Melikertini Engel

Engel, Michael S. & Davis, Steven R., 2021, New genera of melikertine bees with facial modifications in Baltic amber (Hymenoptera: Apidae), Journal of Melittology 2021 (103), pp. 1-52 : 5-10

Tribe Melikertini Engel

Melikertini Engel, 2001a: 112.

Type genus: Melikertes Engel, 1998.

Melikertine bees superficially resemble stingless bees ( Meliponini ), with a general habitus similar to many small meliponine genera. However, Melikertini differ from Meliponini most noticeably by the complete wing venation (reduced in Meliponini ), presence of a supraälar carina (absent in Meliponini ), presence of an auricle (absent in Meliponini ), absence of a penicillum (present in Meliponini ), presence of a single metatibial spur (absent in Meliponini ), toothed pretarsal claws (simple in Meliponini : nota bene, the inner ramus in Melikertini is minute and often difficult to see if not in the proper orientation), and presence of a well-developed sting (vestigial in Meliponini ).

Several Melikertini have unusual morphological modifications. Aside from those taxa discussed herein with facial specializations, Mochlomelikertes Engel et al. has a uniquely modified mesoscutellum that bears a densely setose, elongate, medial extension of the posterior margin that extends caudally over the metanotum, propodeum, and beyond the apical margin of the second metasomal tergum ( Engel et al., 2014). More commonly, however, the unique specializations of melikertines are found in the workers and involve the clypeus, either along the apical margin or, more frequently, the base where it abuts the supraclypeal area ( Figs. 3, 4 , 6 ). In the later instances the clypeus protrudes out and upward in various configurations, with the epistomal sulcus traversing behind the extension (visible also in µCT scans). These clypeal enhancements are elaborated on herein (vide infra).

Additional features of Melikertini include: Generally small- to moderate-sized (ca. 3–8.5 mm in length), with sparse body pubescence. The head is generally about as wide as or slightly wider than long, with a clypeus that is flat to gently convex and only weakly protrudent in profile, not considering the varied clypeal modifications discussed herein. The compound eyes are large, with comparatively straight inner ocular margins, at most with a faint concavity to the margin about midlength or above midlength, and the compound eyes are always bare (unlike Apini and some Meliponini ).

The mandibles are quite typical for Apini and Meliponini , that is they are elongate with a reduction of the outer mandibular grooves and with a distinctively wide rutellar cap that is largely indistinguishable from the remainder of the rutellum ( Figs. 63–70 ). Like Meliponini there is a faint outer groove obliquely traversing the rutellum of the mandible, all other outer mandibular grooves and ridges lost or vestigial. Unlike Meliponini though, this groove gradually weakens further apically and disappears in a straight line with the lower corner of the apical margin, while in stingless bees this groove most frequently arches apically to terminate into the lower margin of the mandible (e.g., Michener & Fraser, 1978; Rasmussen et al., 2017: figs. 3C, 6D; Michener, 2007). There is a small narrow incision along upper part of the apical margin that defines a blunt tooth separate from the lower and much broader apical margin ( Figs. 63–70 ), differing from many Meliponini in this regard who have more distinct teeth, albeit often small and shallowly incised. Some melikertines appear to have two incisions, thus suggesting two teeth along the upper portion of the apical margin, but this can often be difficult to observe, particularly if the mandibles are closed.

The mesosoma is quite typical for corbiculate bees and particularly for Meliponini and Apini . Unlike Meliponini , there is a supraälar carina, in this regard plesiomorphically similar to other corbiculates. The mesoscutellum is like many Meliponini (rather than the more bulbous mesoscutellum of Apini ), with an apical margin that is broadly rounded and may or may not overhang the metanotum.

In general, the form of the protibial calcar that forms part of the strigilis (antennal cleaner) fits nicely into the general patern of changes in the form of this modified spur across corbiculate bee tribes. While Apini , Bombini , and Euglossini have a secondary velum or pronounced lobe in its place (“anterior prong” or “anterior velum”) ( Schöniter & Renner, 1980), Meliponini and Melikertini both lack the secondary velum and have only the primary velum that extends obliquely dorso-posteriorly from the rachis ( Fig. 44 ) (orientation is assuming the leg is outstretched straight and orthogonal to the long axis of the body, meaning that the primary velum extends from the rachis upward and slightly inclined posteriorly). Euglossini and Bombini have serrate margins (“Zahnreihen” sensu Schöniter & Renner, 1980) along the outer margin of the rachis as well as on the inner margin of the malus. Apini , Meliponini , and Melikertini lack these serrate margins ( Fig. 44 ), potentially serving as another synapomorphy for this grouping of advanced eusocial tribes ( Engel, 2001a, 2001b). Where observed, the malus of Melikertini is exceptionally short and simple to virtually absent.

