Heterotrigona (Borneotrigona), Engel, 2019

Engel, Michael S., 2019, Notes on Papuasian and Malesian stingless bees, with the descriptions of new taxa (Hymenoptera: Apidae), Journal of Melittology 2019 (88), pp. 1-25 : 14-21

Borneotrigona Engel , new subgenus

ZooBank: urn:lsid:zoobank.org:act:1278AC40-63E7-410A-A79E-B9B719375A1B

TYPE SPECIES: Trigona hobbyi Schwarz, 1937 . DIAGNOSIS: Heterotrigona hobbyi has long been recognized to be an outlier among Platytrigona and its combination of traits were used by some authors to highlight the lack of distinction between Platytrigona and other groups of Malesian and Papuasian Meliponini (e.g., Michener, 1990). However, it seems clear that H. hobbyi has simply been misplaced. Like Platytrigona , these are larger bees relative to Sahulotrigona and Sundatrigona . The subgenus is similar to Platytrigona in the pubescent basal area of the propodeum, albeit with a slightly broader mediolongitudinal glabrous line ( Fig. 37 ), but the similarity ends there ( Heterotrigona and Sundatrigona lack such pubescence, while Sahulotrigona is largely glabrous except small apicolateral patches of wispy setae) (Sakagami & Inoue, 1987; Inoue & Sakagami, 1993; Engel & Rasmussen, 2017). Unlike Platytrigona , Borneotrigona has distinct erect, stiff, thickened, black, simple bristles ( Figs. 34–36 ) on the head, legs, and most abundantly on the mesosoma, particularly on the mesoscutum and mesoscutellum ( Fig. 37 ). The presence of the thickened bristles in Borneotrigona is similar to those of Heterotrigona s.str. and Sundatrigona (albeit such bristles are sparser in the last subgenus), and it may be that these groups are related. In addition, the posterior glabrate zone of the inner surface of the metatibia is broader than the keirotrichiate zone in Borneotrigona ( Figs. 39, 40 ), while the situation is reversed in Platytrigona . In Borneotrigona the posterior distal angle is subangulate ( Figs. 38–40 ), rather than more angulate in Platytrigona . In the forewing, the basal vein (1M) is distinctly basad 1cu-a and the wing strongly bicolorous in Borneotrigona ( Fig. 41 ), while it is instead confluent or slightly postfurcal and more uniformly colored in Platytrigona ( Figs. 18–21 ).

DESCRIPTION: Workers of moderate size, forewing length (including tegula) approximately 7.3–7.8 mm; integument black, shiny, smooth, with scatered microscopic punctures; integumental maculation absent ( Figs. 34–36 ); setae largely black to dark fuscous except with fine, minute, dark coppery to dark greyish plumose pubescence intermingled on face and mesosoma; erect, stiff, thickened, black, simple bristles on uppermost face, vertex, mesosoma and legs.

Head slightly broader than mesosoma, with face distinctly broader than compound eye length ( Fig. 36 ); vertex short, rounded; ocelloccipital distance slightly greater than one ocellar diameter; interocellar distance approximately 2.0× ocellar diameter; ocellocular distance 2.0× ocellar diameter; scape slightly longer than torulocellar distance; first flagellomere shorter than second flagellomere, second and third flagellomeres equal in length; second through tenth flagellomeres each slightly longer than wide; intertorular distance approximately 0.5× torulorbital distance; upper torular tangent near facial midlength; frontal line weakly impressed, not carinate, extending from median ocellus to near midpoint of toruli; inner orbit of compound eye weakly concave in upper third; gena about as broad as compound eye in profile, posterior border rounded; malar area of moderate length, slightly longer than flagellar diameter, approximately 1.5× flagellar diameter ( Fig. 36 ); labrum weakly convex, short, much wider than long, apical margin medially blunt; mandible bidentate, teeth well defined and incised (as in most other Heterotrigona s.l.), incision between first and second preapical teeth slightly nearly orthogonal; first and second labial palpomeres with several elongate, apically wavy but simple setae (as in most other Heterotrigona s.l.).

Mesoscutum with median sulcus weakly impressed; notauli scarcely evident; parapsidal lines short, about as long as ocellar diameter, and weakly impressed. Mesoscutellum short, ending at profile of metanotum and not distinctly overhanging propodeum, rounded, slightly swollen in profile, with shining transverse depression along mesoscuto-mesoscutellar sulcus. Propodeum long and sloping (strongly declivitous), with distinct angle between basal area and posterior surface, dorsal-facing basal surface about as long as posterior surface; basal area smooth, shining, pubescent except mediolongitudinal glabrous area ( Fig. 37 ), glabrous area slightly wider than medial length of metanotum and much narrower than pubescent areas ( Fig. 37 ); propodeal spiracle elongate, approximately 4.5× as long as wide.

