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Extracted from: 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, 10.0 mm, width across T2, 10.0 mm; front wing length, 15.5 mm, width, 6.0 mm. Head: Frontal outline (excluding mouthparts) roundly trapezoidal, the median height a little less than widest dimension, temples rather broadly rounded, the vertex transversely weakly arcuate; vertical region weakly concave to nearly flat, moderately inclined toward the vertex, well cover ed with small (mostly follicular) punctures; ocular half of ocellocular area with some small punctures, the ocellar half largely smooth; compound eyes only slightly more broadly rounded below than above, their inner margins nearly straight and subparallel; ocelli of medium size, located slightly closer together than their diameters in a weak arc just below the supraorbital line; interocellar and ocellocular lines almost equal; malar space about equal to distance between (and including) mandibular articulations, very weakly convex transversely and practically smooth; labrum little more (if any) than twice as wide as its median thickness, the ventral margin mostly arcuate, the shelf rather short and not very prominent, the normally subconical tubercles (somewhat variable) usually ventrally flattened, irregularly but not strongly punctate, their mesal summits (sometimes quite pointed) separated by an intertuber- cular depression that is about as wide as the length of F2; flagellum slightly more than twice as long as scape, FI about equal to the combined lengths of F2 and F3, the latter only a little longer than quadrate F2; clypeus mesially moderately convex (rather triangularly so) mostly covered with small to medium punctures, sloped laterally and dorsally. Legs: Mesobasitarsite rather rectangular, little more than 3 times longer than the widest dimension which is nearly equal to the length of longest hairs of the posterior fringe, the outer surface nearly flat, the distal end subtruncate; outer hind tibial surface microscopically granulose, nearly flat with at most a very weak convexity extending along the approximate distal half of segment anteriad of the middle, widest part nearly equal to the length of longest of hairs of corbicular fringe; metabasitarsite subrectangular with outer surface being broadly and shallowly concave longi tudinally, the widest part of segment nearly 3 times as wide as the length of longest hairs of the posterior fringe, the distal angles both blunt and about equally extended with a shallow emargination between, the posterior margin rather evenly but not strongly arcuate from base to near distal end. Pubescence: Very fine, rather dense, of moderate length except noticeably shorter on most of the thorax; mesobasitarsal posterior fringe not dense, mostly short, consisting of straight (or nearly so) hairs that are longest along the proximal half of margin; corbicular fringe moderately dense, composed for the most part of long straight (or nearly so) hairs except those at distal end of margin are arcuate; metabasitarsal posterior fringe moderately dense, mostly composed of short hairs that are longest and arcuate along the proximal half of margin, beyond becoming straight and gradually very short at the distal end. Color: Black except typically with a circular or transversely oval patch on the thoracic disc between wing bases, the apical margin (more or less, cf. under Comments) of abdominal T4 and all of T5 white; the sparse short pile of T6 is usually dark but might be pale or whitish; legs black and most, if not all, of the abdominal venter also. Wings uniformly moderately (not deeply) stained with brownish black.
Worker. Length, 14.0 mm; width at wing bases,v>7.0 mm; abdomen, 7.0 mm, width across T2, 7.0 mm; front wing length, 13.0 mm, width, 4.5 mm. This caste does not signi ficantly deviate from the queen in the various comparative morphological features; there is some variation of the punctation especially that on the clypeus, and of the labral tubercles which are usually less prominently produced. The color pattern is essentially like that of the queen.
Male. Length, 16.0 mm; width at wing bases, 7.0 mm; abdomen, 9.0 mm, width across T2, 7.0 mm; front wing length, 13.0 mm, width, 4.5 mm. Head: Frontal outline (excluding mouthparts) very roundly trapezoidal, its widest dimension slightly greater than the median height, the temples rather abruptly rounded, the vertex transversely straight; vertical region flat, very little inclined toward the vertex, mostly covered with small (follicular) punctures; ocelli rather large, situated much closer together than their diameters in an almost straight line considerably below the supraorbital line; ocular half of ocellocular area mostly covered with small to medium (some follicular) punctures; the ocellar half smooth; interocellar line about H times longer than the ocellocular line; compound eyes bulging, about equally rounded above and below, their inner margins nearly straight and virtually parallel; malar space only slightly longer than distance between (and including) mandibular articulations, its surface unevenly flattened and with noticeable microscopic punctures; labrum little more than twice as wide as its median thickness, the ventral margin almost straight but laterally arcuate, the callosities weak and each with few medium punctures, not sharply divided mesially; clypeus rather evenly convex, well covered with small (follicular) punctures, little wider at base than its median height; flagellum little more than 3 times longer than scape, FI equal to F3 which is about li times longer than nearly quadrate F2. Legs: Mesobasitarsite elongate-sub- rectangular, more than 4 times longer than its widest dimension which is approximately i the length of longest hairs of the posterior fringe, the outer surface nearly flat, the sharper distoanterior angle more in advance of the distoposterior one, without much emargination between; hind tibial outer surface shiny, with fine hairs, the widest part of segment equal to little more than \ the length of longest hairs of the posterior fringe; metabasitarsite almost rectangular, about 3 times longer than its widest part which is nearly f the length of longest hairs of the posterior fringe, the posterior margin only weakly arcuate throughout, the outer surface nearly flat, the distal angles about equally produced with a distinct emargination between, distoposterior angle being the more sharply rounded. Pubescence: Fine, rather dense, and even except somewhat longer and looser on most of head, scutellum, and abdomen; mesobasitarsal posterior fringe not dense, composed mostly of long straight (or nearly so) hairs except those along the basal portion of margin are somewhat shorter and those along the distal third are much shorter and sparser; hind tibial posterior fringe long, moderately dense, composed of mostly straight hairs except some of those distally are arcuate near their tips; metabasitarsal posterior fringe similar to that on the mesobasitarsite but somewhat less dense and more extensively long. Genitalia and seventh and eighth abdominal sterna (PI. XIX). Color: Head black; thorax black except for a white circular or transverse oval patch on the disc between the wing bases (c/. under Comments); legs usually entirely black but sometimes some of the hairs on the coxae, trochanters, femora, and tibiae are pale-tipped; abdomen usually black except T4 distally more or less and T5-7 white, but often some of the pile on T1 and the venter will be whitish or pale-tipped (cf. under Comments). Wings uni formly stained with light brown.
Comments. No noteworthy morphological variations have been detected in this taxon except that given in the short redescription of the worker caste. How ever, chromatically the subspecies is very variable, and this is common to all the castes. Sometimes totally black individuals occur in both females and males. I have seen males having their entire bodies clothed with pale-tipped (whitish) black pile but I have never encountered such a condition in the females. Often the white patch on the thoracic disc is small but does vary in size to sometimes being exten sive enough as to include virtually all of the thoracic dorsum. Also the amount of white on the female abdominal T4 might occur only at the distal end of the tergum on individuals that are otherwise typical. Similarly the amount of white on the male abdominal T4 and T5 is variable.
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