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, 25.0 mm; width at wing bases, 10.0 mm; abdomen, 14.0 mm, width across T2, 11.0 mm; front wing length, 19.5 mm, width, 6.5 mm. Head: Frontal outline (excluding mouthparts) rather broadly trapezoidal, only slightly higher than its greatest width, the temples evenly rounded, the occiput only weakly elevated; vertical region for the most part flat, moderately inclined toward the occiput, rather evenly covered with medium punctures; ocular half of ocellocular area mostly minutely punctate, the ocellar half smooth except for few very small punctures outwardly; compound eyes more widely and evenly rounded below than above, their inner margins nearly straight and slightly diverging below; ocelli comparative ly small, positioned in a very weak arc, separated from one another by about their diameters, the lateral ones touching the supraorbital line; malar space about as long as distance between
(and including) mandibular articulations, its surface unevenly convex and mostly bearing very small irregular punctures; labrum about 2i times wider than its greatest thickness, the ventral margin evenly arcuate, the shelf weak and not very sharp, the tubercles covered with small punctures, flat below and with subconical mesal summits that are not deeply and sharply separated; flagellum little more than H times longer than scape, FI at least li times longer than quadrate F2 and considerably longer than F3, being about f as long as F2 and F3 combined; clypeus evenly convex but not strongly bulging, little less than 2\ times as high as widest part near base, well covered with medium and small punctures except sparsely so mid-basally. Legs: Mesobasitarsite rectangular, 4\ times longer than wide, concave longitudinally, the acute disto- posterior angle extended slightly beyond the blunt distoanterior angle, with shallow emargination between; outer surface (corbicula) of hind tibia barely concave to nearly flat on posterior half with a weak convexity longitudinally on most of the anterior half, most of the surface being alutaceous (granulose), not highly polished anywhere, the widest part little less than twice the length of longest hairs comprising most of posterior (corbicular) fringe; metabasitarsite with outer surface widely concave mid-longitudinally, its posterior margin weakly arcuate at basal third, beyond nearly straight to the rather blunt distoposterior angle which is about equally produced as the blunt distoanterior angle, with a somewhat pronounced emargination between, the widest part being about 2i times as wide as longest hairs of the posterior fringe. Pubes cence: Short, dense, and even throughout except somewhat longer and looser on most of scutellum and on abdominal T4-5; mesobasitarsal posterior fringe short and even; corbicular fringe comprised of hairs that are mostly straight and of medium length except very short basally, distally longest and arcuate, rather dense; metabasitarsal posterior fringe moderately dense, short, longest basally and gradually diminishing in length beyond, becoming very short and sparse or absent distally. Color: Entirely black except abdominal T3 which is golden- yellow. Wings brownish black to black with some violaceous reflection under certain lights.
Worker. Length, 16.5 mm; width at wing bases, 8.0 mm; abdomen, 8.0 mm, width across T2, 7.0 mm; front wing length, 15.0 mm, width, 5.0 mm. Except for its smaller size, it closely resembles the queen in structural details save for a few minor points; for example, often the clypeus might be less densely punctate than in the queen, and the outer surface of the hind tibia is sometimes more strongly and evenly convex longitudinally as well as more shiny. Color: Like that of the queen except often the yellow on abdominal T3 is noticeably paler.
Male. Length, 14.5 mm; width at wing bases, 6.5 mm; abdomen, 7.0 mm, width across T2, 6.0 mm; front wing length, 14.0 mm, width, 4.5 mm. Head: Frontal outline (excluding mouth- parts) roundly trapezoidal, slightly wider than median height, the upper angles (temples) rather sharply rounded, the occiput straight to at most only weakly arcuate; vertical region virtually flat, mostly covered with medium punctures, only weakly sloped toward occiput; ocelli situated in a weak arc and separated from each other by noticeably more than their diameter, the lateral ones fully touching the supraorbital line; most of the ocular half of ocellocular area with some medium to small punctures, the ocellar half smooth and impunctate; compound eyes distinctly more broadly and evenly rounded below than above, their inner margins subparallel; malar space about H times as long as the distance between (and including) mandibular articulations, the surface rather unevenly convex and for the most part minutely punctate; labrum pubescent, little more than twice as wide as thick, mostly flat, the callosities much reduced and irregularly punctate, the ventral margin nearly straight; most flagellar segments very weakly crenulate, FI subequal to F2 and little more than i that of F3. Legs: Meso basitarsite narrow elongate-rectangular, at least 4 times longer than widest part, little wider than longest hairs of posterior fringe, only weakly concave longitudinally, the distal end nearly truncate with both distal angles about equally extended; metabasitarsite subrectangular, about 3i times longer than the widest part, close to i wider than longest hairs of posterior fringe, the posterior margin only weakly arcuate at basal third, beyond nearly straight, the outer surface shallowly to rather deeply concave longitudinally, the distoposterior angle somewhat sharper than distoanterior one, both about equally extended with shallow emargination between. Pubescence: Similar to that of queen and worker but somewhat shorter and less dense especially on the abdomen; mesobasitarsal posterior fringe rather short, moderately dense, longest on the basal half and gradually diminishing in length distally, all hairs being virtually straight; meta- basitarsal posterior fringe short, straight and rather even except distal hairs somewhat shorter, noticeably less dense than fringe on corresponding segment of middle leg. Genitalia, seventh and eighth abdominal sterna (PI. XV). Color: Black except traces of yellowish brown (especially intermixed with the black on the clypeus) and traces of fine pallid pubescence elsewhere on the face, and all abdominal T3 and at least the basal portion of T4 yellow. Wings infumated as in the queen and worker though somewhat less deeply so.
Comments. Exceptforvariabilityofsizeofworkerssomeofwhichmightnot be much more than i that given above, both queen and worker are reasonably stable structurally as well as in color pattern. The male, on the other hand, is subject to a certain degree of chromatic variation whereby yellow might occur on the basal middle of abdominal T2 as well as over all of T3-4; perhaps there is even more extensive color variability but too few males are at hand to permit a more informative statement in this regard.
Reputedly taken in Ecuador, I have before me a male in too poor condition for complete description; it is close to mexicanus to which I had long considered it to belong. However, after critical study of the specimen, noticeable structural differences were noted concerning the genitalia, antenna, malar space, and puncta- tion on the head; the color is at variance in that the face bears more pallid pubes cence, the pronotum has numerous yellowish hairs, traces of yellow occur on abdominal T2, all of T3-4 are yellow, there are traces of burnt sienna on T6 and abdominal segment 7 is largely of this color; most of the abdominal sterna have pale pile on their distal margins. This might represent an undescribed species, or if it is mexicanus it shows that considerable variation can occur especially in the male.