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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, 22.5 mm; width at wing bases, 9.0 mm; abdomen, 12.0 mm, width across T2, 9.5 mm; front wing length, 16.5 mm, width, 6.5 mm. Head: Frontal out line (excluding mouthparts) vertically elongate (as in borealis), slightly higher than wide, temporal area strongly arcuate; vertical region mostly flat, finely punctate and polished except concave area laterad of lateral ocelli; ocular two-thirds of ocellocular area irregularly weakly punctate, the ocellar one-third smooth and polished; compound eye more arcuately rounded above than below, about 3 times higher than wide; lateral ocelli moderate in size, the median larger, all located equidistant apart in a weak arc near the supraorbital line; clypeus trapezoidal, distinctly wider basally than high, moderately convex, medially smooth and polished; labrum 1 wider than thick, the triangular rounded tubercles weakly punctate laterally to nearly smooth, the inner angles directed mesolaterally and separated by length of pedicel; malar space almost twice higher than distance between (and including) mandibular articulations, sinuate sub-ocular surface weakly punctate, the convex extremity more or less reticulate; flagellum slightly less than twice as long as scape, FI 2i times longer than distal width, about as long as lengths of quadrate F2-3 combined. Legs: Mesobasitarsite subrectangular, narrowed proximally, less so distally, nearly 3 times longer than widest dimension, its outer surface weakly broadly concave, completely covered with fine dense recumbent pubescence, the distoposterior angle moderately sharply acute, the posterior margin more strongly arcuate on proximal half, the margin nearly straight on distal half; metabasitarsite elongate rectangular, little more than 3 times longer than widest dimension, entirely covered with short strong recumbent pubescence on the outer surface, the posterior margin weakly arcuate basally, straight to spinose distoposterior angled the outer surface nearly flat. Pubescence: Moderately fine throughout, dense and even except longer, looser and more uneven on face, vertex, scutellum posteriorly and on abdominal terga, especially on T5 and laterally on T4 and 3, T6 mostly with short stiff hairs and lacking coverage as on preceding terga; corbicular fringe long and loose, mostly weakly arcuate, the longest hairs about equal to greatest tibial width; metabasitarsal posterior fringe comparatively short, that along the greater median border longer, stouter and only weakly arcuate, the remaining fringe distally progressively shorter, straighter. Color: Head pallid yellow except brownish on gena; thorax above anteriad of tegulae and mesopleuron mostly ochraceous yellow, the moderately wide interalar black band intermixed with yellowish laterally, the scutellum yellow; abdominal Tl-5 orange-yellow, T6 mostly blackish above, basolaterally with longish pubescence, more distally with dense short orange-yellow intermixed with irregularly longer darker hairs. Wings moder ately infuscated throughout.
Worker. Length, 18.0 mm; width at wing bases, 6.5 mm; abdomen, 10.5 mm, width across T2, 6.5 mm; front wing length, 15.0 mm, width, 5.7 mm. Morphologically similar to the queen except comparatively smaller throughout. Color: Like that of the queen.
Male. Length, 17.5 mm; abdomen, 10.5 mm, width across T2, 6.2 mm; width at wing bases, 5.5 mm; front wing length, 13.5 mm, width, 5.0 mm. Head: Frontal outline (excluding mouthparts) more or less triangular, broadly arcuate above, little less than twice as high as widest dimension; compound eyes elongate, unevenly oval, slightly less than twice as high as greatest width; vertical region irregularly flat and variously punctate except smooth and polished laterad of ocelli which are located just below supraorbital line, the ocelli about as far apart as f the widest diameter of the median one; mandibular space irregularly convex with weak de pressions especially anteriorly, without noticeable pile except posteriorly, the ventrodistal width (including mandibular articulations) nearly equal to the length; labrum flat, without noticeable callosities, the ventral margin bilobed medially; flagellum little more than 3i times longer than scape, FI nearly equal to F3, combined with F2 subequal to F3 plus F4. Legs: Mesobasitarsite subrectangularly elongate, 3£ times longer than its widest dimension, the outer surface evenly concave, the distal angles more or less acute and subequal; metabasitarsite rectangular, 3i times longer than wide, exterior surface moderately concave mid-longitudinally, completely covered with short recumbent pile, the acute distoposterior angle and the more rounded distoanterior one equally extended, the posterior margin weakly evenly arcuate, with a short irregular fringe. Pubescence: Longer and more shaggy on most parts than that of either queen or worker. Genitalia, seventh and eighth abdominal sterna (PI. XII). Color: Drab yellow on head intermixed with some black; thorax mostly yellow except paler on anterior thoracic dorsum, the interalar black band not well defined; distal parts of legs (chitinous) brownish
ferruginous; pubescence mostly black on coxae and femora, otherwise on legs (tibiae and tarsi) yellow to ferruginous; abdominal Tl-7 mostly ochraceous yellow, the corresponding sterna lighter. Wings lightly infumated, more so over the proximal two-thirds.
Comments. This species is very closely allied to distinguendus (Mor.) and might be only a junior synonym. Further study is warranted to determine the true taxonomic status of both taxa. As far as I have been able to distinguish between this species and distinguendus is a matter of the small differences in the lengths of cor responding proximal flagellar segments, especially in the male.
This species, like its close relative borealis (Kby.), exhibits only slight morpho logical variations that are within my species concept. However, the color of the pubescence on the head and thorax is especially variable in degree of paleness to the extent that any chromatic decision, let alone that based strictly on morphological differences, must await further study.
Over sympatric areas, captured (presumably) senile specimens of borealis that are encountered are often mistaken for this species, and vice versa; evidently, hybridization occurs in such sympatric distributional areas, ruling out senility as the logical explanation for specific convergence. The numerous questionable specimens seen from such locations are sufficient to indicate hybridization.
Most often in the male, frequently in the worker and less often in the queen, the dorsal thoracic pile anteriad of the interalar band varies from whitish yellow to a deeper shade, approaching the uniform typically golden-yellow of borealis.
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