What is the fundamental difference in reproductive organization between Bombus and Psithyrus bees?

*Bombus* forms colonies with worker castes; *Psithyrus* is solitary.

The core distinction between the two groups lies in their social structure and methods of reproduction. Standard bumblebees (*Bombus*) are highly social insects that form colonies underground, where a queen initiates the nest and relies on sterile female worker castes to perform all necessary foraging for the brood. Conversely, cuckoo bumblebees, often classified under the subgenus *Psithyrus*, operate entirely differently; they are solitary and function as cleptoparasites. They do not establish nests, nor do they possess or maintain any worker caste whatsoever. The *Psithyrus* female lives alone, only interacting with another species' nest for the sole purpose of reproduction and resource hijacking.