Key to Alpinobombus species for males

1 Cheek> 2.0× as long (oculo-malar distance) as broad (breadth of the mandible at its base between and including condyles); penis-valve head on its outer edge proximal to the apex by a distance approximately equal to the breadth of the penis-valve head without a tooth, rarely with a very small point without an underlying convex bump that does not break the longer curve of the outer penis-valve head (maximum size in Fig. 144).......................................................... 2

- Cheek <2.0× as long as broad; penis-valve head on its outer edge proximal to the apex by a distance approximately equal to the breadth of the penis-valve head with a strong spinose tooth often placed on a convex bump on the edge of the penis-valve head (minimum size in Fig. 145)......................................................................... 4

2 (1) North America, hair of the face below antennal base with a large patch predominantly yellow (Figs. 112‒115); mandibular pos- terior fringe (‘beard’) unusually sparse, but the outer surface of the mandible with an extremely dense covering of short even branched orange hairs (which may appear dark if matted), forming a pronounced thick dense broad rounded pad extending from the mandibular articulations to just short of the mandibular teeth (Fig. 149)............................. B. kluanensis

- North America or Europe or Russia, hair of the face almost entirely black (Figs. 119‒123, 127‒128); mandibular posterior fringe long and dense, but the outer surface of the mandible with any short branched hairs mostly black or dark, longer, and not forming a pronounced pad (Fig. 148)...................................................................... 3

3 (2) Europe or Russia, hair of the pale bands usually dark chestnut brown (or pale yellow from Wrangel Island) (Figs. 127‒128, 137)..................................................................................... B. hyperboreus

- North America or Greenland, hair of the pale bands usually pale straw yellow (Figs. 1 19‒123, 136)............. B. natvigi

4 (1) North America, gonostylus inner (medial) edge between the anterior and posterior inner corners nearly straight, the proximal (anterior) corner not projecting (Fig. 134); hair of the band between the wing bases and on T4 usually predominantly black, at most with only a very few pale hairs (Figs. 101‒103)............................................... B. neoboreus

- North America or Europe or Asia, gonostylus inner edge between the anterior and posterior inner corners strongly concave or sinusoidal, the proximal corner projecting (Figs. 129‒133); hair of the band between the wing bases either black or with yellow intermixed and on T4 either black or with patches of either orange or yellow or white (Figs. 18‒21, 32‒38, 52‒58, 71‒76, 85‒88).............................................................................................. 5

5 (4) Cheek> 1.5× as long (oculo-malar distance) as broad (breadth of the mandible at its base between and including condyles); gonostylus inner (medial) edge strongly and consistently concave, the submarginal groove broad and sharply defined, the anterior and posterior inner corners strongly extended (Fig. 146); metasoma hair with yellow and black patterns (if present) sharply defined............................................................................................. 6

- Cheek <1.5× as long (oculo-malar distance) as broad (breadth of the mandible at its base between and including condyles); gonostylus inner edge either nearly straight, or weakly concave with a convexity near its midpoint, the submarginal groove indistinct, the anterior and posterior inner corners weakly extended (Fig. 147); metasoma hair with yellow and black patterns (if present) weakly defined and diffuse with widely scattered pale hairs.......................................... 7

6 (5) Europe or Russia; gonostylus inner (medial) edge deeply concave almost as a semi-circle (Figs. 71‒76, 132).... B. balteatus

- North America; gonostylus inner edge shallowly concave (Figs. 85‒88, 133)............................. B. kirbiellus

7 (5) Hind tibia outer surface with distinct microsculpture and matte, hair of the median anterior margin of T2 either usually in part black or entirely yellow (Figs. 32‒38, 52‒58); penis valve ventro-lateral process without a pronounced distal lateral shoulder; gonostylus posteriorly more than half as broad as anteriorly (Figs. 130‒131)...................................... 8

- Hind tibia outer surface without microsculpture and shining, hair of the median anterior margin of T2 almost entirely orange (Figs. 18‒21); penis valve ventro-lateral process with a pronounced distal lateral shoulder; gonostylus posteriorly less than half as broad as anteriorly (Fig. 129)................................................................... B. alpinus

8 (7) Europe or Russia (Figs. 32‒38, 130).......................................................... B. pyrrhopygus

- North America or Greenland (Figs. 52‒58, 131)...................................................... B. polaris