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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, 20.0 mm; width at wing bases, 9.0 mm; abdomen, 11.0 mm, width across T2, 10.0 mm; front wing length, 18.0 mm, width, 6.0 mm. Head: Frontal outline (excluding mouthparts) roundly trapezoidal, little wider than its median height, temples rather sharply rounded, the vertex transversely nearly straight; vertical region mostly flat, only slightly inclined toward the occiput, covered with small (follicular) punctures obliquely of each lateral ocellus; ocular half of ocellocular area with some small irregular punctures and very few just inside the ocellar half; compound eyes noticeably more broadly and evenly rounded below than above, their inner margins for the most part subparallel, evenly incurved above; ocelli of medium size, separated from one another by little more (if any) than i their diameters* positioned in a weak arc distinctly below the supraorbital line; interocellar and ocellocular lines about equal; malar space little longer than distance between (and including) mandibular articulations, the outer surface nearly flat and with some microscopic punctures below; labrum nearly twice as wide as its median thickness, the ventral margin mostly broadly arcuate, the shelf not sharp or very conspicuous, rather short, the tubercles only weakly conical, flattened, and irregularly sculptured (punctate) below and more coarsely so above, their mesal summits separated by a rather shallow intertubercular depression; flagellum little more than If times longer than scape, FI about equal to the combined lengths of F2 and F3, the latter approxi mately li times longer than quadrate F2; clypeus little wider near base than median height, for the most part the median area rather evenly but not strongly convex and covered with well-separated small irregular punctures, laterally slanted. Legs: Mesobasitarsite rectangular, 4 times longerJhan widest dimension which is about f- as wide as the length of longest hairs of the posterior fringe, the rounded distoposterior angle slightly in recess of the blunt distoanterior angle, without much emargination between; outer surface of hind tibia (corbicula) micro scopically granulose and with only a very weak convexity mid-longitudinally, the widest part of segment nearly equal to the length of longest hairs of the posterior fringe; metabasitarsite subrectangular, its outer surface longitudinally shallowly concave, posterior margin evenly but not strongly arcuate from base to distal end, the widest part of segment nearly 3 times wider than the length of longest hairs of the posterior fringe, both distal angles blunt and about equally extended with only a very shallow emargination between. Pubescence: That on head medium in length, mostly rather loose and uneven; that on thorax short, dense, and even except longer and looser on most of the scutellum; that on most of the abdomen of medium length, dense and even except on most of T4-6 where it is looser; mesobasitarsal interior fringe not dense, rather scattered, composed of straight or nearly straight hairs that are long along the proximal half of margin, beyond gradually becoming short and sparser distally; corbicular fringe rather dense, long except at very base of segment, consisting of straight hairs except those along the approximate distal one-third of margin are arcuate; metabasitarsal posterior fringe moderately dense, short, composed mostly of weakly arcuate hairs, longest along the approximate proximal half of margin, beyond gradually becoming very short distally. Color: Head, thoracic pleura, venter, and a moderately wide interalar band black, remainder of thoracic dorsum (anteriad of tegulae) and the scutellum golden-yellow; legs black; abdominal T1 golden-yellow but often tinged with pale orange, T2 often with more or less the proximal half light orange blending to golden-yellow across the distal margin but sometimes entirely light orange, T3 usually with the proximal half (more or less) light orange to golden-yellow followed by some black then by some whitish pile along the distal margin but sometimes entirely light orange to golden-yellow, T4 usually with the proximal third (or more) black followed by white or whitish but sometimes this tergum is almost totally black except for traces of whitish along the distal margin, T5-6 white or whitish, the abdominal venter black though sometimes a few whitish hairs might be present distally on the penultimate sternum. Wings stained with brownish black, usually being somewhat paler at places in the discal area and having some violaceous reflection under certain lights.
Worker. Length, 13.0 mm; width at wing bases, 6.5 mm; abdomen, 6.0 mm, width across T2, 7.5 mm; front wing length, 13.0 mm, width, 4.5 mm. Morphologically essentially like the queen and the relative features given are proportionately the same but more diminutive. The color pattern and wing color are also like those of the queen, including the same degree of color variability on the abdomen. For reference to range in size of this caste, see Comments.
