subequal in width in middle portion, prosternal process bordered at apex; apical margin of abdominal ventrite VII widely and rather deeply emarginate in male; a member of the O. longicornis species group, distinguished by its aedeagus constricted subapically in dorsal view. Length: 12.5 mm; width: 5.5 mm. Habitus as in Figure 10. Description. Dark brown or black, but palpi and femora yellowish brown; trochanters, coxae and lateral pronotal margins reddish brown.
Wrinkles and punctures: surface smooth and impunctate; head and pronotum faintly striate, odd elytral intervals (3, 5 and 7) with an irregular row of fine punctures. Surface strongly shiny.
Microsculptural meshes densely isodiametric on elytra, denser and more transverse on pronotum and head.
Pronotum strongly transverse, PW/PL=1.63, sides evenly rounded, widest at about middle, both basal and fore margins beaded, lateral expanded margins well defined, wide, uneven, smooth and rather flat; both fore and hind angles rounded; disc moderately convex, both transverse impressions not well defined; basal foveae small, middle line distinct.
Elytra ovate (EL/WL=1.55), strongly convex, basal border complete; sides slightly expanded in middle portion, nearly parallel-sided, widest at middle; striae deep, punctate-striate, intervals distinctly convex; intervals 2, 4 and 6 subequal in width, each wider than intervals 1, 3, and 5, respectively, but less than twice as wide; odd intervals with more distinct fine punctures; apex roundly truncate, inner angle broad, without tooth; interval 3 with three well marked setiferous pores (but middle pore absent from left elytron in the holotype and an additional fourth pore present on left elytron in one of the paratypes); interval 7 simple, wide and not carinate, without seta.
Legs moderate, fore tibia with outer angle nearly rectangular, blunt, outer margin faintly serrate; middle and hind coxae smooth and glabrous; middle and hind tibia slender, apex slightly dilated; middle tibia not dilated or curved in middle portion; hind apical tibial spurs very long and sharp; tarsomere 1 much longer than 2, tarsomere 3 slightly longer than 4, tarsomere 4 deeply emarginate at apex (about 1/3 deep as the joint); all tarsal claws strongly pectinate; hind femur moderately dilated, with 2 posterior setae on ventral.
Prosternal process well bordered at apex. Apical margin of abdominal ventrite VII widely and rather deeply emarginate in male.
Male genitalia ( Figure 22): Aedeagus elongate, expanded in median portion, sinuate before apex which is more or less bent and pointed at tip; in dorsal view, distinctly constricted before apex, the apical lamella long and slender, 3.4 times as long as wide.
Female. Unknown. Remarks. This species is a member of the O. longicornis group, but is easily distinguished from other members by its long and slender apical lamella, together with aedeagus more or less constricted before apex in dorsal view.
Distribution. Thailand. Known only from the type locality. Diagnosis. Medium sized, labrum with apical margin straight, prosternal process well bordered at apex, apical margin of abdominal ventrite VII widely emarginate; a member of the O. longicornis species group, and easily recognized by its stout and more or less broad apical lamella. Length: 13.5 mm; width: 6.0 mm. Habitus as in Figure 11. Description. Black, clypeus, mouthparts (except tips of mandibles) and palps, antennomere 1, and coxae, trochanters and femora of legs yellowish brown.
Wrinkles and punctures: surface impunctate except elytral intervals 3, 5 and 7 with tiny, and sparse punctures arranged in a row, head wrinkled, pronotum faintly striate.
Microsculptural meshes isodiametric on elytra, faint or more or less irregular on head and pronotum.
Pronotum strongly transverse, PW/PL=66/45, strongly convex; sides evenly rounded, widest at about middle, both basal and fore margins beaded, lateral expanded margins well defined, with few transverse striae, slightly reflexed, and uneven, fore and hind angles rounded; fore transverse impression indistinct, hind one faint, basal foveae small, but well marked, middle line distinct.
Elytra ovate, convex, basal border complete, sides slightly expanded, not parallel at middle, striae deep, intervals very convex, intervals subequal in width in middle; apex roundly truncate; interval 3 with three setae, all are well marked; interval 7 not carinate, without seta.
