Key to Females

For definitions of morphology characters, go to female morphology page of this site.

1. Female: antenna with 12 segments; abdomen with six visible tergites 2
1’. Male: antenna with 13 segments; abdomen with seven visible tergites No key provided. (To key males to species, use Williams et al. 2014.)
 
2(1). Outer surface of hind tibiae concave and shiny; pollen baskets present; social species 3
2’. Outer surface of hind tibiae convex and hairy; pollen baskets absent; social parasitics 35
 
3(2). T2 and T3 with red or orange hairs, sometimes with yellow hairs intermixed at least in the middle 4
3’. T2 and/or T3 with yellow or black hairs 10
 
4(3). Scutum in front of the wing bases with yellow and black hairs intermixed, giving cloudy appearance 5
4’. Scutum in front of the wing bases with predominantly yellow or pale yellow hairs 6
 
5(4). T2 with black hairs at least in the middle; exoskeleton of hind leg basitarsus brownish-orange and often lighter than the tibia; corbicular fringes orange or brown Bombus bifarius Cresson
5’. T2 orange; corbicular fringes black, sometime with orange Bombus melanopygus Nylander
 
6(4’). Cheek (i.e. malar space) distinctly shorter than wide; face with predominantly black hairs Bombus rufocinctus Cresson
6’. Cheek as long as wide; face with yellow hairs, at least centrally 7
 
7(6’). Scutellum with yellow hairs only Bombus huntii Greene
7’. Scutellum with yellow or pale yellow hairs divided by a line or triangle of black hairs 8
 
8(7’). T2 with black hairs at least in the middle; exoskeleton of hind leg basitarsus brownish-orange and often lighter than the tibia Bombus bifarius Cresson
8’. T2 orange 9
 
9(8’). T5 with yellow hairs along the edges; hairs long and uneven; coxae with predominately black hairs Bombus sylvicola Kirby
9’. T5 black; hairs short and even; coxae often with yellow hairs Bombus ternarius Say
 
10(3’). Cheek longer than wide (though sometimes only slightly longer than wide and appearing square) 11
10’. Cheek as long as or shorter than wide 25
 
11(10). Face with pale white or yellow hairs, sometimes with black hairs intermixed, giving cloudy appearance 12
11’. Face with predominantly black hairs 16
 
12(11). T3 and T4 black Bombus flavifrons Cresson
12’. T3 and T4 orange or yellow or brownish 13
 
13(12’). T3 and T4 orange 14
13’. T3 and T4 yellow or brownish 15
 
14(13). Scutum in front of the wing bases with black and yellow hairs intermixed, giving cloudy appearance Bombus flavifrons Cresson
14’. Scutum in front of the wing bases with predominantly yellow hairs Bombus centralis Cresson
 
15(13’). T5 yellow or brownish Bombus appositus Cresson
15’. T5 usually mostly black Bombus borealis Kirby
 
16(11’). T5 and T6 with orange or pale orange hairs Bombus kirbiellus Curtis
16’. T5 and T6 with black hairs 17
 
17(16’). T3 with mostly black hairs 18
17’ T3 with yellow hairs at least in the middle 21
 
18(17). T4 black 19
18’. T4 yellow Bombus californicus Smith
 
19(18). T2 with yellow hairs medially and black hairs along the sides (yellow often in the shape of a “W”) Bombus bimaculatus Cresson
19’. T2 yellow 20
 
20(19’). Sides of the thorax with yellow hairs predominating Bombus vagans Smith
20’. Sides of the thorax usually with black hairs predominating Bombus perplexus Cresson (possibly in MT)
 
21(17’). Hairs between the wing bases completely or predominantly black 22
21’. Hairs between the wing bases predominantly yellow; if black hairs are present they form a small circle medially Bombus nevadensis Cresson
 
22(21). T4 black 23
22’. T4 yellow 24
 
23(22). T1 usually with yellow hairs; surface of clypeus with many pits and punctures Bombus pensylvanicus (DeGeer)
23’. T1 with black hairs, at least in the middle, sometimes with yellow hairs intermixed; surface of clypeus smooth and shiny Bombus auricomus (Robertson)
 
24(22’). T2 yellow Bombus fervidus (Fabricius)
24’. T2 with yellow hairs medially and black hairs laterally Bombus californicus Smith
 
25(10’). Cheek shorter than wide 26
25’. Cheek as long as or slightly longer than wide 31
 
26(25). Hairs between the wing bases completely or predominantly black 27
26’. Hairs between the wing bases predominantly yellow; if black hairs are present they form a small circle in the middle of the thorax 30
 
27(26). T1 yellow Bombus rufocinctus Cresson
27’. T1 black 28
 
28(27’). T2 yellow 29
28’. T2 with black hairs or, if some yellow, along back margin only Bombus occidentalis Greene
 
29(28). T3 black Bombus cryptarum (Fabricius) (possibly in MT)
29’. T3 yellow Bombus terricola Kirby
 
30(26’). T3 yellow, at least in the middle. Bombus morrisoni Cresson (possibly in MT)
30’. T3 black Bombus griseocollis (DeGeer)
 
31(25’). Scutellum with yellow or pale yellow hairs divided by a line or triangle of black hairs Bombus bifarius Cresson
31’. Scutellum not as above (with yellow hairs only, or yellow and black hairs intermixed throughout) 32
 
32(31’). T2-6 black Bombus impatiens Cresson (non-native, introduced in MT)
32’. T2-6 with some yellow and/or orange 33
 
33(32’). Scutum in front of the wing bases with black and yellow hairs intermixed, giving cloudy appearance 34
33’. Scutum in front of the wing bases with yellow hairs only Bombus frigidus Smith
 
34(33). Scutellum distinctly darker (with many black hairs intermixed with yellow) than anterior scutum Bombus sitkensis Nylander
34’. Scutellum and scutum in front of the wing bases not as above (either with scutellum and scutum in front of the wing bases similar in appearance, or with the scutum in front of the wing bases appearing darker than scutellum) Bombus mixtus Cresson
 
35(2’). Occiput with predominantly black hairs; hairs of face predominantly black around bases of antennae; S6 with strongly produced lateral ridges 37
35’. Occiput with predominantly yellow hairs 36
 
36(35’). Hairs of face predominantly black around bases of antennae; ridges along the edge of S6 not apparent (there may be small bumps); T6 often tightly curled under the abdomen Bombus flavidus Eversmann
36’. Hairs of face predominantly yellow around bases of antennae; S6 with evenly rounded side ridges Bombus insularis (Smith)
 
37(35). Hairs on the side of the thorax predominantly yellow; T4 yellow but with a triangle of black hair medially; large and obvious ridges along the edge of S6, extending beyond T6 Bombus suckleyi Greene
37’. Hairs on the side of the thorax predominantly black, T4 yellowish-white to white, at least medially; large and obvious ridges along the edge of S6, but these ridges barely extending laterally beyond T6 Bombus bohemicus Seidl (possibly in MT)


Definitions of morphology characters can be found on the female morphology page of this site.