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, 24.5 mm; width at wing bases, 11.0 mm; abdomen, 13.5 mm, width across T2, 11.5 mm; front wing length, 19.0 mm, width, 7.0 mm. Head: Frontal outline (excluding mouthparts) rather narrowly trapezoidal, rounded over the temples and arcuate across occiput, little higher medially than greatest width; vertical region moderately inclined toward the occiput, rather densely covered with small and medium punctures; ocular half of ocellocular area irregularly covered with microscopic to small punctures, the ocellar half largely smooth; ocelli positioned in a weak arc, little closer together than their diameters, lateral ones touching the supraorbital line; compound eyes considerably more rounded below than above, their inner margins subparallel; malar space nearly H times longer than distance between (and including) mandibular articulations, the surface rather evenly and weakly convex with few very small punctures over lower half; labrum nearly twice as wide as thickest part, its ventral margin broadly arcuate, the irregularly arcuate shelf sharp, the tubercles for the most part rather coarsely punctate, their subconical mesal summits separated by a distinct but not sharp depression; flagellum about II times longer than scape, FI almost twice as long as quadrate F2 and distinctly longer than rectangular F3, being subequal to combined lengths of F2 and F3; clypeus for the most part rather evenly but not strongly convex, well covered with small and medium irregular punctures more sparsely so mid-ventrally, nearly II times wider near the base than its median height. Legs: Mesobasitarsite subrectangular, little less than 5 times longer than widest part, its posterior margin slightly arcuate, its outer surface weakly concave longitudinally to nearly fiat, the spinate distoposterior angle extended noticeably beyond the blunt distoanterior one; outer surface of hind tibia for the most part rather shiny, weakly and unevenly convex longitudinally, the broadest part little greater than length of longest hairs comprising the posterior fringe; metabasitarsite with somewhat alutaceous outer surface broadly concave longitudinally, the posterior margin weakly arcuate along basal third then nearly straight beyond, the acute disto posterior angle noticeably extended beyond the blunt distoanterior angle, with shallow emargina- tion between. Pubescence: Moderately short, dense and of even length on most of body except somewhat longer and looser on the scutellum; corbicular fringe rather dense, most hairs of medium length and only weakly arcuate except distal ones; metabasitarsal posterior fringe with longer hairs only at proximal third of the segment, these of only medium length, remainder of fringe very short. Color: Head, rather broad interalar band with arcuate anterior margin and posteriorly encroaching onto scutellum, and legs black; remainder of thorax, abdominal Tl-3 and abdominal venter (mostly) tawny yellow, abdominal T4-6 rufescent. Wings evenly in- fumated with light brown except streak in anterior area of the front wing marginal cell grading from dark to pale brown.
Worker. Length, 15.5 mm; width at wing bases, 7.5 mm; abdomen, 7.0 mm, width across T2, 8.0 mm; front wing length, 14.0 mm, width, 5.0 mm. Very similar to the queen in all im portant respects except that the given structural features are more diminutive. With regard to color pattern the interalar black “band” is most often more or less circular with somewhat less definitive outline; in the marginal cell the streak referred to above in the queen is usually entirely pallid.
Male. Length, 15.0 mm; width at wing bases, 7.0 mm; abdomen, 8.0 mm, width across T2, 7.0 mm; front wing length, 14.0 mm, width, 5.0 mm. Head: Frontal outline (excluding mouth- parts) roundly trapezoidal, weakly arched medially above, about as wide as medium height; vertical region nearly flat and not strongly inclined toward the occiput, well covered mostly with irregular medium punctures; ocelli situated in nearly a straight line just below or forward of the supraorbital line, slightly closer together than their diameters; ocellar half of ocellocular area smooth and polished, the ocular half with some obscure small punctures; compound eyes little more broadly rounded below than above, their inner margins subparallel except weakly con vergent above; malar space about li times as long as distance between (and including) mandi bular articulations, weakly convex and having only some microscopic punctures; labrum little more than twice as wide as thick, its ventral margin broadly arcuate, the callosities very weak and not sharply separated from each other; flagellum near 5i times longer than scape, most of its segments very weakly crenulate, FI subequal to rectangular F2, their combined lengths equal to F3. Legs: Mesobasitarsite narrow rectangular, at least 3i times longer than its widest part, its outer surface weakly concave longitudinally to nearly flat, the distal angles being equally extended with a shallow emargination between, the distoposterior one only noticeably sharper; meta- basitarsite subrectangular, its outer surface for the most part broadly concave longitudinally, the posterior margin weakly but rather evenly arched from base to near distal end which has a shallow emargination between the equally extended angles, the distoposterior one being the sharper. Pubescence: On body similar to that of the queen and worker except somewhat shorter especially that on the abdomen; posterior fringe of hind tibia short and rather even throughout, the same fringes on both meso- and metabasitarsite very short and even. Genitalia and seventh and eighth abdominal sterna (PI. XIV). Color: Similar to that of the queen and worker except the face has
a noticeable amount of fine pale hairs intermixed with the black; coxae, trochanters, and (usually) all femora especially ventrally and/or laterally with some pale hairs; the interalar black band is narrower and (usually) less well defined; abdominal T5-7 are rufescent (often less vividly so); the front wing marginal cell has only a slight trace of the streak mentioned in the other castes.
Comments. Structurally, for the most part, this taxon has its closest ally in M. (M.) pullatus (Fkln.), but in coloration is entirely unlike that species. Despite some variation in color, this presents no problem in identification; sometimes the interalar band is variable in width to a limited degree, form and definity, especially among workers and males. Often some of the yellow, especially that on the abdomen tends to have a very faint reddish tinge under certain lights, and the rufescens on the distal abdominal terga might be considerably diluted especially in the male.
Senile specimens of this species are sometimes encountered and, in my opinion, part of the specimens described by Friese (1911a: 457) as robustus v. sulfuratus are in reality seniles that should be assigned to opifex;these have been examined. For a fuller discussion of the confusion resulting from the description of sulfuratus see Comments under Pyrobombus (C.) tucumanus (Vach.) (Milliron: in press).* My 1960 lectotype of robustus v. sulfuratus Fr. is from Salta, Argentina, 2500, 3-1905, Steinbach, which is in the Zoologische Museum, Humbolt Uuniversitat, East Berlin, and is being assigned to this latter species (i.e., tucumanus).