Legs moderate, middle tibia slightly curved in middle, abruptly and slightly dilated at apex; middle coxae glabrous in median portion; hind tibiae slender, slightly dilated only at apex; hind tibial spurs long and sharp; tarsomere 3 much shorter than tarsomere 4 (almost 1:1.5), tarsomere 4 symmetrically and shallowly emarginate at apex (depth of emargination equal about 2/5 length of the joint); femora rather slender, hind femur with two setae posteriorly; all tarsal claws strongly pectinate.
Prosternal process well bordered at apex. Apical margin of abdominal ventrite VII widely emarginate.
Male genitalia (Figure 23): Median lobe of aedeagus moderately stout for the group, slightly dilated in middle portion, gradually constricted towards apex in lateral view; in dorsal view apical lamella stout and somewhat expanded at tip.

Female. Unknown.
Remarks. This species is a member of the O. longicornis species group, but is easily separated from other members by its stout apical lamella of aedeagus.
Distribution. Thailand. Know only from the type locality. Diagnosis. A stout and broad species; eyes very large and prominent, pronotum and elytra strongly convex; labrum straight at front; mentum asetose; lateral expanded margin of pronotum tapered from base to front, not reflexed; elytra well bordered at base, apex roundly truncate, inner angle broad; interval 3 with three setiferous pores, interval 7 normal; prosternal process well bordered at apex; ventrite VII in male distinctly emarginate; fore tibia with outer angle nearly rectangular, blunt, and not protruded, outer margin not serrate; middle and hind tibiae slender; hind tibial spur very long and sharp, hind tarsomere 3 much longer than 4, tarsomere 4 shallowly emarginate; all tarsal claws very strongly pectinate; femora moderately dilated; hind femur with two posterior setae. Length: 13.0-14.0 mm; width: 6.3-6.5 mm. Habitus as in Figure 12. Description. Black on upper and lower surfaces, except margin of pronotum, antennae (1-2 much lighter than other antennomeres), palpi, and labrum brown, coxae, trochanters and femora yellowish, tibiae and tarsi dark brown. Surface smooth and impunctate; head intricately striate, pronotum very finely striate; odd elytral intervals (3, 5, 7) with distinct fine punctures in an irregularly row. Surface strongly shiny. Microsculptural meshes densely isodiametric, clear on elytra, but faint on pronotum and head.
Pronotum strongly transverse, PW/PL=1.57, sides evenly rounded, widest at about middle, both basal and fore margins beaded, lateral expanded margins well defined, flat and distinctly tapered from base to front, wide and smooth; both fore and hind angles rounded; disc strongly convex, fore transverse impression indistinct, basal transverse impression moderate, basal foveae distinct and deep.
Middle and hind coxae smooth and glabrous. Legs moderate, fore tibiae with outer angle nearly rectangular, blunt, outer margin not serrate; middle and hind tibiae slender, apex slightly dilated, apical spurs very long and sharp; tarsomere 3 much longer than 4, tarsomere 4 shallowly emarginate at apex; all tarsal claws strongly pectinate; hind femur with 2 posterior setae on ventral.
Prosternal process well bordered at apex. Apical margin of abdominal ventrite VII widely but shallowly emarginate in male.
Male genitalia ( Figure 24): Median lobe long and distinctly expanded in median portion, upper margin abruptly sinuate, apex gradually tapered; ventral margin sinuate, dorsal opening long and wide; the apical lamella quite elongate, two times as long as wide, and tip rounded, and nearly parallel-sided.
Distribution. Cambodia and Thailand.

Orthogonius kirirom Tian & Deuve, 2008
Remarks. In one of the specimens from sample T2068, the ligula is very thin and narrow, 6-setose at apex (rather than 4-setae as Ron Felix's noted label), but all other characters are typical for the species. Therefore, we treat it as an abnormal individual.

Orthogonius longicornis
Length: 9.0-13.5 mm; width: 4.0-5.0 mm. Habitus as in Figure 13-15. Description. Body dark brown to yellow (That means the coloration is variable for some species of Orthogoniini, and the size too, not only shapes, but legs, pronotum, elytra, head and so on as well).
Body with varied coloration: from yellowish to black. Upper surface smooth and glabrous, impunctate (but punctate in one paratype), elytral intervals each with an irregular row of tiny punctures along median portion; moderately shiny.
Microsculptural meshes densely isodiametric on elytra, irregularly and densely on head and pronotum.
