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, 32.0 mm; width at wing bases, 10.0 mm; abdomen, 16.0 mm, width across T2, 14.0 mm; front wing length, 22.0 mm, width, 8.0 mm. Head: Frontal outline (excluding mouthparts) elongate trapezoidal, about 11 times higher than greatest width through compound eyes; vertical region weakly concave laterad of ocelli, behind rather strongly inclined to occipital area; ocelli located in nearly straight line, the lateral ones almost touching supraorbital line, somewhat closer together than their diameter, the interocellar and ocellocular lines about equal; ocular half of ocellocular area with miscroscopic punctation, the ocellar half mostly smooth and polished, the vertex laterad and behind well covered with microscopic punc tures; compound eye little less than twice as high as its greatest width, more broadly rounded below than above, the inner margin for the most part subparallel with its counterpart to weakly convergent dorsally; clypeus about as high as its basal width, rather evenly convex and well covered with fine punctures; labrum about twice as wide as thick, its ventral margin and prominent shelf both broadly arcuate, the tubercles flattened and irregularly sculptured, their pointed summits nearly as far apart as length of FI; malar space nearly If times longer than distance between (and including) mandibular articulations, weakly and irregularly convex, shin ing and with weak sculpture and some microscopic hairs; flagellum near If times longer than scape, FI about If times longer than F2 and f the combined length of F2 plus F3. Legs: Meso- basitarsite subrectangular, little more than 3 times longer than greatest width, its outer surface broadly and shallowly concave longitudinally, the bluntly rounded distoanterior angle consider able in recess of the spinate distoposterior angle; most of outer hind tibial surface microscopically punctate, longitudinally convex just anteriad of middle then nearly flat behind; metabasitarsite with outer surface evenly but not deeply concave mid-longitudinally, well covered with fine recurved pubescence, about 3 times as long as the widest dimension, its posterior margin not strongly but for the most part evenly arcuate, noticeably emarginate between the nearly equally extended broader distoanterior angle and the acute distoposterior angle; epipygium (abdominal T6) with prominent median carina on distal half. Pubescence: Fine, dense, longer and looser on scutellum and on most of abdomen; corbicular fringe rather dense, the longest hairs nearly equal in length to the widest tibial dimension, none strongly arched except distal ones, those comprising the proximal fourth (or third) finer, shorter, straighter and more dispersed; mesobasitarsal pos terior fringe similar, but more loosely arranged; metabasitarsal posterior fringe rather dense, short and even throughout comprised mostly of weakly arcuate hairs. Color: Vertex mostly, occiput, all of thorax to just above coxae and abdominal Tl-5 orange ferruginous; remainder of head, all the venter, legs and abdominal T6 black. Wings rather deeply infumated throughout.
Worker. Length, 19.0 mm; width at wing bases, 8.0 mm; abdomen, 10.0 mm, width across T2, 9.0 mm; front wing length, 15.0 mm, width, 5.0 mm. The morphology of this caste is similar to that of the queen though usually the labral tubercles and carina on the epipygium are notice ably reduced. The coloration is like that of the queen.
Male. Length, 22.0 mm; width at wing bases, 8.0 mm; abdomen, 12.0 mm, width across T2, 9.0 mm; front wing length, 17.0 mm, width, 5.0 mm. Head: Frontal outline (excluding mouth- parts) narrowly trapezoidal, about as high as its greatest width; compound eye nearly 3 times higher than widest dimension, below noticeably more broadly rounded than above, the inner margin weakly concave, the outer margin evenly arcuate; vertex nearly flat, rather strongly in clined toward occiput, covered with medium punctures; ocular half of ocellocular area with small punctures, these much sparser on ocellar half; ocelli positioned in a nearly straight line adjacent to supraorbital line, slightly closer together than their diameter, the interocellar line nearly equal to the ocellocular line; malar space about II times longer than distance between (and including) mandibular articulations, weakly convex, with some microscopic punctures (those anteriad noticeably larger) and minute recurved hairs; labrum nearly 3 times wider than median thickness, its ventral margin widely arcuate, covered with prominent pubescence below its weak callosities; flagellum 4 times longer than scape, FI about I as long as F2 which is about f- the length of F3, FI and F2 combined only noticeably longer than F3. Legs: Mesobasitarsite subrectangular, 4 times longer than wide, its outer surface weakly concave mid-longitudinally, the rounded disto posterior angle little extended beyond the less evenly rounded distoanterior angle, with only a shallow emargination between; outer surface of hind tibia finely alutaceous, unevenly and weakly concave, both anterior and posterior fringes rather long and relatively dense; metabasitarsite subrectangular, nearly 3i times longer than widest dimension, the outer surface rather shallowly concave mid-longitudinally, the distal angles about equally extended with weak emargination between, the posterior margin only weakly arched basally to nearly straight beyond, the fringe thereon relatively short (shortest at very base), not dense, mostly composed of straight hairs the longest not exceeding i widest part of segment. Pubescence: In character similar to that of queen except somewhat more even in length save for longer scutellar pile. Genitalia, seventh and eighth abdominal sterna (PI. XIV). Color: Like that of queen except that the abdominal dorsum is concolorous orange ferruginous with only few inconspicuous intermixed dark hairs at or near middle of T6-7.
Comments. Many queens of this species, though variable in length to some extent, under 32.0 mm, are the largest and bulkiest bumblebees found in the Western Hemisphere. Workers range in length from about 10 to 20 mm. Morphologically there seems to be little variation except for degree and coarseness of punctation, and prominence of the median epipygial carina referred to in the queen; often this is very weak especially on workers. The color pattern of younger specimens (not callows) is stable, but senile individuals of each caste often have the orange ferruginous faded to very pale or whitish yellow.