Megarthrus bimaculatus Fauvel

(figs 3, 6–14)

Megarthrus bimaculatus Fauvel: 1904, 87; Cuccodoro 2003, 367, figs 10–11 (redescription). Type locality: Ceylon. Holotype female (ISNB).

Specimens examined. 33: SRI LANKA: " Ceylon, 22.iii.1882 | I.R.Sc. N. B. #17.479" (ISNB, holotype female). Additional specimens, 32: Central: Hakgala, 1700–1800m, 28.i.1970, leg. Besuchet, Löbl & Mussard, #30a [sifting in ravine, NE slope] (MHNG, 1 male and 1 female); same data, but 1800m #30c [sifting in primary forest above Botanical Garden] (MHNG, 1 male); Hanguranketa, 750m, 27.i.1970, leg. Besuchet, Löbl & Mussard, #28 [sifting bark and Polypores on old log] (MHNG, 2 males and 2 females]; Madulkele, c. 1000m, 27.i.1964, leg. Mussard (MHNG, 1 male); above Mululla, 750m, 4.ii.1970, leg. Besuchet, Löbl & Mussard, #45 [sifting in forest above Mululla] (MHNG, 1 female); Nuwara Eliya, 1950m, 29.i.1970, leg. Besuchet, Löbl & Mussard, #33 [sifting in forest at inferior forest edge, at foot of Pidurutalagala] (MHNG, 1 female); same data, but 15.ii.1970, #69 (MHNG, 1 male); Pidurutalagala, 2200m, 29.i.1970, leg. Besuchet, Löbl & Mussard, #32 [sifting in forest, on SW slope] (MHNG, 1 female); above Talatuoya, 850–1000m, 27.i.1970, leg. Besuchet, Löbl & Mussard #27a [sifting in remnants of forest] (MHNG, 1 female); Uva: Haputale, 1350m, 23.i.1970, leg. Besuchet, Löbl & Mussard, #19 [sifting forest leaf litter in ravine] (MHNG, 12 males and 7 females).

Diagnosis. Body (fig. 3) predominantly yellow brown, with broad transverse subapical dark spot on each elytron; combined length of head, pronotum and elytra = 1.50–1.70 mm; maximal pronotal width = 0.80–0.90 mm; lateral outlines of frons markedly concave to apex truncate; occipital ridge well marked, with anterolateral portions sinuate, extended from lower eye margins dorsally to nearly third of neck width, then sharply recurved posteroventrally back to upper level of eyes, posterolateral portions connected dorsally with irregular transverse ridge; antennae with short and dense pubescence present only on antennomeres 8–11; antennomeres 9–10 symmetrical.

Male: carina of anterior frontal edge raised, occasionally sinuate; protarsomeres 1 without tenent setae; protarsomeres 5 without flat apophysis; metatibiae slightly curved subapically, with adventral row of peg-like setae on slightly less than apical half; mesotrochanters with 1-3 peg-like setae; abdominal sternites IV–V unmodified; abdominal sternite VIII with well delimited discoidal hyaline medial impression; aedeagus as in figs 6–9.

Female: anterior frontal edge slightly carinate, evenly; abdominal tergite VIII as in figs 13, 14); valvifers fused dorsally on entire length, each with dorsal apophysis projecting posteriorly near mid length (figs 11, 12); dorsal part of genital segment as in fig. 10.

Distribution and natural history. The species is restricted to Sri Lanka (Central and Uva Provinces), where specimens were collected in forest leaf litter at elevations ranging from 750 to 2200 m a.s.l.

Comparisons and diagnostic notes. Megarthrus bimaculatus, M. lanka sp. n., M. narendrani sp. n., M. nilgiriensis sp. n., M. rufomarginatus, M. sumatrensis Cameron and M. vastus Wendeler are the only members of the genus with short and dense pubescence restricted to the antennomeres 8 to 11. These species also uniquely share the presence of externally recurved posterolateral branches of the occipital ridge. These species also notably have in common female valvifers fused dorsally on entire length, and the presence in male of a well delimited discoidal hyaline medial impression on the abdominal sternite VIII. Among them only M. bimaculatus, M. narendrani sp. n. and M. nilgiriensis sp. n. have bicoloured elytra and dorsally projecting apophysis on female valvifers. These three species can be easily distinguished by the shape of their aedeagus and valvifers (see comparisons and diagnostic notes under M. narendrani sp. n.).