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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.
D escription. Queen. Length, 23.0 mm; width at wing bases, 8.5 mm; abdomen, 12.0 mm, width across T2, 9.5 mm; front wing length, 18.0 mm, width, 6.5 mm. Head: Frontal outline (excluding mouthparts) trapezoidal, above rather evenly arcuate, only slightly higher than greatest width, the vertical region inclined toward occiput, irregularly punctate; ocellar half of ocellocular area smooth and polished, the ocular half with only microscopic punctation; ocelli situated in very weak arc just below supraorbital line, closer to one another than shorter diameter of a lateral ocellus; compound eyes little more broadly rounded below than above, their inner margins nearly subparallel; malar space about equal to distance between (and including) mandibular articulations, the surface weakly convex and at most only microscopically punctate; labrum subrectangular, little more than twice as wide as greatest thickness, its ventral margin rather evenly arcuate, the tubercles well separated, not strongly conical mesally and for the most part rather coarsely punctate; flagellum nearly twice as long as scape, FI nearly li times longer than almost quadrate F2, distinctly longer than F3 and subequal to F2 and F3 combined; clypeus moderately and rather evenly convex, for the most part irregularly punctate except less so medially below, little wider below than its median height. Legs: Mesobasitarsite sub- rectangular, nearly 4 times longer than widest dimension, its outer surface shallowly concave mid-longitudinally, the blunt distoanterior angle noticeably in recess of the spinate distoposterior angle; outer surface of hind tibia for the most part rather evenly convex and nearly smooth mid-longitudinally, its widest part little less than longest hairs of corbicular fringe; metabasitarsite with outer surface microscopically alutaceous, broadly concave from near base to distal end, posterior margin arched basally but nearly straight beyond, the acute distoposterior angle pro duced very little if any beyond blunt distoanterior angle. Pubescence: Rather short, even and dense except slightly longer on face, vertical region, scutellum and on abdominal T4-5; corbicular fringe of medium length, shortest basally, most hairs only slightly curved; metabasitarsal posterior fringe rather short, longest basally, beyond gradually diminishing in length to distal end. Color: Black, except golden-yellow on thoracic dorsum anteriad of tegulae, scutellum behind wide interalar (black) band and on distal two-thirds of abdominal T3. Wings deeply infuscated to blackish, not iridescent or with violaceous reflection.
Worker. Length, 16.0 mm; width at wing bases, 7.0 mm; abdomen, 8.0 mm, width across T2, 7.5 mm; front wing length, 14.0 mm, width, 5.0 mm. Except for smaller size, this caste is structurally similar to the queen but the relative features are more diminutive. Chromatically the yellow pattern (usually) is a more pallid yellow, and the interalar (black) band somewhat narrower.
Male. Length, 17.0 mm; width at wing bases, 7.0 mm; abdomen, 9.0 mm, width across T2, 8.0 mm; front wing length, 16.0 mm, width, 5.0 mm. Head: Frontal outline (excluding mouth- parts) rather narrowly trapezoidal, only little higher than widest dimension, the vertical region only weakly convex, mostly covered with medium punctures; ocellocular area largely smooth; ocelli positioned closer together than their diameters and in a weak arc, the lateral ones touching supraorbital line; compound eyes somewhat more broadly rounded below than above, their inner margins about subparallel to slightly divergent below; malar space little longer than distance between (and including) mandibular articulations, evenly and weakly convex, smooth except for few small punctures; labrum subrectangular with broadly arcuate ventral margin, mostly pubescent and with very weak callosities; flagellum 4 times longer than scape, most of its seg ments weakly crenulate, FI subequal to F2 and considerably shorter than F3 being little more than I the combined lengths of F2 and F3. Legs: Mesobasitarsite subrectangular, little more than 4 times longer than widest part, the acute distoposterior angle somewhat extended beyond the more or less rounded distoanterior angle, the outer surface mid-longitudinally concave; metabasitarsite narrowest at base, 4 times longer than widest dimension, the outer microscopically alutaceous surface mid-longitudinally concave, evenly weakly arcuate from base to distal end, the distal angles similar to those of mesobasitarsite but posterior one somewhat less acute. Pubescence: On head, thorax, and abdomen similar to that of the queen but slightly looser; posterior mesobasitarsal fringe longest along proximal half to short at distal end of segment; metabasitarsal posterior fringe only long on proximal third and gradually becoming short at distal end of segment. Genitalia, seventh and eighth abdominal sterna (PI. XV). Color: Similar to that of queen except for the following: Trace of intermixed fine pallid pile on the face; black on vertex intermixed with noticeable amount of yellow; little more downward extension of yellow mixed with the black anteroventrad of tegulae; small amount of yellow near base and laterally on abdominal T4; some burnt sienna distally on abdominal T7 and S7. Wing color similar to that of the queen.
Comments. Otherthanforsizeandsmalldifferencesinpunctation,especially on the clypeus, and differences in size of the queen epipygial convexity, little morpho logical variation was detected in this taxon. Queens are somewhat larger or smaller than the length given for the hypotype. Workers vary from being considerably larger than given, and I have seen some no larger than i the given length of the hypotype. As for the male no notable size variation has been detected by me, though it might occur. Some color variation has been encountered in all castes, especially with respect to the width of the black interalar band; on most specimens it is quite broad, while on others it is of medium width. Sometimes the black encroaches considerably onto the scutellum in both queens and workers, and occasionally in the male the interalar band is not very sharply defined. The yellow of the worker and often of the male is lighter than that normal for the queen (golden-yellow), but sometimes the latter might have rather brownish yellow. Some males have very little or no yellow on abdominal T4 and/or extensive burnt sienna on both T6-7.
This species has been much confounded with M. (M.) atratus (Fkln.) since South American specimens of the latter have been identified as medius which is not known to occur on that continent (see Map 22). Although Cresson gave Utah as the type locality of medius, this taxon does not occur anywhere in the United States.
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