Psithyrus in the context of Cuckoo bees

A bumblebee (or bumble bee, bumble-bee, or humble-bee) is any of over 250 species in the genus Bombus, part of Apidae, one of the bee families. This genus is the only extant group in the tribe Bombini, though a few extinct related genera (e.g., Calyptapis) are known from fossils. They are found primarily in the Northern Hemisphere, although they are also found in South America, where a few lowland tropical species have been identified. European bumblebees have also been introduced to New Zealand and Tasmania. Female bumblebees can sting repeatedly, but generally ignore humans and other animals.

Most bumblebees are eusocial insects that form colonies with a single queen. The colonies are smaller than those of honey bees, growing to as few as 50 individuals in a nest. Cuckoo bumblebees are brood parasitic and do not make nests or form colonies; their queens aggressively invade the nests of other bumblebee species, kill the resident queens and then lay their own eggs, which are cared for by the resident workers. Cuckoo bumblebees were previously classified as a separate genus, but are now usually treated as members of Bombus.

The Bombini are a tribe of large bristly apid bees which feed on pollen or nectar. Many species are social, forming nests of up to a few hundred individuals; other species, formerly classified as Psithyrus cuckoo bees, are brood parasites of nest-making species. The tribe contains a single living genus, Bombus, the bumblebees, and some extinct genera such as Calyptapis and Oligobombus. The tribe was described by Pierre André Latreille in 1802.