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, 26.5 mm; width at wing bases, 11.0 mm; abdomen, 13.5 mm, width across T2, 12.0 mm front wing length, 20.5 mm, width, 7.5 mm. Head: Frontal outline (excluding mouthparts) roundly trapezoidal, above weakly arcuate, temples somewhat bulging and sharply rounded, little higher medially than widest dimension; vertical region mostly weakly concave and for the most part covered with medium and small punctures, moderately inclined toward the occiput, with a rather striking cresent-shaped elevation above; ocular half of ocellocular area mostly covered with minute to microscopic punctures, the ocellar half smooth except for few small separate punctures outwardly; compound eyes distinctly more broadly and evenly rounded below than above, their inner margins subparallel except evenly convergent above; ocelli rather large, located in a weak arc distinctly below the supraorbital line and separated from one another by about i their diameters; malar space nearly equal in length to distance between (and including) mandibular articulations, rather weakly and somewhat uneven ly convex, shining and bearing microscopic punctations; labrum about twice as wide as its great est thickness, the ventral margin arcuate, the shelf prominent but not especially sharp, tubercles roughly sculptured with their subconical mesal summits well separated by a moderately deep depression; flagellum If times longer than scape, FI about H times longer than F2 which is distinctly shorter than F3, the combined lengths of F2 and 3 not much in excess of the length of FI; clypeus nearly li times wider near base than median height, most of the middle portion weakly convex to nearly flat forming a trapezoidal appearance with the smaller more or less arcuate dimension above, for the most part not densely covered with small to medium punctures. Legs: Mesobasitarsite subrectangular, about 3i times longer than widest part which is nearly l i times the length of longest hairs in posterior fringe, the outer surface broadly concave longi tudinally, the distoposterior angle acute but not sharply so, extended considerably beyond the blunt distoanterior angle, with a narrow emargination between; outer surface of hind tibia weakly convex longitudinally to nearly flat, somewhat alutaceous (microscopically granulose), the broadest portion about 11 times wider than length of longer hairs of the posterior (corbicular) fringe; metabasitarsite with posterior margin for the most part straight being abruptly and strongly arcuate near base, the outer surface broadly concave longitudinally, the broadest portion about 3 times wider than length of longest hairs of posterior fringe, the subacute disoposterior angle extended considerably beyond the blunter distoanterior one, with a shallow emargination between. Pubescence: Short, dense, even throughout; corbicular fringe dense, comprised mostly of straight hairs of medium length except somewhat shorter toward base and longer, evenly arcuate ones distally; metabasitarsal posterior fringe dense, of nearly straight hairs which are longest basally and gradually diminished in length toward distal end of the segment. Color: Jet-black throughout. Wings uniformly black with some violaceous reflections under certain light.
Worker. Length, 18.0 mm; width at wing bases, 8.5 mm; abdomen, 9.0 mm, width across T2, 8.5 mm; front wing length, 16.5 mm, width, 6.0 mm. Disregarding difference in size, the features of this caste are so similar to those of the queen that they need not be treated in detail; generally these are more diminutive, and sometimes they are less clearly evident; for example, the weakly convex to flattened condition on the clypeus might be more difficult to detect, the limits of any depressed or flattened area on the vertical region might not be so sharply defined, and sometimes the posterior margin of the metabasitarsite is not as abruptly and strongly arcuate near the base. Often the outer surface of the hind tibia is somewhat more shiny, and on certain parts the punctation is subject to some variation.
Male. Length, 18.5 mm; width at wing bases, 8.0 mm; abdomen, 9.5 mm, width across T2, 8.0 mm; front wing length, 17.0 mm, width, 6.0 mm. Head: Frontal outline (excluding mouth- parts) rather narrowly trapezoidal, nearly straight above and evenly rounded at the temples, about as high as its widest dimension; vertical region nearly flat and well covered with mostly medium punctures, weakly inclined toward occiput; ocular half of ocellocular area with minute punctures, the ocellar half largely smooth; ocelli situated in a weak arc, separated from one another by slightly less than their diameters, distinctly below the supraorbital line; compound eyes somewhat more broadly and more evenly rounded below than above, their inner margins almost parallel; malar space little longer than distance between (and including) mandibular articulations, weakly convex, with some minute punctures; labrum little more than twice as wide as thick, its ventral margin broadly arcuate, its surface mostly flat and with very weak finely punctate callosities; clypeus weakly convex over most of its median area, the sides prominently sloped; flagellum about 4 times longer than scape, FI as long as or slightly longer than F2 and little more than j- of F3, being distinctly more than i the combined lengths of F2 and F3. Legs: Mesobasitarsite narrow elongate rectangular, at least 4 times longer than widest part which is little wider than longest hairs of posterior fringe, the outer surface broadly concave longitudinally, distoposterior angle only slightly sharper than the distoanterior one, and both about equally extended with shallow emargination between; outer surface of hind tibia rather weakly and unevenly convex; metabasitarsite with broadest part nearly 4 times as wide as longest hairs of posterior fringe, its outer surface for the most part weakly concave to nearly flat then rising rather strongly to the posterior margin which is weakly and evenly arched along the proximal half, beyond less arcuate to nearly straight, the distal angles about equally extended with shallow emargination between, both subacute, the distoposterior one being somewhat more rounded. Pubescence: Length, density, and evenness similar to that given for the queen. Genitalia, seventh and eighth abdominal sterna (PL XV). Color: Entirely black except face with some fine pallid pubescence intermixed, and sometimes part of the pile on leg bases will be at least pale tipped, and that along the posterior margins of some abdominal sterna (especially the distal ones) will be pale. Wings black but not as deeply so as those of the females.
Comments. This taxon is very distinctive and easy to recognize. In the female the median area of the clypeus is, for the most part, weakly convex to nearly flat and does not vary widely in this respect as to be unreliable. The genitalia of the male is very characteristic and does not closely resemble that of other species.
Chromatic aberrations are not often encountered in this species. I have found only a very few non-striking ones on which one or more isolated yellow hairs occur almost anywhere on the abdominal dorsum, especially along the extreme lateral limits of a tergum.