''Bombus bifarius'', the two-form bumblebee, is a species of

eusocial Eusociality ( Greek 'good' and social) is the highest level of organization of sociality. It is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping generations wit ...

bumblebee of the subgenus ''Pyrobombus''. ''B. bifarius'' inhabits mountainous regions of western North America, primarily the states of

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

and

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

. Its common name refers to a historical artifact, in that it was believed that this species had a color polymorphism, with a red-tailed nominate form and a black-tailed ''nearcticus'' form, present in the species. This polymorphism has recently been shown to belong instead to a cryptic sister species, '' Bombus vancouverensis'', that occupies almost the entirety of the range of what was formerly classified as ''bifarius''; true ''bifarius'' only has a red form, so it is not "two-formed" at all.Ghisbain, G., J. D. Lozier, S. R. Rahman, B. D. Ezray, L. Tian et al. (2020) Substantial genetic divergence and lack of recent gene flow support cryptic speciation in a colour polymorphic bumble bee (''Bombus bifarius'') species complex. Syst. Entomol. 45: 635–652. doi:10.1111/syen.12419

Taxonomy and phylogeny

''Bombus bifarius'' was first described by

Ezra Townsend Cresson Ezra Townsend Cresson (18 June 1838, in Byberry19 April 1926, in Swarthmore) was an American entomologist who specialized in the Hymenoptera order of insects. He wrote ''Synopsis of the families and genera of the Hymenoptera of America, north of ...

in the 1878 ''Proceedings of the Academy of Natural Sciences of Philadelphia''. It is a member of the order

Hymenoptera Hymenoptera is a large order of insects, comprising the sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are parasitic. Females typi ...

and the family

Apidae Apidae is the largest family within the superfamily Apoidea, containing at least 5700 species of bees. The family includes some of the most commonly seen bees, including bumblebees and honey bees, but also includes stingless bees (also used for ...

, which also includes orchid bees, honey bees, and bumblebees. It was long believed that ''B. bifarius'' expressed varying color patterns, with the two most observed being the nominate and nearcticus color patterns. Red-tailed specimens classified as ''B. bifarius'' are found from Colorado all the way north to British Columbia, but have been found to comprise two different species (''bifarius'' and '' Bombus vancouverensis'') while the ''nearcticus'' color pattern, primarily found in western Canada, Alaska, California, Oregon, Idaho, Montana, and Washington, belongs to ''vancouverensis'' and not to ''bifarius''. Because of differences in genetic structuring between these populations in various geographic locations, there had been debate as early as 2013 suggesting these two major color pattern polymorphisms might represent more than one biological species.Lozier, J.D., Strange, J.P. & Koch, J.B. (2013) Landscape heterogeneity predicts gene flow in a widespread polymorphic bumble bee, ''Bombus bifarius'' (Hymenoptera: Apidae). Conservation Genetics, 14, 1099–1110. As of 2020, the genetic differentiation between ''bifarius'' and ''vancouverensis'' has been confirmed and characterized.

Description and identification

''Bombus bifarius'' has a relatively small body size ranging from for workers and for queens, with short, even hair covering their bodies. Hair on the faces of ''B. bifarius'' individuals is usually yellow or white in color and sometimes exhibits black coloration on the top of the head. In at least the lowermost third of the

thorax The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main di ...

, there is also black coloration. The hindlegs and

pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...

baskets are brownish-orange, and the hairs on metasomal tergites 2 and 3 are red.

Sexual dimorphism

Males are similar in size to female workers, ranging from . Their eyes are also similar in size and shape to those of their female counterparts. Colorations on their bodies are similar to those of workers and the queens.

Distribution

''Bombus bifarius'' is found in mountainous regions of western North America, limited to northern

Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

, northern

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

, and southern

Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...

; populations from all other areas formerly classified as ''bifarius'' belong instead to ''B. vancouverensis''.

References

{{Taxonbar, from=Q4676197 Bumblebees Hymenoptera of North America Insects described in 1878