Gnathoncus semimarginatus Bickhardt, 1920

Lackner, Tomáš, 2020, A review of Gnathoncus of Southeast Asia (Coleoptera: Histeridae: Saprininae), Acta Entomologica Musei Nationalis Pragae 60 (1), pp. 397-409 : 405-408

Gnathoncus semimarginatus Bickhardt, 1920

( Figs 56 , 59, 60–69)

Gnathoncus semimarginatus Bickhardt, 1920: 29 (original description).

Gnathoncus semimarginatus : KRΥƵΗΑΝοඏඌΚιΙ +| RΕιർΗΑRƉτ (1976): 115 (keyed); MΑƵUR (1984): 103 (catalogue); MΑƵUR (1997): 215 (catalogue); MΑƵUR (2004): 92 (catalogue); MΑƵUR (2011): 176 (catalogue); LΑർΚΝΕR et al. (2015): 114 (catalogue).

Type locality. ‘Shi-wan-tze’ (North India or China).

Type material examined. HοΓοτΥΡΕ: ♀, mounted on a triangular mounting point, right protarsus missing, ‘Shi-wan-tze | Nord-Indien’ [written] || ‘G. semimargi- | natus m. | H. Bickhardt det. 1919’ ([printed-written]

|| ‘Type’ [brick-red, printed label] || ‘ HOLOTYPUS | Gnathoncus | semimarginatus | Bickhardt, 1920 | labelled by MNHUB 2008’ [red label, printed] || ‘D08-086’ [yellow, pencil-written label] ( MFNB).

Note. Although BιർΚΗΑRƉτ’ඌ (1920) description mentions ‘China’ as the type locality, the actual specimen bears the label ‘Nord Indien: Shi-wan-tze’ (this locality has not been located). This species was described based on a single specimen, which is the holotype by original designation.

Redescription. Body ( Fig. 56 ): PEL: 3.10 mm; APW: 1.10 mm; PPW: 2.20 mm; EL: 2.20 mm; EW: 2.60 mm. Body oval, moderately convex, slightly flattened from above, cuticle piceous black; legs, mouthparts and antennae dark brown.

Head. Antennal scape ( Fig. 60 ) somewhat thickened, lower margin carinate, with few short setae; antennal club ( Fig. 61 ) oval, without visible articulation, entire surface covered with dense short setae, intermingled with sparse setae; sensory structures of antennal club in form of two horizontal slit-like orifices on dorsal side of club, otherwise not examined. Mouthparts: mandibles with rounded outer margin curved inwardly, acutely pointed, sub-apical tooth on inner margin very small; labrum flattened, punctate, with only very shallow median excavation; labral pits with single short seta in each pit; terminal labial palpomere elongated, its width about one-fourth of its length; mentum ( Fig. 62 ) sub-quadrate, anterior angles slightly produced, anterior margin with shallow emargination, medially with two setae, lateral margins with two rows of short sparse ramose setae, disc of mentum glabrous; cardo of maxilla with few short setae laterally; stipes triangular, with three short setae; terminal maxillary palpomere elongated, its width about one-fourth of its length, approximately three times as long as penultimate. Clypeus ( Fig. 60 ) large, rectangular, flattened dorsally, rounded laterally, with dense punctures separated by about their own diameter; frons with coarse round to ellipsoid punctures separated by about their own diameter; eyes convex, well visible from above.

Pronotum. Pronotal sides ( Fig. 56 ) in basal half moderately convergent anteriorly, thence strongly narrowing apically; apical angles obtuse, anterior emargination for head deep, almost straight medially; marginal pronotal stria present only on basal pronotal half (Fig. 59), thin, interrupted behind head; pronotal disc laterally covered with deep round punctation, punctures separated by about their own diameter, medially punctures become finer and sparser; scutellum very small, but visible.

Elytra. Epipleuron with coarse and dense ellipsoid punctures separated by about their own diameter; marginal epipleural stria doubled, weakly impressed but complete, in punctures; marginal elytral stria well impressed, slightly carinate and in punctures, shortly continued as weakened apical elytra stria, next obliterated; humeral elytral stria weakly impressed on basal third, crossed by number of fine oblique rugae; internal subhumeral stria present medially, well impressed, rather long; elytral disc with four dorsal elytral striae I–IV, first longest, deeply impressed on basal half, carinate, weakening on apical half, reaching about two-thirds of elytral length apically, second, third and fourth striae about the same length, reaching about elytral half apically, fourth stria in deep punctures, basal end of fourth stria forms small hook, between that and sutural stria present short characteristic hooked appendix; basal end of sutural elytral stria also with small hook; sutural elytral stria very short, reaching only about one-tenth of elytral length basally, then obliterated. Elytral punctation on apical half coarse and dense, punctures separated by about their own diameter; on elytral intervals 1–3 punctation much sparser, punctures separated by several times their diameter, on fourth elytral interval punctures become microscopic and very sparse; before elytral apex punctures laterally forming tiny elongate strioles.

