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, 22.0 mm; width at wing bases, 8.0 mm; abdomen, 11.0 m m , width across T2, 9.0 mm; front wing length, 18.5 mm, width, 7.5 mm. Head: Frontal out line (excluding mouthparts) narrowly trapezoidal, slightly higher than widest dimension; vertical region weakly irregularly concave, rather strongly inclined toward occiput; ocelli situated in weak arc somewhat below supraorbital line, closer together than their diameter, the interocellar line slightly greater than the ocellocular line; ocular half of ocellocular area with microscopic puncta- tion, the ocellar half mostly smooth and polished; compound eyes each little more than twice as high as widest measurement, the upper outline somewhat more narrowly rounded than that below, the inner margins for the most part subparallel and only weakly convergent above; clypeus rather evenly convex, little broader basally than median height, its disc mostly smooth with some weak punctures, laterally and above with small irregular punctures; labrum well covered with strong hairs below, little more than twice as wide as median thickness, its ventral margin broadly arcuate, the shelf depressed, the tubercles not very prominent, with coarse punctures above and only limited smooth area below, their mesal summits separated a distance about equal to length of F2; malar space only perceptibly longer than distance between (and including) mandibular articula tions, its surface only weakly convex and for the most part smooth; flagellum nearly twice as long as scape, FI subequal to combined lengths of F2 and 3, F2 subequal to F3. Legs: Mesobasitarsite slightly more than 3i times longer than greatest width, its outer surface shallowly concave mid- longitudinally, the abruptly rounded distoanterior angle considerably in recess of the spinate distoposterior angle; outer surface of hind tibia microscopically granulate, weakly convex mid- longitudinally from base to about f its length, its widest part little less than longest hairs of corbicular fringe; metabasitarsite with finely pubescent and mostly microscopically granulose outer surface broadly concave mid-longitudinally, its posterior margin noticeably arcuate near base and beyond nearly straight, the acutely rounded distoposterior angle extended beyond bluntly rounded distoanterior angle. Pubescence'. Rather short on head and thorax, even, fine and dense, longer on most of abdomen; corbicular fringe moderately long and dense comprised of mostly straight hairs or nearly so; metabasitarsal posterior fringe not dense with weakly arcuate to nearly straight hairs, longest on basal half becoming gradually shorter beyond. Color: Head, thorax, legs and abdominal Tl-3 black (as well as their corresponding sterna); last 3 abdominal segments ferruginous orange. Wings very dark with some violaceous reflection under certain light.
Worker. Length, 17.0 mm; width at wing bases, 7.0 mm; abdomen, 9.0 mm, width across T2, 8.0 mm; front wing length, 16.0 mm, width, 6.0 mm. Except for size, morphologically very similar to the queen. Character of pubescence and coloration like those of the queen.
Male. Length, 18.0 mm; width at wing bases, 7.0 mm; abdomen, 9.0 mm, width across T2, 7.5 mm; front wing length, 16.0 mm, width, 5.5 mm. Head: Frontal outline (excluding mouth- parts) rather narrowly trapezoidal, above broadly arcuate, slightly wider than median height; compound eye somewhat bulging, twice as high as broad, nearly equally rounded above and below, the inner margin for the most part parallel with its counterpart; vertex weakly convex, evenly sloped toward occiput, to large extent covered with irregular punctures behind ocelli; ocellocular area mostly smooth to microscopically granulose, the ocular half with only few small punctures; ocelli located noticeably below supraorbital line in the form of an arc, slightly closer together than their diameter, the interocellar and ocellocular lines equal; malar space about H times longer than distance between (and including) mandibular articulations, very weakly un evenly convex, shiny but with minute shallow punctures and microscopic hairs; labrum little more than twice as wide as its median thickness, the ventral margin nearly straight except lateral corners are arcuate, the lower half covered with somewhat coarse sculpture and rather strong hairs, the weak callosities largely smooth and with only few punctures; flagellum little more than 4 times longer than scape, FI perceptibly longer than F2, together only slightly longer than F3. Legs: Mesobasitarsite subrectangular, about 4 times longer than its greatest width, the outer sur face broadly and shallowly concave mid-longitudinally, the distoposterior angle acute, the disto anterior one broadly rounded with only slight emargination between these angles; metabasitarsite also subrectangular but slightly less than 4 times longer than widest part, the microscopically alutaceous outer surface shallowly convex mid-longitudinally, with fine reclining pubescence, the distal angles about equally extended with slight emargination between, the distoposterior angle sharper than the distoanterior one, the posterior margin only weakly arched with rather dense and even short fringe most of which is not longer than i of widest part of segment. Pubescence: Character like that of female except on thorax somewhat longer, looser and a little uneven in length. Genitalia, seventh and eighth abdominal sterna (PI. XV). Color: Like that of female except base of abdominal T4 narrowly clothed with black pile, the remainder of its pubescence and all on T5-7 ferruginous. Wing color very much the same as that of the female.
Comments. Except for differences in body length, especially of the worker, which range from ca. 8.0 mm to near the size of small queens, I cannot detect any significant morphological variations in the castes of this species. Its coloration likewise is notably constant throughout its range.
The color pattern of this taxon is very much like that of Pyrobombus (C.) brachycephalus (Handl.), a species to which it bears no close relationship.