Melikertine bees have a distinctive metatibia, the metatibia is consistently long and slender, scarcely expanding along its length for the corbicula ( Figs. 5 , 19–20 , 46–50 , 53, 54 ). In comparison to Meliponini , the metatibia in this regard is superficially similar to the legs of meliponine males, some Lestrimelita Friese, or those Trigona Jurine of the hypogea species group, although melikertines were definitely pollen-collecting and probably also resin as it is not uncommon to find pollen or sometimes what seems to be resin in their corbiculae. While pollen is clear, potential resin masses in the corbiculae are more difficult to discern in some specimens owing to the resin being identical to the resin in which they are entombed. Nonetheless, a clear globular mass that looks like the surrounding resin but is typically faintly darker and distinguished by a subtle line demarcating it from the matrix resin may at times be found in the corbicula. The separation of these is analogous to those internal lines discernable between different resin flows. The subtle coloration differences could result from the resin in the corbicula having been partly processed by the bees during collection through the addition of specific enzymes. Alternatively, the color and faint line demarcating the surface of the mass in the corbicula from the resin matrix could reflect nothing more than the fact that the collected resin had already begun to harden relative to the subsequent resin flow that ensnared the bee. Bees collect resin that has already been exuded from tree wounds (sometimes inflicted by the bees themselves, with the resulting resin-secreting surfaces maintained and defended for days or weeks by repeated chewing: Schwarz, 1948; Howard, 1985) and begun hardening due to exposure to air ( Leonhardt & Blüthgen, 2009), resulting in different stages of curing for the resin. Owing to the lack of metatibial expansion and the presence of an auricle on the metabasitarsal base, which thereby necessitates some corresponding surface on the apical margin of the metatibia with which to press, the posterior margin rounds down to a blunt posterior angle, the angle representing the outer rim of that surface facing the auricle. In this context, the metatibia of Melikertini is more similar to that of Apini or Bombini . The metatibia’s posterior apical corner also differs in this context from the more typical apically broadened metatibia with an angulate, even sharply angled, posterior corner found in some Meliponini , or when the posterior border is broadly rounded it is almost always associated with a greatly expanded corbicular portion of the metatibia (e.g., Cephalotrigona Schwarz : refer to images in Engel & Rasmussen, 2021). The corbicular surface is not depressed or even sometimes faintly convex, like much of the outer surface of the metatibia and again superficially resembling the hind legs of meliponine males or robber bees. The corbicula is often elongate and largely only demarcated by the patern of setae on the outer surface, rather than a distinctly depressed surface typical of most corbiculate bees. As noted, the apex of the metatibia does have a slightly depressed, well-delimited surface defining the upper bounds of a pollen press and a distinct auricle is present proximally on the metabasitarsus to form the lower portion of the press ( Figs. 43 , 53, 54 ). Unlike Meliponini there is no penicillum, although in Amelikertotes there is a small patch of long setae that extends apically from the outer, anterior, apical corner of the metatibia. These setae do not have the form of penicillum, instead having the form typical of most generally straight or slightly arched setae, but given their placement it is tempting to speculate such a patch as a precursor to an eventual penicillum. Overall, it is possible that the general form of the metatibia is a synapomorphy for Melikertini as it seems to be a derived condition relative to the general form of corbiculae and metatibiae of other corbiculate tribes.

Other leg features of the tribe include the presence of a well-developed mesotibial spur (variable, but many times greatly reduced and vestigial in Meliponini : Oliveira, 2002), a single metatibial spur ( Figs. 19 , 43 ), pretarsal claws with a minute inner ramus (often exceedingly difficult to see without the right orientation), and an arolium is always present and sometimes quite large.

The general patern of wing venation is largely typical for those corbiculate bees with complete distal wing venation (distal wing venation reduced in Meliponini ). The marginal cell is large and narrowly to broadly rounded apically, sometimes feebly appendiculate, and typically offset from the anterior wing margin to varying degrees. The marginal cell is always longer than the distance from its apex to the wing apex. The pterostigma is always well developed and of a moderate size, and typically around 2.5–3× longer than wide, with a minute prestigma, and with r-rs arising near the midpoint (sometimes slightly basad or distad) of the pterstigma. The second submarginal cell is always produced posteriorly where it meets 1m-cu, and 1m-cu is angulate apically where it meets 2M. The hind wing has a distinct jugal lobe that is broadly and deeply incised, and there are a reduced number of distal hamuli (relative to Euglossini , Bombini , Apini , Electrobombini, and Electrapini, with the exception of Thaumastobombus Engel). The wing membranes lack alar papillae or infuscation patterns, always being hyaline.

