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Extracted by H. E Milliron. A Monograph of the Western Hemisphere Bumblebees (Hymenoptera: Apidae; Bombinae) II. The Genus Megabombus Subgenus Megabombus. The Entomological Society of Canada, Ottawa 1970. pp. 239-330.
Description. Queen. Length, 21.0 mm; width at wing bases, 10.0 mm; abdomen, 11.0 mm; width across T2, 11.0 mm; front wing length, 18.0 mm, width, 6.5 mm. Head: Frontal outline (excluding mouthparts) roundly trapezoidal, the widest part slightly more than median height, temples broadly rounded, the vertex transversely weakly arcuate; vertical region irregularly flat, well covered with small (follicular) punctures, moderately inclined toward the occiput; ocular half of ocellocular area with some small punctures, the ocellar half largely smooth; compound eyes almost equally rounded above and below, their inner margins only slightly incurved, subparallel; ocelli of medium size, separated from one another by about I their diameters in a nearly straight line distinctly below the supraorbital line; occellocular line subequal to the interocellar line; malar space only slightly longer (if any) than distance between (and including) mandibular articulations, the surface mostly smooth, irregularly nearly flat; labrum little more than twice as wide as its median thickness, the ventral margin mostly arcuate, the shelf rather short and not sharp, the bluntly conical tubercles irregularly punctate and without a sharp deep intertubercular depression between; flagellum nearly 21 times longer than scape, FI subequal to F2 plus F3, the rectangular latter about 11 times longer than quadrate F2; clypeus for the most part rather strongly and evenly convex, with some small and medium well separated punctures, the widest part near base a little wider than median height. Legs: Mesobasitarsite rectangular, nearly 41 times longer than widest part which is somewhat less than the length of longest hairs of the posterior fringe near base, the outer surface of segment mostly shallowly concave longitudinally, both distal angles more or less rounded and equally extended with a shallow emargination between; outer hind tibial surface granulose, having a weak rather irregular longitudinal convexity anteriad of middle, the widest part of segment about f the length of longest hairs of the posterior fringe; meta- basitarsite subrectangular, most of its outer surface broadly concave longitudinally, the widest part of segment at least 21 times greater than the length of longest hairs of the posterior fringe, the posterior margin only weakly arcuate proximally beyond very weakly arcuate to nearly straight, the rather rounded distoposterior angle extended noticeably beyond the blunt disto- anterior one with a shallow emargination between. Pubescence: Fine, dense, rather short and even except somewhat longer and looser on most of head, posteriorly on the scutellum and some abdominal terga especially T4-5, absent on most of the middle of Tl; mesobasitarsal posterior fringe rather dense consisting of straight (or mostly so) hairs the longest of which are along the proximal one-third of margin beyond gradually becoming shorter; metabasitarsal posterior fringe not dense, consisting of straight or nearly straight hairs that are longest along the proximal half of margin, gradually becoming very short and sparser at the distal end of segment. Color: Head black; thoracic dorsum golden-yellow (sometimes pale) except an interalar band (variable in width) and remainder of thorax black; legs black; abdominal Tl
(usually) with some golden-yellow laterad of middle, the sides or anterolaterally black (often Tl is entirely black; T2 pale golden-yellow, or with patch of yellow laterad of middle the remainder black (often T2 is entirely black); T3 with proximal half or more typically pale golden yellow, usually with at least some black laterally and distally or with a mixture of yellow and black on these areas (on some specimens T3 is totally black); T 4-6 usually entirely white or whitish; abdominal venter dark, but usually with at least some whitish (pale) hairs on S3-5. Wings stained with brown, often having paler areas in some of the discal cells, usually with an iridescent sheen under certain lights.
Worker. Length, 12.5 mm; width at wing bases, 6.5 mm; abdomen, 6.0 mm, width across T2, 6.5 mm; front wing length, 11.0 mm, width, 4.0 mm. Except for the much smaller dimensions, the diagnostic structural features and other characteristics are in relation to one another similar to those detailed for the queen. Color: Head and thorax colored as in the queen; the relative amounts of yellow to black on the abdomen is just as variable as in the queen and specimens of this caste can be found to match any given above; abdominal T4-6 is also usually always white or whitish. Wings colored like those of the queen.
Male. Length, 13.5 mm; width at wing bases, 7.0 mm; abdomen, 6.5 mm, width across T2, 7.0 mm; front wing length, 12.0 mm, width, 4.0 mm. Head: Frontal outline (excluding mouthparts) roundly trapezoidal, the widest dimension equal to the median height, the narrow temples abruptly rounded, the vertex transversely almost straight; vertical region flat, not much inclined toward the occiput, well covered with small (follicular) punctures; ocelli some what large, situated by slightly less than i their diameters apart in a weak arc distinctly below the supraorbital line; ocellocular area smooth; interocellar line slightly more than twice as long as the ocellocular line; compound eyes somewhat bulging, noticeably more broadly rounded above than below, their inner margains straight, parallel; malar space in length only slightly (if any) exceeding the distance between (and including) mandibular articulations, practically impunctate and only very weakly convex transversely; labrum little more than twice as wide as its median thickness, its ventral margin straight or nearly so with rounded corners, the surface mostly covered with medium to small punctures, the callosities distinct but rather weak; flagellum 2.86 times longer than scape, FI and rectangular F3 nearly equal, the latter about i longer than rectangular F2. Legs: Mesobasitarsite narrowly elongate- subrectangular, nearly 5.6 times longer than the widest part which is about 1 the length of longest hairs of the posterior fringe, the outer surface only weakly concave longitudinally, the subacute distoanterior angle considerably advanced beyond the rounded distoposterior angle, without any emargination between; outer surface of hind tibia rather strongly convex longi tudinally, microscopically granulose, the widest part about equal to f the length of longest hairs of the posterior fringe; metabasitarsite subrectangular, its outer surface longitudinally rather broadly concave, the widest part equal to little more than i the length of longest hairs of the posterior fringe, the posterior margin evenly (but not strongly) arcuate from base to distal end, the distoposterior angle sharper than the distoanterior one, both extended about equally with a distinct emargination between. Pubescence: Fine, rather dense and even except some what longer and looser on much of the scutellum and on most of the abdomen, sparse to absent on the middle of Tl; mesobasitarsal posterior fringe not dense, relatively long except shorter at the proximal and distal ends of margin, composed mostly of weakly arcuate or straight hairs; posterior fringes of both the hind tibia and metabasitarsite relatively long, not
dense, consisting of straight or weakly arcuate hairs, those at the base and distal end of both segments shorter. Genitalia and seventh and eighth abdominal sterna (PI. XVII). Color: Head black with some intermixed yellowish pile on vertex and face; thoracic dorsum yellow (often golden) with an interalar black band (sometimes not sharply delimited), remainder of thorax and legs black; abdominal Tl laterally with black and yellow intermixed (or entirely black); T2 yellow with some black basolaterally or laterally, black with patches of yellow in the middle or entirely black; T3 yellow with some black at the sides to entirely black; T4 black; T5 mostly black with some whitish pile distally; T6-7 white or whitish; abdominal venter mostly black with some whitish pile especially laterally on the three distal sterna.
Comments. I have not detected any significant structural variation in any caste of this taxon which is one of the most variable (if not the most variable) chromatically in the Neotropical Region; it has caused bumblebee taxonomists much difficulty. The variability of the color pattern is alluded to at various places in the redescription above and need not be repeated here. Friese (1904h: 188) described this species as a variety of a close ally, P. (C.) robustus (Sm.).
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