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Subgenus Psithyrus

Bombus insularis Bumblebee - Bombus flavidus Which Bombus, please? - Bombus citrinus Bombus fervidus? - Bombus citrinus - male Lemon Cuckoo Bumble Bee - Bombus citrinus - male Unknown Bumblebee - Bombus citrinus  Lemon Cuckoo Bumble Bee - Bombus citrinus Bombus insularis - Indiscriminate Cuckoo Bumble Bee? - Bombus insularis - male
Classification
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Hymenoptera (Ants, Bees, Wasps and Sawflies)
No Taxon (Aculeata - Ants, Bees and Stinging Wasps)
No Taxon (Apoidea (clade Anthophila) - Bees)
Family Apidae (Cuckoo, Carpenter, Digger, Bumble, and Honey Bees)
Subfamily Apinae (Honey, Bumble, Longhorn, Orchid, and Digger Bees)
Tribe Bombini (Bumble Bees)
Genus Bombus (Bumble Bees)
No Taxon Subgenus Psithyrus
Synonyms and other taxonomic changes
used to be treated as a separate genus (discussion + list and distribution of world spp.)(1)
Explanation of Names
Psithyrus Lepeletier 1833
Numbers
6 spp. in our area, ~30 spp. total(2)
Size
Males much smaller than females
Identification
Females differ from those of all other Bombus in having the outer surface of the hind tibia convex and densely hairy (flat or concave and largely bare in other, pollen-collecting Bombus). Males have small heads and often have conspicuous bare patches on the apical terga medially.
Range
holarctic, most diverse in Eurasia; in our area, widespread: 4 spp. are transcontinental in the north and range south along the Rockies (except B. ashtoni), B. citrinus ne. US + adjacent Canada, B. variabilis QC & e. half of the US to AZ(2)
Food
Social parasites of other bumble bees
Life Cycle
There is no worker caste, the females invade Bombus nests and lay eggs there to be reared by the hosts(1)
Internet References
Pedersen B.V. (1996) A phylogenetic analysis of cuckoo bumblebees (Psithyrus, Lepeletier) and bumblebees (Bombus, Latreille) inferred from sequences of the mitochondrial gene cytochrome oxidase I. Mol. Phylog. Evol. 5: 289‒297 (Full text)