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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.
D escription. Queen. Length, 24.0 mm; width at wing bases, 12.0 mm; abdomen, 13.0 mm, width across T2, 13.0 mm; front wing length, 19.0 mm, width, 7.0 mm. Head: Frontal outline (excluding mouthparts) roundly trapezoidal, the median height little more than the widest dimension, the temples rather broadly rounded, the vertex nearly straight across; vertical region slightly concave to almost flat, for the most part densely covered with small irregular (follicular) punctures, only moderately inclined toward the occiput; ocular half of ocellocular area bearing some microscopic punctures, the ocellar half largely smooth; com pound eyes noticeably more broadly and evenly rounded below than above, their inner margins rather subparallel below and evenly incurved above; ocelli rather large, situated much closer together than their diameters in a weak arc far below the supraorbital line; interocellar line about H times longer than ocellocular line; malar space little less than distance between (and including) mandibular articulations, weakly convex transversely and with only a few microscopic punctures; labrum little wider than its greatest thickness, the ventral margin mostly arcuate, the shelf not strong and sharp, the tubercles rather coarsely punctate, flattened below, their mesal summits separated by a somewhat deep intertubercular depression that is about as wide as F2 is long; flagellum nearly If times longer than scape, FI nearly equal to the combined lengths of F2 and F3, the latter noticeably longer than the short rectangular F2; clypeus more strongly convex above than below but for the most part weakly convex, the middle part with only inconspicuous small punctures. Legs: Mesobasitarsite rectangular, nearly 5 times longer than its widest part which is about equal to the length of longest hairs of the posterior fringe, the outer surface shallowly concave longitudinally, the distoposterior angle somewhat sharper than the distoanterior one, both about equally extended with a shallow emargination between; outer hind tibial surface mostly granulose with a weak longitudinal convexity just anteriad of middle, the widest part about equal to the length of longest hairs of corbicular fringe; metabasitarsite subrectangular with its posterior margin weakly arcuate at the basal third beyond to distal end, its outer surface broadly concave longitudinally, the widest part at least 2i times wider than the length of longest hairs of the posterior fringe, the distal angles both rounded and equally extended with a shallow emargination between. Pubescence: Fine, dense, and rather short except somewhat longer and looser posteriorly on scutellum, on most of the distal half of abdomen, and on the abdominal venter. Color: Head, thorax (except for an intermixture of cinereous pile across the anterior part of mesoscutum), and legs black; abdominal T1 and most of T2 black, the distal margin (mostly) of T2 and all of T3-6 white, the proximal half of the abdominal venter covered predominantly with black or brownish black, the distal half mostly with palish pile. Wings black or brownish black with some violaceous reflection under certain lights.
Worker. Length, 17.0 mm; width at wing bases, 9.0 mm; abdomen, 9.0 mm, width across T2, 9.0 mm; front wing length, 17.0 mm, width, 6.0 mm. Morphologically this caste is very similar to the queen but the diagnostic features are usually proportionately less. Many, some of which being much smaller, have the intermixed cinereous pile involving as much as all of the thoracic dorsum, and the wings are frequently noticeably paler than those of the queen.
Male. Length, 18.0 mm; width at wing bases, 8.0 mm; abdomen, 9.0 mm, width across T2, 8.5 mm; front wing length, 15.0 mm, width, 5.0 mm. Head: Frontal outline (excluding mouthparts) roundly trapezoidal, the narrow temples rounded, weakly arcuate across the vertex, slightly wider than the median height; vertical region for the most part covered with small (follicular) punctures and only slightly inclined toward the occiput; ocelli large, positioned in a weak arc far below the supraorbital line and removed from one another by much less than their diameters; ocellocular area much narrower than the diameter of a lateral ocellus, smooth and slightly ridged adjacent to the compound eye; interocellar line slightly more than twice as long as the ocellocular line; compound eyes more broadly rounded above than below, moderately swollen, their inner margins nearly straight and convergent above; malar space little more than £ as long as the distance between (and including) mandibular articulations, weakly convex and mostly smooth; labrum nearly 2£ times wider than thick, nearly flat with very weak callosites, the ventral margin nearly straight except arcuate at the corners; flagellum about 3£ times longer than scape, FI subequal to F3 which is nearly 1£ times longer than rectangular F2. Legs: Mesobasitarsite subrectangular, nearly 4 times longer than its widest part which is slightly less than £ the length of longest hairs of the posterior fringe, the outer surface nearly flat, the distoposterior angle rounded and considerably in recess of the stronger distoanterior angle; hind tibial outer surface minutely granulose, longi tudinally convex near the center where it is sparsely pubescent; metabasitarsite nearly rec tangular, its posterior margin only weakly curved, the outer surface flat except longitudinally concave posteriorly, nearly 4 times longer than its widest part which is little more than £ the length of longest hairs of the posterior fringe, the sharper distoposterior angle and blunt distoanterior one about equally extended with a shallow emargination between. Pubescence: Rather short, fine, and dense except somewhat longer and looser on the vertex, most of the scutellum, and distal half of the abdomen; metabasitarsal posterior fringe composed for the most part of rather long straight hairs, moderately dense; except proximally and distally the rather loose posterior fringe consists of long straight or weakly bent hairs. Genitalia and seventh and eighth abdominal sterna (PI. XVI). Color: Head tawny yellow except for some intermixed black hairs on the vertical region and temples (sometimes these areas are pre dominantly or almost entirely black); thorax tawny except for some intermixed black hairs on dorsum between wing bases (this intermixture is sometimes much more extensive anteriad and posteriad and might even occur below the wing bases); legs mostly black with considerable tawny yellow on coxae, trochanters, and femora; abdominal Tl-2 tawny yellow though sometimes only the very base of T2 is so colored, the remainder of terga black and virtually concolorous with the abdominal venter, which is mostly brownish black to black. Wings uniformly infumated with light brown, much paler than wings of the females.
Comments. This dichromatic species is a large and striking one especially in the queen. Structurally there seems to be little variation. Most of the chromatic variability that occurs in the worker has been indicated in the redescription; some times the cinereous pile on the thoracic dorsum might occur in the queen un commonly and usually less extensively. The male is quite variable with respect to shade of yellow, amount of black on the thoracic dorsum, and the amount of yellow on the abdomen.
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