Head stout, slightly wider than long, eyes very large and strongly prominent; frons and vertex moderately convex, frontal impressions small and fovea-like, clypeus bisetose, surface with a transverse impression and a median fovea near base; labrum sexsetose, frontal margin almost straight; ligula short, quadrisetose (sexsetose in a few individuals) at apex; palpi slender, subcylindrical, maxillary palpomere 4 longer than 3, palpomere 3 glabrous, except several setae at apex; maxillary palpomere 4 glabrous with very short setae; labial palpomere 3 slightly shorter than palpomere 2, palpomere 3 with a few setae at base; labial palpomere 2 bisetose in inner margin, and with two or three additional setae at subapex and apex; palpiger asetose, mentum and submentum each with one pair of setae; mentum without median tooth. Antennae moderate, ex- tended to middle of elytra; pubescent from basal 1/3 of antennomere 4; antennomere 3 slightly shorter than antennomere 4.
Pronotum transverse, widest at about middle, PW/PL=1.17-1.22, disc slightly and evenly convex, both angles broadly rounded, both basal and fore margins beaded, lateral expanded margins wide, rather flat or somewhat reflexed; fore and hind transverse impressions distinct, basal foveae well-marked.
Elytra elongate ovate, EL/EW=1.7; moderately convex, basal border complete, shoulders broadly square; sides more or less parallel at middle, widest at about middle; striae deep, punctate-striate; intervals moderately convex, and subequal in width in middle; apex quite narrowly and obliquely truncate, outer angle well marked, inner angle sharp and denticulate; interval 3 with two setiferous pores (the middle pore absent); interval 7 simple.
Prosternal process unbordered at apex. Apical margin of abdominal ventrite VII of male narrowly but distinctly emarginate at apical margin.
Male genitalia (Figure 25): the median lobe of the aedeagus somewhat stout, slightly or evenly expanded at middle portion; dorsal opening wide and long; in dorsal view apical lamella very short, , broadly pointed at apex.
Remarks. This species differs from the Perakean species, O. perakensis Tian & Deuve, 2006, by its slender and flat body; and is easily separated from O. perroti Tian & Deuve, 2006 by the shape of its elytral apex.
Variability. To treat this species is somewhat a challenge, because of the variability in several important characters such as coloration, shape of pronotum, elytral apex, aedeagus and middle tibia, and seta number on ligula. Several species might be "recognized" if there were only a few individuals available. Fortunately the large series of the specimens make it possible to realize the complicated variations of this species. The variations appeared in the following aspects: (1) sized: 9.0-13.5 mm; (2) coloration: from pure yellowish (4 ex), brown (24 ex), dark brown (13 ex), then to black (10 ex). Among Chinese specimens, five are bicoloured on the elytra (Fig.  15); (3) generally the apex of elytra of this species distinctly obliquely truncate, but in one male paratype the outer angle of apical elytra rounded and less sinuate, and inner angle less pointed; (4) pronotum: narrow to wide, and intermediate shapes, occurring in specimens of different size and coloration; (5) middle tibia: slightly curved (majority) or distinctly curved (11 ex); slightly dilated (29 ex) or strongly dilated in median portion; (6) aedeagus: stouter (6 ex) or a little more elongate at apex in dorsal view; (7) ligula: generally quadrisetose, only sexsetose in three specimens in Thianland species (but on the contrary, sexsetose in most Chinese specimens); and (8)  less than that of these two other countries. Malaise traps are mainly used to collect Hymenoptera and Diptera, but they are also efficient when used to catch beetles like Orthogonius species that are strong fliers. The majority of the specimens collected in the course of this study were collected in Malaise traps.
Orthogonius specimens were only collected in ten of the 25 national parks sampled in Thailand. Only one species was collected in two parks, viz. O. longicornis in Phu Phan and O. sp. in Phu Ruea; two species were collected in three parks (Nam Nao, Tat Ton and Pha Hin Ngam); and four, five and six species were collected in Phu Kraudueng, Khao Yai and Pha Taem, respectively. Thung Salaeng Luang National Park holds the richest fauna for Orthogonius, with 16 species recorded from that park ( Figure 38).
Almost all records of Orthogonius species from the TIGER project provide new distribution records for the genus. Detailed collecting data make it possible to analyze Although Orthogonius beetles are able to fly, their dispersal ability appears to be limited because the majority of the species of this genus represented in Thailand are endemic, with just a few (six) species known to occur also in other countries nearby, such as Vietnam, Cambodia and China.