Male. Length, 15.0 mm; width at wing bases, 7.5 mm; abdomen, 7.0 mm, width across T2, 6.5 mm; front wing length, 14.0 mm, width, 4.0 mm. Head: Frontal outline (excluding mouthparts) roundly trapezoidal, noticeably wider than the median height, the very narrow temples abruptly rounded, the vertex transversely straight; vertical region flattened, mostly covered with small (follicular) punctures, little if any inclined toward occiput; ocelli of medium size, located from one another by distinctly less than k their diameters, in a weak arc much below the supraorbital line; ocellocular area largely smooth but traversed by a ridge adjacent to eye margin; interocellar line slightly more than twice as long as ocellocular line; compound eyes bulging, more broadly rounded above than below, their inner margins straight and almost parallel; malar space not quite equal to distance between (and including) man dibular articulations, its outer surface at most only very weakly convex transversely, with some minute punctures posteriorly; labrum about 2k times wider than its median thickness, the ventral margin straight except arcuate at the corners, the surface mostly flat with very weak callosities, each having a few small inconspicuous punctures; flagellum 3 times longer than
scape, FI equal to F3 which is nearly lk times longer than rectangular F2. Legs: Mesobasi tarsite narrowly subrectangular, little less than 5k times longer than widest part which is about i the length of longest hairs of the posterior fringe, the outer surface nearly flat, the disto- posterior angle arcuate and considerably in recess of the blunt distoanterior angle, without any emargination between; outer surface of hind tibia moderately convex longitudinally near the middle, microscopically granulose and loosely pubescent, the widest part of segment about equal to f the length of longest hairs of the posterior fringe; metabasitarsite subrectangular, its widest part about i the total length and at least equal to f the length of longest hairs of the posterior fringe, the outer surface broadly and shallowly concave longitudinally, the posterior margin straight to about distal third of segment which at that point becomes attenuated, the more or less rounded distal angles about equally extended with a shallow emargination between. Pubescence: That on head and thorax moderately short, fine, dense, and even except somewhat longer and looser on much of the scutellum; that on abdomen of medium length, fine, dense, and rather even except somewhat longer and looser on T4-7; mesobasitarsal posterior fringe moderately dense, consisting for the most part of long nearly straight hairs except those along the approximate distal one-third of margin are gradually shorter and arcuate near their tips; hind tibial posterior fringe moderately dense, composed largely of long nearly straight hairs except those at the proximal and distal ends of margin are shorter, most of those at the distal end being arcuate or bent near their tips; metabasitarsal posterior fringe not dense, consisting of long straight to weakly arcuate hairs, some along the very proximal portion of margin and those at the distal end are noticeably shorter, usually some of the latter being bent near their tips. Genitalia and seventh and eighth abdominal sterna (PI. XVII). Color: Head black with considerable intermixed whitish pile on the face, especially on the clypeus, and some on the vertex; thoracic pleura, venter, and a rather broad interalar band black, remainder of thoracic dorsum yellow; legs black; abdominal T1 usually entirely yellow, T2 and T3 pale drab orange frequently with an irregular intermixture of pale yellowish pile, T4 predominantly black usually with an intermixture of yellowish hairs laterally, T5 mostly with an intermixture of black and drab yellowish pile, T6 mostly whitish but often a few blackish hairs intermixed, T7 largely covered with whitish tinged with pale yellow. Wings colored very much like those of the females.
C o m m e n t s . This species is rather constant morphologically but there is some variability in the distinctness of the punctation on the clypeus. Workers vary considerably in length from 17 or 18 mm down to about 9 or 10 mm; larger workers usually are difficult to distinguish from small queens. As can be judged from the redescription, this taxon is chromatically quite variable especially on the abdominal dorsum; this involves the amount or extent of orange and yellow on Tl-3, and of yellow and/or black on T4. I have seen some females with abdominal T4 entirely black and others with this tergum entirely yellow. With respect to the forementioned terga, both females and males show similar color variability. The amount of white or whitish pile on abdominal T4-6 (females) and T5-7 (males) is also subject to variation, typically being quite prominent but sometimes it is greatly reduced and replaced by black or blackish pile.
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