Forewing extending well beyond apex of metasoma, with 2Rs, 1rs-m, 1m-cu, apical half 3M, 4M, apical third 1Cuβ, 2Cu, 3Cu, and 2cu-a indicated by brownish nebulous traces ( Fig. 41 ); fenestrae demarcated by faint white spectral lines on infuscate-clear wing membrane; membrane in costal, radial, first cubital cells and anal area bordering cells darkly infumate, slightly less infumate beyond 1M and 1cu-a and blending to clear by tangent with pterostigmal apex, thus wing membrane strongly bicolorous (proximally dark, apically clear); prestigma exceptionally short, less than anterior width of 1Rs; pterostigma slender; marginal cell slightly more than four times as long as maximum breadth, separated from wing apex by slightly less than its maximum width, with apex broadly open, opening about one-third maximum marginal cell width, with nebulous, angled, appendiculate apex to 4Rs, without nebulous 2r-rs; 1M basad 1cu-a, thus short 2M+Cu present, shorter than 1cu-a; submarginal angle (i.e., anterior angle between 1Rs and Rs+M) approximately 101°; M obtusely angled at 1m-cu (i.e., angle between 2M and 3M); 3M tubular in basal half, then nebulous; 2Rs with short basal nebulous stub, otherwise weakly nebulous, angulate; 1rs-m faintly nebulous, straight; r-rs distinctly longer than 3Rs. Hind wing with 8 distal hamuli; radial and cubital cells closed by nebulous veins.

Metatibia approximately 2.75× as long as greatest width, elongate subtriangular ( Fig. 38 ); posterior margin gently recurved with subangulate distal angle ( Figs. 38–40 ) (angulate in Platytrigona ), setae along posterior margin and upper outer surface mostly plumose; outer surface weakly concave apically, with corbicula occupying apical third; apical margin transverse; inner surface with narrow, elevated keirotrichiate zone and broad posterior subglabrous zone, with abrupt clivulus; keirotrichiate zone narrower than posterior glabrate zone ( Figs. 39, 40 ) (broader than in Platytrigona ), width of keirotrichiate zone in the subapical region (wider portion) distinctly less than length of apical glabrate zone ( Figs. 39, 40 ); penicillus and rastellar comb present, each composed of stiff setae. Metabasitarsus weakly trapezoidal, with posterior margin gently arched, distal angle not projecting ( Figs. 38, 39 ); outer surface with fine basal posterior fimbriate line bordering depression ( Fig. 38 ) (present in all Heterotrigona s.l. and several Asiatic genera, but lacking in nearly all New World lineages with some exceptions such as Oxytrigona Cockerell ); inner surface with short basal sericeous area ( Fig. 39 ).

Metasoma narrow, terga largely smooth and shining, with scatered appressed, minute, apically directed setae, intermingled with erect to suberect short black setae apically on tergum IV and more extensively on terga V and VI; postgradular surfaces of terga V and VI with short, erect to suberect, fuscous plumose setae.

ETYMOLOGY: The new subgeneric name is a combination of Borneo, the island from which the group is known, and Trigona Jurine. The gender of the name is feminine.

INCLUDED SPECIES: The subgenus presently includes only the type species, H. (Borneotrigona) hobbyi from northern Borneo [ Brunei and Malaysia (Sarawak and Sabah)] .

COMMENTS: One hypothesis, mentioned above under Platytrigona , is that Borneotrigona , Heterotrigona s.str., and Sundatrigona form a clade alongside another composed of Platytrigona and Sahulotrigona . Another possibility to be explored is that Borneotrigona is the sister group to a Platytrigona + Sahulotrigona clade as evidenced by the propodeal setation. Under such a scenario, the stout bristles of Borneotrigona , Heterotrigona s.str., and Sundatrigona would potentially be plesiomorphic for the genus, and subsequently lost in the Papuasian subgenera. To date, definitive species of Platytrigona and Sahulotrigona have not been included in phylogenetic estimates so these hypotheses await testing by either morphological, behavioral, molecular, or, ideally, combinations of these data.

Figures

Figures 36–41. Details of worker of Heterotrigona (Borneotrigona) hobbyi (Schwarz). 36. Facial view. 37. Mesoscutellum, metanotum, and basal area of propodeum. 38. Outer surface of metatibia and metatarsus. 39. Inner surface of metatibia and metatarsus. 40. Inner surface of metatibia and metatarsus with major zones colored (using color scheme of Engel & Rasmussen, 2017; Rasmussen et al., 2017): yellow = posterior glabrate zone, green = keirotrichiate zone, blue = apical glabrate zone. 41. Forewing.

Figures 34–35. Worker of Heterotrigona (Borneotrigona) hobbyi (Schwarz). 34. Lateral habitus. 35. Dorsal habitus.

Figures 18–21. Forewings species of subgenus Platytrigona Moure. 18. Heterotrigona (Platytrigona) planifrons (Smith). 19. H. (P.) flaviventris (Friese). 20. H. (P.) keyensis (Friese). 21. Paratype of H. (P.) lamingtonia (Cockerell).