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, 8.0 mm; abdomen, 12.0 mm, width across T2, 10.0 mm; front wing length, 16.0 mm, width, 6.0 mm. Head: Frontal outline (excluding mouthparts) trapezoidal, about as high as wide, dorsolaterally rather strongly rounded; vertical region weakly concave laterad of ocelli; ocelli positioned in a weak arc, re moved from one another by less than their widest diameter, the interocellar line about equal to the ocellocular line, the lateral ones contiguous to supraorbital line; ocular half of ocellocular area with only microscopic punctures, the ocellar half mostly smooth and polished, the vertex laterad and behind with irregular medium punctures; outline of compound eye noticeably more rounded below than above, the inner margin straight except weakly incurved above, the eye about 3 times higher than greatest width; clypeus near base slightly wider than median height, rather evenly convex and mostly well covered with medium irregular punctures; labrum little more than twice as wide as thick, its ventral margin subtruncate, the sharp labral shelf arcuate, the labral tubercles transverse and somewhat flattened, with irregular medium punctures, separated from each other by about the length of FI; malar space nearly li times longer than distance between (and including) mandibular articulations, smooth, weakly convex with some microscopic hairs; flagellum little more than li times longer than scape, FI nearly If times as
long as F2, subequal to F2 and F3 combined, F3 distinctly longer than F2. Legs: Subrectan- gular mesobasitarsite about 4 times longer than its greatest width, the outer surface moderately concave mid-longitudinally, the acutely rounded distoanterior angle in recess of the spinate distoposterior angle; outer hind tibial surface evenly convex to near distal end, microscopically alutaceous; metabasitarsite about 3 times longer than widest dimension, with outer surface broadly concave mid-longitudinally, finely alutaceous, bearing some recumbent microscopic hairs, the blunt distoanterior angle in recess of the acute distoposterior angle, distal margin broadly emarginate, the posterior margin arcuate basally becoming nearly straight over distal half. Pubescence: Compact, moderately fine, of rather even length throughout except longer on head, posteriorly on scutellum and parts of legs, densest on thorax; corbicular fringe rather dense, hairs weakly arcuate except those distally, the longest about equal to widest corbicular dimension; hairs comprising metabasitarsal posterior fringe longest and more arcuate basally, beyond gradually becoming short toward distal end. Color: Head black, thorax yellow (some times ochraceous yellow) except for black interalar band of medium width (sometimes broader); legs black; abdominal Tl, T2 basally (especially medially) and distally (especially distolaterally) and T3 yellow, the remainder of abdomen black (for variability cf. under Comments). Wings strongly, evenly infumated.
Worker. Length, 15.0 mm; width at wing basas, 6.0 mm; abdomen, 7.0 mm, width across T2, 7.0 mm; front wing length, 13.0 mm, width, 5.5 mm. Morphological features, including character of pubescence, of this caste are relatively like those given for the queen. Coloration of typical specimens is also very similar to that described for the queen (atypical specimens with more extensive yellow patterns are more frequently encountered in this caste; cf. under Com ments).
Male. Length, 15.0 mm; width at wing bases, 5.0 mm; abdomen, 8.0 mm, width across T2, 7.0 mm; front wing length, 14.0 mm, width, 4.5 mm. Head: Frontal outline (excluding mouth- parts) narrowly trapezoidal, about as high as widest dimension; compound eye little more than twice as high as greatest width, its outline only slightly more broadly rounded below than above, the inner eye margins almost parallel; vertex weakly concave laterad of ocelli; ocellocular area largely smooth and polished with only few microscopic punctures; ocelli situated in very weak arc near supraorbital line, removed from one another by somewhat less than their diameter, the interocellar line equal to the ocellocular line; malar space about li times as long as distance between (and including) mandibular articulations, weakly convex, polished, with microscopic punctations; labrum nearly 2\ times wider than median thickness, mostly well covered with pubescence except the weak polished callosities; flagellum about 3f times longer than scape, FI nearly equal to F2, together about equal to F3. Legs: Mesobasitarsite subrectangular, nearly 4f times longer than greatest width, distal end emarginate with equally produced angles the pos terior one more sharply rounded, the outer surface weakly concave mid-longitudinally; met- abasitarsite subrectangular, about 4 times longer than greatest width, the outer surface broadly concave mid-longitudinally, its distal end shallowly emarginate, the posterior margin weakly arcuate. Pubescence: Similar in character to that described for the queen. Genitalia, seventh and eighth abdominal sterna figured (PI. XIV). Color: Very much like that detailed'for the queen (for variability c/. under Comments). Wings (usually) somewhat paler.
Comments. Except for differences in size, especially in the worker caste, I have not detected any significant morphological variation in this species, which is, chromatically, one of the most variable in South America. On light colored specimens the interalar black band varies considerably in width and to some degree in definity. The abdominal yellow usually is evident on at least terga 1 and 3; sometimes it is nearly or completely replaced by black on terga 1 and 2, while other specimens might have terga 1-3 all yellow, particularly in the workers. Melanism has been en countered in the male. Such specimens of this sex at hand indicate a condition whereby the color is totally black or one that shows only traces of yellow on the anterior part of the thoracic dorsum and on abdominal T3. I have not found this degree of melanism in the female though possibly it does occur; if so, such in dividuals might easily be confused with M. (M.) atratus (Fkln.), a species in which similar coloration is common.