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,21.0mm;widthatwingbases,10.0mm;abdomen,10.0mm, width across T2, 10.5 mm; front wing length, 20.0 mm, width, 6.5 mm. Head: Frontal outline (excluding mouthparts) narrowly trapezoidal, broadly arcuate above, only slightly higher than greatest width, vertical region noticeably inclined toward occiput, for the most part rather densely covered with small to medium punctures; ocular half of ocellocular area with some small distinct punctures, the ocellar half smooth; ocelli situated in weak arc just below supraorbital line, the distance between each little less than their diameters; compound eyes little more broadly rounded below than above, their inner margins for the most part subparallel and weakly convergent above; malar space about equal in length to distance between (and including) mandibular articulations, its outer surface somewhat unevenly convex and with some minute punctures; labrum little wider than it greatest thickness, with broadly arcuate ventral margin, with both labral shelf and tubercles prominent, the latter coarsely punctate but not especially flattened below and with well-separated conical mesal summits; flagellum about If times longer than scape, FI much longer than either F2 or F3 and very nearly equal to their combined lengths; clypeus for the most part rather weakly convex, sloping laterally, well covered with a mixture of small and medium punctures, about li times wider near base than its median height. Legs: Mesobasitarsite subrectangular, little less than 4 times longer than greatest width, its outer surface weakly concave longitudinally, the spinate distoposterior angle extended considerably beyond blunt distoanterior angle, outer surface of hind tibia weakly alutaceous anteriorly and distally otherwise shining, unevenly convex longitudinally, the widest part being near If times wider than longest hairs of corbicular (posterior) fringe; metabasitarsite with outer surface broadly concave longitudinally and mostly alutaceous, the posterior margin arcuate only near base, beyond nearly straight, the blunt disto anterior angle considerably in recess of sharply acute distoposterior angle with distinct emargina- tion between. Pubescence: Rather short, dense, being only somewhat longer and looser on head, posterior region of scutellum and over most of abdominal dorsum; corbicular fringe rather dense, of medium length, most hairs except distal ones being nearly straight; metabasitarsal posterior fringe rather short becoming gradually shorter distally, the hairs mostly very weakly arcuate to straight. Color: Entirely black. Wings black with noticeable violaceous reflection, under certain lights tending to display slight coppery tinge.
Worker. Length, 17.0 mm; width at wing bases, 7.0 mm; abdomen, 9.0 mm, width across T2, 6.5 mm; front wing length, 15.5 mm, width, 5.0 mm. Essentially like the queen in all respects except the relative morphological features usually are more diminutive.
Male. Length, 14.5 mm; width at wing bases, 6.0 mm; abdomen, 7.0 mm, width across T2, 6.0 mm; front wing length, 14.5 mm, width, 5.0 mm. Head: Frontal outline (excluding mouth- parts) mostly roundish trapezoidal, broadly arched above and behind eyes, slightly wider than median height, vertical region weakly convex and covered for the most part, with small and medium punctures; ocelli situated in nearly a straight line, little closer together than their diameters, the latter ones touching the supraorbital line; ocular half of ocellocular area with very few small to medium irregular punctures; compound eyes nearly as broadly rounded above as below, their inner margins almost parallel; malar space little longer than distance between (and including) mandibular articulations, its surface weakly irregularly convex, rather sparsely covered for the most part with small punctures; labrum about twice as wide as thick, its ventral margin broadly arcuate, its surface more or less flat and irregularly sculptured, the callosities much reduced; flagellum little more than 5 times longer than scape, most segments slightly crenulate, FI subequal to F2 and about one-half of F3, FI and F2 combined about equal to F3. Legs: Mesobasitarsite rectangular, somewhat less than 4 times longer than widest part, its outer surface shallowly concave longitudinally, the distal end weakly emarginate with distoanterior angle only slightly produced beyond the more acute distoposterior angle; metabasitarsite subrectangular, most of its outer surface rather deeply concave longitudinally, its posterior margin weakly but rather evenly arcuate from base to distal end which is emarginate between the evenly extended distal angles, the distoposterior one more acute than the distoanterior angle. Pubescence: Very similar to that of queen but not quite as dense, especially on the thorax; mesobasitarsal posterior fringe of medium and rather even length throughout; metabasitarsal posterior fringe of even and short length throughout. Genitalia, seventh and eighth abdominal sterna (PI. XIII). Color: Black except some finer pile on face and some pubescence on venter, including leg bases, slightly tinged with greyish, most of that distomedially on last abdominal sternum very pale ferruginous
(or golden) in the form of a dense patch. Wings colored like those of the queen.
Comments. This species is close to M. (M.) morio (Swed.) with which it frequently has been confused. From that species it can be separated in the females, along with other features, by the shorter malar space, the sharper metabasitarsal distoposterior angle and the somewhat less violaceous reflections on the wings; the male can be distinguished by the sagittal heads of the genitalia having their mesal margins devoid of any serrations, most of the flagellar segments are more pronounced ly crenulate than those of morio, and the posterior fringes on parts of the legs referred to in the redescription are different.
Some workers seen are only one-half or less the size given for that caste.