The metasoma is quite like most Meliponini , although the sting apparatus is clearly not reduced, with many melikertines preserved with the simple sting everted and with the associated sheaths visible.

The diversity of melikertine bees as currently understood is summarized in table 1. The most commonly encountered melikertine has been S. micheneri , a distinctive species that can be found in comparatively larger numbers than other Melikertini . The growing number of well-preserved specimens has made it possible to make more extensive comparisons between this species and the newly discovered species reported herein. Conversely, some species are exceptionally rare, such as Roussyana palmnickenensis (Roussy) and Melissites trigona Engel ( Engel, 2001a). Presently, all Melikertini are known only as inclusions in amber, while other extinct tribes from the same epoch are found as both inclusions and compressions (e.g., Wappler & Engel, 2003; Wappler et al., 2015; Wedmann et al., 2009).

Figures

Figures 3–5. Worker of Succinapis micheneri Engel (SEMC B-018). 3. Facial view. 4. Dorsal view of head and mesoscutum. 5. Right lateral view.

Figures 6–8. Worker of Succinapis micheneri Engel (SEMC B-017). 6. Oblique facial view. 7. Outer (prolateral) view of metatibia and metabasitarsus. 8. Detail of fringe setae of metatibial posterior/upper margin; white arrow indicates a deceptive internal microfracture of resin trail with branched fractal patern (such fracture planes and trails are a common taphonomic structure among or even running alongside many fine setae in amber inclusions, and can give such setae an artificially plumose appearance; accordingly, care must be taken before concluding whether or not amber-included setae are branched); black arrow highlights one of many truly plumose setae, separate from the internal microfracture planes.

Figures 63–70. PPC-SRµCT scan of mandibles of worker of Amelikertotes clypeata (Engel), new combination; lower left worker from figure 29. Right mandible rotated axially in left column, left mandible rotated axially in right column. 63. Outer (slightly ventral oblique) view of right mandible. 64. Outer (slightly ventral oblique) view of left mandible. 65. Dorsal view of right mandible. 66. Dorsal view of left mandible. 67. Inner view of right mandible. 68. Inner view of left mandible. 69. Ventral view of right mandible. 70. Ventral view of left mandible.

Figures 40–45. PPC-SRµCT scan of fore- and midleg structures of worker of Amelikertotes clypeata (Engel), new combination; upper right worker from figure 29. 40. Foreleg, outer (prolateral) view. 41. Foreleg, anterior view. 42. Foreleg, inner (retrolateral) view. 43. Magnified view of metatibial-metabasitarsal joint showing auricle and rastellum. 44. Magnified view of protibial calcar. 45. Midleg. Note that scan resolution was insufficient to resolve most fine setae.

Figures 19–20. Inner (retrolateral) surfaces of metatibiae of Haidomelikertes, new genus, and Aethemelikertes, new genus. 19. Haidomelikertes uraeus, new species, holotype worker (SMF Be 14263a). 20. Aethemelikertes emunctorii, new species, holotype worker (SMF Be 14262a).

Figures 46–50. PPC-SRµCT scan of hind leg structures of worker of Amelikertotes clypeata (Engel), new combination; upper right worker from figure 29. 46. Right hind leg, outer (prolateral) view. 47. Right hind leg, inner (retrolateral) view. 48. Left metatibia, view along posterior edge. 49. Left hind leg, inner (retrolateral) view. 50. Left hind leg, outer (prolateral) view, note debris packed into surface of corbicula. Note that scan resolution was insufficient to resolve most fine setae.

Figures 51–56. PPC-SRµCT scan of worker of Amelikertotes clypeata (Engel), new combination; lower left worker from figure 29. 51. Left lateral view. 52. Dorsal view. 53. Outer (prolateral) view of hind leg. 54. Inner (retrolateral) view of hind leg. 55. Ventral view. 56. Right lateral view. Scale bar at right for 53 and 54, scale bar in center for 51, 52, 55, and 56. Note that scan resolution was insufficient to resolve most setae of the body and legs (e.g., posterior fringe of fine setae on metatibia is not resolved).