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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, 26 mm; width at wing bases, 10.5 mm; abdomen, 14.0 mm; width across T2, 11.0 mm; front wing length, 18.0 mm, width, 6.5 mm. Head: Frontal outline (excluding mouthparts) broadly trapezoidal, the upper corners broadly arcuate, per ceptibly wider than high; vertical region weakly concave to nearly flat laterad of ocelli, the ocular two-thirds of ocellocular area covered with moderately deep rather coarse well-separated irregular punctures, the ocellar one-third smooth and polished; compound eye compared with other species rather smaller, its outline almost 3 times higher than greatest width, subacutely rounded above, more widely rounded below; ocelli moderate in size, about equidistant from one another, situated in nearly transverse position, the lateral ones touching supraorbital line; clypeus about as high as wide hasally, rather strongly evenly convex medially, its disc nearly completely covered with medium to small separated irregular punctures; labrum about 2\ times wider than thick, its ventral margin broadly arcuate, the outer surface covered with moderately long, stiff, irregular pubescence, each of the prominent tubercles concave medially, roughly sculptured, the tubercular depression deep, the strongly developed mesal tubercular prominences as far apart as length of F3; malar space only perceptibly longer than distance between (and including) mandibular articulations, its surface unevenly convex with microscopic pubescence medially and posteriorly, otherwise smooth and polished; flagellum slightly shorter than twice as long as scape, FI little longer than distal width, longer than rectangular F2 which is much shorter than F3. Legs: Mesobasitarsite subrectangular, narrowed proximally, 3 times longer than widest dimension, its outer surface broadly concave especially mid-longitudinally, nearly truncate distally, covered with dense fine pilose pubescence on most of distal third; metabasi- tarsite subrectangular, little less than 3 times longer than widest dimension of segment, its posterior margin evenly weakly arcuate basally to nearly straight distally, the outer surface shallowly concave. Pubescence: Rather fine, dense and even except that on most of head, scutellum and abdominal T4—5 longer, coarser and looser; corbicular fringe long and loose throughout, nearly straight except arcuate distally, averaging distinctly longer than greatest width of segment; metabasitarsal posterior fringe long, arcuate, loose hairs intermixed with short weakly arcuate ones on proximal two-thirds of segment, beyond progressively shorter to absent distally. Color: Head black with only slight intermixture of brownish yellow on vertex; thorax above and upper half of mesopleuron ochraceous yellow except wide interalar black band extending over anterior one-third of scutellum, the thorax below (including metapleura) black; abdomimal Tl-3 ochraceous yellow, T4 black with some burnt sienna distally, T5 intermixed with black and burnt sienna, all prominent hairs on T6 with burnt sienna tinge. Wings moder ately infumated throughout with pale reddish brown.
Worker. Length, 12.5 mm; width at wing bases, 6.5 mm; abdomen, 7.0 mm; width across abdominal T2, 6.5 mm; front wing length, 14.0 mm, width, 4.5 mm. Morphologically like that of the queen in all respects except in diminutive relation of corresponding features. Color: Like that of the queen except more often abdominal T5-6 predominantly covered with burnt sienna pile.
Male. Length, 12.5 mm; abdomen, 6.0 mm; width across abdominal T2, 5.5 mm; width at wing bases, 6.0 mm; front wing length, 15.5 mm, width, 4.8 mm. Head: Frontal outline (excluding mouthparts) broadly triangulate, about as high as greatest width, the upper angles rather broadly rounded; compound eye nearly 2i times higher than widest part, more acutely rounded above than below; vertical region nearly flat, the ocelli situated transversely on the supraorbital line, ocular area coarsely irregularly sculptured except smooth and polished about ocelli and on mesal half of ocellocular area, the ocelli removed from one another by slightly more than their diameter, interocellar line about equal to ocellocular line; malar space 11 times longer than distance between (and including) mandibular articulations, convex except area below compound eye, smooth and posteriorly covered with microscopic pubescence; labrum 21 times wider than thick, the ventral margin very shallowly emarginate medially, broadly arcuate at lateral corners, the transverse callosities weak and confluent mesially, the outer surface generally weakly irregularly sculptured, mostly pubescent; flagellum H times longer than scape, FI in length about equal to F2, combined only slightly longer than F3. Legs: Mesobasitarsite subrectangularly elongate, nearly 4 times longer than greatest width of segment, its outer surface mid-longitudinally concave, the distal angles nearly equal; metabasitarsal out line subrectangular, distally truncate, little more than 3 times longer than greatest width of segment, its outer surface nearly flat, the posterior margin weakly arcuate throughout, posterior fringe long (about H times longer than widest dimension of segment) except progressively shorter distally, the external surface uniformly covered with microscopic recumbent hairs. Pubescence: Generally long and loose, that on head and thorax above more even, shaggy and uneven on thoracic pleura, legs and abdomen. Color.’ Pale yellow on clypeus, vertical region and thoracic dorsum (including upper pleura) except broad indefinite interalar black band (sub- dominantly intermixed with tawny yellowish hairs), the thoracic venter mostly tawny yellow; legs to tarsi predominantly tawny yellow, the tibial and metabasitarsal fringes rufescent tinged; abdominal Tl-3 pale ochraceous yellow, T4 basally black the remainder burnt sienna distally intermixed with some black, T5-7 covered with vivid pale burnt sienna. Wings lightly uniformly infumated with pale reddish brown. Genitalia, seventh and eighth abdominal sterna (PI. XII).
Comments. Morphologically there is evidently only slight variations within the castes (or sexes), usually these with respect to differences in body length and compactness; body size and other morphological features of the worker are relatively proportional to those of the queen. The male is rather constant in all critical char acters, such as the appearance of the flagellum, distal abdominal sterna and the genitalia. Chromatic variability is in keeping with that of most species, melanism being encountered in populations of certain environmental areas. Certainly I have encountered melanism in all of the arctic and subarctic species of bumblebees in the Western Hemisphere. The more robust and compact melanic form of this species described as neoboreus by Sladen (1919b: 28G) belongs to this species. The proximal three female abdominal terga of strenuus are typically entirely covered with ochraceous yellow, but sometimes T3 is black laterally and distally; abdominal T3 of the male is likewise suffused with black which is sometimes predominant. Usually the distal abdominal terga (T5-6 in female, T5-7 in male) are covered with varying amounts of burnt sienna to rufescent pubescence, but sometimes only the ultimate terga are so colored wholly or in part, otherwise occasionally these are mostly to entirely black (especially with regard to the form heretofore known as neoboreus) .
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