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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, 22.0 mm; width at wing bases, 11.0 mm; abdomen, 11.0 mm, width across T2, 12.0 mm; front wing length, 19.0 mm, width, 7.0 mm. Head: Frontal outline (excluding mouthparts) trapezoidal, the median height slightly greater than the widest dimension, temples broadly rounded, the vertex transversely only weakly arcuate; vertical region nearly flat and moderately sloped toward the occiput; ocular half of ocellocular area rather densely covered with minute punctures, the ocellar half largely smooth; compound eyes little more broadly and evenly rounded below than above, their inner margins subparallel; ocelli small, separated from one another by little less than their diameters in a weak arc just below the supraorbital line; ocellocular and interocellar lines about equal; malar space little longer than distance between (and including) mandibular articulations, mostly impunctate, not strongly or evenly convex; labrum about twice as wide as its median thickness, the ventral margin mostly arcuate, the tubercles not prominent, pointed, or conical but rather irregularly and coarsely punctate above, more or less flat beneath, their mesal summits not separated by a sharp deep intertubercular depression; flagellum nearly If times longer than scape, FI subequal to combined lengths of F2-3, the latter about If times longer than quadrate F2; clypeus evenly but not strongly convex, well covered with mostly small punctures, the widest part near base equal to the median height. Legs: Mesobasitarsite with outer surface flat to broadly and weakly concave, the posterior margin arcuate along the basal half then straight beyond, the distal end nearly truncate, the widest part of segment about If times wider than the length of longest hairs of the posterior fringe; outer surface of hind tibia entirely finely granulose, longitudinally weakly convex anteriad, longitudinally broadly weakly concave posteriad, the widest part of segment about equal to the length of longest hairs of the posterior fringe; metabasitarsite subrectangular, its outer surface broadly and shallowly concave longitudinally, the widest part nearly twice as wide as the length of longest hairs of the posterior fringe, the posterior margin for the most part weakly arcuate, the moderately acute distoposterior angle extended notice ably beyond the blunt distoanterior angle with only a shallow emargination between. Pubescence: Very fine, dense, and rather even except somewhat longer on the scutellum and dorsally on most of the abdomen especially on T4-5; mesobasitarsal posterior fringe moderately short, dense, composed mainly of straight hairs; corbicular fringe mostly rather long and dense, composed of straight to weakly arcuate hairs of which the distal ones are more strongly arcuate and progressively shorter; mesabasitarsal posterior fringe not dense, composed of rather short, mostly straight hairs along basal half of margin, beyond becoming progressively shorter and sparser. Color: Head, thorax, legs, and abdominal T1 (except for some rufescent pile distolaterally) black; abdominal T2-3 bright ferruginous (or copper red), T4 mostly paler ferruginous with some of the distal pile being even paler with tinges of yellowish or whitish, T5 mostly whitish or with tinges of yellowish, T6 largely dark, the abdominal venter mostly dark except some pile on S4-6 pallid (yellowish with rufescent tinge). Wings lightly and uniformly stained with brown.
Worker,, Length, 18.5 mm; width at wing bases, 8.5 mm; abdomen, 9.5 mm, width across T2, 9.0 mm; front wing length, 13.0 mm, width, 4.5 mm. Except for overall size difference, the morphological features of this caste are very similar to those of the queen, and the color pattern is the same but abdominal T2-4 usually have the intensity of vivid ferruginous (or copper red) equal, the pile on T5 is also ferruginous with only a slight tendency to pallidness, and the abdominal venter is more extensively darker but not com pletely so especially distally.
Male. Length, 17.0 mm; width at wing bases, 7.0 mm; abdomen, 9.0 mm, width across T2, 7.5 mm; front wing length, 18.0 mm, width, 4.5 mm. Head: Frontal outline (excluding mouthparts) somewhat roundly trapezoidal, the widest part equal to the median height, the rather narrow temples arcuately rounded, the vertex transversely almost straight; vertical region nearly fiat, for the most part well covered with small (follicular) punctures, moderately inclined toward the occiput; ocelli small, situated considerably closer together than their diameters in a weak arc much below the supraorbital line; ocular half of ocellocular area with few medium and small punctures, the ocellar half mostly smooth; interocellar line a little longer than ocellocular line; compound eyes slightly bulging, somewhat more broadly and evenly rounded below than above, their inner margins almost straight and nearly parallel; malar space a little longer than distance between (and including) mandibular articulations, very weakly convex, smooth; labrum about twice as wide as its median thickness, mostly flat, the ventral margin straight except rounded at corners, the callosities very weak and incon spicuous; flagellum nearly 3 times longer than scape, FI only slightly longer than rectangular F3 which is about li times longer than quadrate F2. Legs: Mesobaritarsite rather narrow elongate-subrectangular, its outer surface nearly flat, the widest part about equal to f the length of longest hairs of the posterior fringe, the blunt distoanterior angle extended some what beyond the rounded distoposterior angle without much of an emargination between; outer surface of hind tibia very weakly and microscopically granulose, weakly convex longi tudinally anteriad to about the distal fourth, the widest part nearly equal to the length of longest hairs of the posterior fringe; metabasitarsite nearly rectangular, its outer surface broadly and shallowly concave longitudinally, at the middle about as wide as f length of longest hairs of the posterior fringe, the distal end of segment subtruncate. Pubescence: In character similar to that of queen or worker. Genitalia and seventh and eighth abdominal sterna (PI. XX). Color: Head, thorax, legs, abdominal Tl, T2 along its base and extending posteriad medially, abdominal sterna (mostly) black; remainder of abdominal T2 and T3-5 vivid ferruginous (or copper red), T6-7 mostly whitish with some pile basolaterally on T6 ferruginous tinged with paler tips. Wings only slightly paler than those of the queen and worker.
Comments. There does not seem to be any appreciable structural varia bility in this taxon. The smallest workers I have seen attain only about one-half the size of the dimensions given in the redescription of that caste. The abdominal color pattern is subject to some degree of variation especially in the worker; Tl-2 might be predominantly to entirely black, and on the other hand T2 might be predominantly to completely vivid ferruginous; also, there might be a difference in the amount or extent of pale (whitish or yellowish) pile on T4-5. This species is closely related to P. (C.) baeri (Fr.) structurally and in similarity of color pattern.
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