Propygidium ( Fig. 63 ) covered with very dense and coarse punctures separated by about half their own diameter, interspaces with alutaceous microsculpture; pygidium ( Fig. 63 ) convex medially, covered with very coarse and dense round punctures separated by about half their own diameter, interspaces with alutaceous microsculpture.

Prosternum. Anterior margin of median portion of prosternum ( Fig. 64 ) rounded; prosternal process flat, broad, dorsally and laterally covered with coarse and dense punctures separated by about 1.5 times their diameter; carinal prosternal striae well impressed, carinate, sub-parallel on basal two-thirds, thence strongly convergent anteriorly, terminating in deep tiny doubled prosternal fovea; lateral prosternal striae very short, strongly convergent anteriorly, reaching carinal prosternal striae in apical two-thirds of prosternal process.

Mesoventrite. Anterior margin of mesoventrite ( Fig. 65 ) almost straight; marginal mesoventral stria well impressed, slightly carinate; mesoventral disc flat, covered with very coarse and dense round punctures separated about their own diameter; meso-metaventral suture very thin, straight, anterad of that present undulate, inwardly bent meso-metaventral stria; intercoxal disc of metaventrite laterad of lateral metaventral stria and before metacoxa with deep round punctures separated by about their own diameter, medially punctation becomes sparser and finer and completely disappears on short narrow area around median line. Lateral metaventral stria ( Fig. 66 ) well impressed, carinate, straight, shortened; lateral disc of metaventrite flat, with deep ellipsoid large punctures, separated by about their own diameter; metepisternum evenly covered with much coarser and denser punctation, punctures separated by less than half their own diameter.

Abdomen. Intercoxal disc of first abdominal ventrite with lateral depressions, almost completely striate laterally; surface laterally and basally covered with coarse and dense punctures, medially punctation becomes finer and sparser.

Legs. Protibia ( Fig. 67 ) slightly dilated and flattened, outer margin with five conspicuous teeth diminishing in size in proximal direction topped by minute denticles; protibial spur rather large, hooked, inserted near tarsal insertion; protarsal groove rather deep; setae of outer row sparse, regular; protibial stria carinate, complete; setae of median row dense, short; tarsal denticle single, conspicuous and thin. Posterior surface of protibia ( Fig. 68 ) with row of widely-spaced minuscule denticles; apex of protibia with three minuscule apical denticles, posterior protibial stria complete, terminating in several inner posterior denticles; inner row of setae double, thin. Mesotibia ( Fig. 69 ) slender, outer margin with approximately nine short denticles growing in size in proximal direction, outer row of setae regular, rather short; setae of median row microscopic; posterior mesotibial stria almost complete; mesotibial spur short and stout. Anterior face of mesotibia with dense row of rather long and thick setae near outer margin, another row of microscopic setae present medially; anterior mesotibial stria complete, terminating in several inner anterior denticles. Metatibia slenderer than mesotibia with five widely-spaced denticles on outer margin growing in size in proximal direction; anterior face of mesotibia with double row of dense regular microscopic setae; terminal metatarsomere approximately twice as long as two preceding, metatarsal claws very short, shorter than half its length.

Differential diagnosis. An unusually large species (PEL= 3.10 mm), distinguished easily from all SE Asian congeners by shortened marginal pronotal stria that is completely absent on its posterior half (compare Figs 58 and 59; the name semimarginatus is very appropriate here). This species somewhat resembles G. nannetensis in its sheer size and elytral punctation but differs from it in shortened marginal pronotal stria as well as obliterated apical elytral stria. Unfortunately, the only known specimen is a female.

Biology. Unknown.

Distribution. Known only from the holotype, which was collected either in North India or in China.

Figures

Figs 56–57.Habitus of SEAsian Gnathoncus species. 56 – G. semimarginatus Bickhardt, 1920, dorsal habitus; 57 – G. vietnamicus Kryzhanovskij, 1972, dorsal habitus. 58 – G. brevisternus Lewis, 1907, anterolateral view showing marginal pronotal stria; 59 – G. semimarginatus, the same.

Figs 60–69. SEM micrographs of Gnathoncus semimarginatus Bickhardt, 1920. 60 – head, dorsal view; 61 – antennal club, ventral view; 62 – mentum; 63 – propygidium and pygidium; 64 – prosternum; 65 – mesoventrite; 66 – lateral disc of metaventrite and metepisternum; 67–68 – protibia (67 – dorsal view; 68 – ventral view); 69 – mesotibia, dorsal view.