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Extracted from: Milliron H.E., (1973). A Monograph of the Western Hemisphere Bumblebees (Hymenoptera: Apidae; Bombinae). The Entomological Society of Canada, No. 89.
Description. Queen. Length, 23.0 mm; width at wing bases, 9.0 mm; abdomen, 12.0 mm, width across T2, 10.5 mm; front wing length, 19.0 mm, width, 7.5 mm. Head: Frontal outline (excluding mouthparts) subtrapezoidal, the dorsal outline bluntly rounded dorsolaterally, slightly wider than high, the vertical contour subconvex; ocellocular area deeply concave near lateral ocellus, mostly smooth and polished except for weak, shallow, irregular punctations medially; outine of compound eye narrowly convex above, below more broadly evenly rounded, the inner margin nearly straight except above weakly concave, nearly twice as high as greatest width; ocelli situated in an arc slightly below supraorbital line, less than their diameters apart, the median one noticeably larger anteroposteriorly; vertical region behind irregularly weakly sculptured; clypeus rather evenly convex except laterally, nearly as high as greatest width, mostly roughly irregularly punctate but smoother mid-ventrally; labrum with broadly arcuate ventral margin, mostly covered with short stiff hairs, the broad tubercles nearly confluent mesially, more or less subtriangular and irregularly sculptured; malar space about as long as width between (and including) mandibular articulations, its median sur face mostly polished and with limited minute pubescence; flagellum slightly more than If times longer than scape, FI If times longer than distal width, distinctly longer than sub- rectangular F2, perceptibly longer than rectangular F3, subequal to F2-3 combined. Legs: Mesobasitarsite subrectangular, nearly 3 times longer than greatest width, its outer mid longitudinal surface weakly evenly concave, covered with short stiff pubescence except posterior fringe longer, the distoposterior angle acute; metabasitarsite 2f times longer than greatest width, its posterior margin strongly arcuate basally, beyond weakly so to nearly straight, the outer sur face mid-longitudinally widely evenly concave, the distoposterior angle subacute and extending somewhat beyond the rounded distoanterior angle, the posterior fringe stiff, slightly arcuate, of moderate length and longest over the basal half of segment. Pubescence: Moderately long, dense and even except somewhat longer and looser on scutellum, sparse on head; corbicular fringe moderately dense, longer and more arcuate distally, the longest hairs subequal to widest dimension of tibia which is weakly unevenly convex and minutely aciculate over its mid-longi tudinal outer surface. Color: Pronotum (including lobes to near tegulae), anterior one-third of mesoscutum, most of scutellum and abdominal T3 golden-yellow (sometimes lemon-yellow), otherwise entirely black except for (sometimes) few yellowish hairs mesolaterally on abdominal T2; interalar black band wide, not sharply defined, the black pubescence conspicuously infused with the yellow on the mesoprescutum and that on the scutellum. Wings deeply infumated throughout, violaceous.
Worker. Length, 15.0 mm; width at wing bases, 6.0 mm; abdomen, 6.0 mm, width across T2, 6.5 mm; front wing length, 13.0 mm, width, 5.0 mm. Overlooking diminutive differences in measurements, very much like the queen, except that the pubescence is somewhat longer, more uneven and shaggy throughout. Color: Like that of the queen, except lighter yellow on ab dominal T2 medially, over most of T3 distally, and on all of T4.
Male. Length, 13.0 mm; width at wing bases, 5.5 mm; abdomen, 6.5 mm, width across T2, 6.0 mm; front wing length, 13.5 mm, width, 5.0 mm. Head: Frontal outline (excluding mouthparts) ovate trapezoidal, nearly as high as greatest width, the upper (temporal) angles rather weakly evenly arcuate; compound eye narrowly rounded above, more evenly and broadly arcuate below, the inner margin nearly straight, the posterior margin broadly evenly convex; vertical region nearly flat, mostly irregularly punctate except smooth and polished about ocelli; the latter positioned in a transverse arc, all about equidistant apart, the lateral ones about touching the supraorbital line; ocellocular area smooth and polished except for some irregular sculp ture posteriorly, weak anteriorly; malar space about as long as distance between (and including) mandibular articulations, the surface convex and polished, with some irregular punctations, and some microscopic pile especially posteriorly; labrum subrectangular with nearly straight ventral margin arcuate only at lateral corners, irregularly punctate, evenly covered with short hairs, medially transversed concavely between weak indefinite callosities; flagellum nearly 2f times longer than scape, FI slightly shorter than F2, both combined nearly equal to F3 or F4, the suc ceeding flagellar segments each progressively shorter and somewhat narrower. Legs: Mesobasi tarsite elongate, subrectangular, and slightly more than 4 times longer than greatest width, outer surface mid-longitudinally moderately evenly concave, the distal angles subarcuately rounded, the posterior one sharper and more advanced; metabasitarsite subrectangular, about 3f times longer than wide, mid-longitudinally broadly evenly concave, the posterior margin evenly arcuate, distally nearly truncate except the subarcuate distoposterior angle sharper and somewhat extended beyond the rounded distoanterior angle, the posterior fringe rather short and uniform. Pubescence: Moderately long, mostly uniformly dense and compact, shaggier and longer on thoracic dorsum. Genitalia, seventh and eighth abdominal sterna (PI. XV). Color: Head black with general admixture of yellowish-gray subpubescence; pronotum, mesoscutum anteriad of tegulae, thoracic pleura above, scutellum, sparsely on abdominal T1 and mid-proximally on T2 as well as all of T3 lemon-yellow, the ill-defined (black) interalar band, thoracic venter, legs and remainder of abdomen black. Wings uniformly reddish brown throughout.
Comments. This species is not exceptionally variable structurally. Some times the labral tubercles are weak and completely convergent mesially. The relative lengths of the flagellar segments do not differ to any degree; but the depth of the vertical concavity (especially of the queen), its sculpture, and that of the female clypeus do vary somewhat. Chromatically, this is the most variable of all Neotropical species. Throughout the eastern range, extending into northern Argentina, the coloration as given is commonly encountered. Yet, queens taken in eastern South America have pubescence predominantly lemon-yellow on the thorax and abdomen, just as do many workers and males. Totally black specimens commonly occur throughout the species range. These constitute the major part of most collections of the species. On some specimens collected in the northwestern range (especially in Colombia) the abdominal areas normally covered with yellow might have this color replaced (at least in part) by coppery red, but such deviants are not regarded as representatives of subspecific rank because the gradation of replacement is from slight to extensive and seems to be an unstabilized and more or less erratic condition, suggesting no more than chromatic aberrations induced by climatic effects in these areas. Sometimes the males have ferruginous pile laterally on abdominal T6-7, but are otherwise entirely black.
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