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, 21.0 mm; width at wing bases, 9.0 mm; abdomen, 11.0 mm, width across T2, 10.0 mm; front wing length, 20.0 mm, width, 7.0 mm. Head: Frontal outline (excluding mouthparts) narrowly trapezoidal, about 11 times higher than greatest width; vertical region strongly sloped toward occiput; ocelli rather small, located in weak arc just below supra orbital line, distinctly closer together than their diameter, the interocellar line slightly less than ocellocular line; ocular half of ocellocular area with small punctures, the ocellar half mostly smooth; compound eye twice as high as widest dimension, somewhat more narrowly rounded above than below, the inner margin weakly curved to nearly straight and subparallel to its counterpart; clypeus only slightly higher medially than its width at base, its disc not strongly but rather evenly convex and covered with small to medium irregular punctures; labrum slightly less than twice as wide as its median thickness, the ventral margin rather evenly arcuate, the labral shelf weak, tubercles not strongly produced, with their ventral surfaces irregularly punctate and their mesal summits slightly less far apart than length of FI and separated by a rather shallow depression; nearly smooth malar space only weakly convex over its ventral half, nearly twice as long as distance between (and including) mandibular articulations; flagellum nearly 2i times longer than scape, FI almost twice as long as F2 and subequal to combined lengths of F2-3. Legs: Mesobasitarsite subrectangular, about 4 times longer than widest part, its outer surface rather deeply concave mid-longitudinally, the distoposterior angle sharply acute and extended somewhat beyond the bluntly rounded distoanterior angle, with subarcuate emargination between; most of hind tibial outer surface faintly granulose to nearly smooth, with mid-longitudinal con vexity from near base to about distal third; metabasitarsite with outer surface broadly concave longitudinally, microscopically granulose, for the most part covered with fine recumbent pubes cence intermixed with some stronger inclined hairs, the posterior margin weakly arcuate at basal third, beyond nearly straight, the acute distoposterior angle extended slightly beyond bluntly rounded distoanterior angle with an angulate emargination between. Pubescence: Moderately long, fine, dense, longest on face, vertex, scutellum, and abdomen; corbicular fringe rather dense with most hairs only weakly arcuate to nearly straight except arcuate distal ones, the longest somewhat longer than widest part of corbicula; metabasitarsal posterior fringe longest basally, gradually declining in length distally and becoming sparser. Color: Head, thorax, legs, ab dominal T1 and T6, and venter black or blackish; abdominal T2-5 ferruginous (or light coppery red).Wings uniformly stained with brown.
Worker. Length, 18.0 mm; width at wing bases, 8.0 mm; abdomen, 9.0 mm, width across T2, 8.0 mm; front wing length, 16.0 mm, width, 5.5 mm. Morphologically like the queen except the characteristic features are more diminutive. Pubescence and coloration like those of the queen.
Male. Length, 17.0 mm; width at wing bases, 7.0 mm; abdomen, 9.0 mm, width across T2, 7.5 mm; front wing length, 14.0 mm, width, 5.0 mm. Head: Frontal outline (excluding mouth- parts) narrowly trapezoidal, about li times higher than greatest width, the upper angles rather strongly arched; compound eyes little more narrowly curved above than below, the inner margins nearly parallel, the outer margins broadly and rather weakly convex; vertical region nearly flat, strongly inclined toward occiput, mostly smooth and polished about ocelli to finely punctate elsewhere; ocelli about equal in size, distinctly closer together than their greatest diameter, posi tioned in a weak arc, the lateral ones definitely touching supraorbital line; ocellocular area mostly smooth with small irregular punctures present on scarcely more than the ocular third; malar space fully H times as long as distance between (and including) mandibular articulations, its surface at most only weakly convex, smooth and bearing some microscopic hairs posteriorly; labrum subrectangular, its ventral margin nearly straight, about twice as wide as its greatest thick ness, with weak callosities at basolateral corners and only moderately pubescent; flagellum slightly less than 4 times longer than scape, FI and F2 about equal, each nearly twice as long as thick, combined distinctly longer than F3; clypeus perceptibly higher than basal width, its surface rather evenly though not strongly convex, for the most part covered with medium punctures. Legs: Mesobasitarsite subrectangular, about 5 times longer than greatest width, most of outer surface mid-longitudinally concave, the bluntly acute distoposterior angle not extended beyond distoanterior angle; metabasitarsite nearly 4 times longer than greatest width, its outer surface moderately concave longitudinally, the distoanterior angle only slightly in recess of acute disto posterior angle, posterior margin weakly but rather evenly arcuate for the most part. Pubes cence: That on clypeus not dense and fine, similar to that on remainder of head, that on thoracic dorsum somewhat longer and less compact than on the queen but otherwise similar through out; metabasitarsal posterior fringe short, straight or nearly so with longest hairs on basal third about equal to I widest dimension of segment. Genitalia, seventh and eighth abdominal sterna (PI. XIV). Color: Like that of the queen except few blackish hairs distally on abdominal T6 and all of T7 black. Wings only slightly paler than those of the queen.
Comments. Except for some variation in size of workers, little structural variation has been detected in the limited material of this taxon available for study. However, with respect to color most, if not all, of the normally black coloration of the queen (possibly worker and male also) might appear as dark or blackish cinereous pubescence, especially that on the thorax.