linguistic:paraphrasing, structural:summarization
Cuckoo bumblebees

Bumblebees of the subgenus Psithyrus (known as 'cuckoo bumblebees', and formerly considered a separate genus) are brood parasites, sometimes called kleptoparasites, in the colonies of other bumblebees, and have lost the ability to collect pollen. Before finding and invading a host colony, a Psithyrus female, such as that of the Psithyrus species of B. sylvestris, feeds directly from flowers. Once she has infiltrated a host colony, the Psithyrus female kills or subdues the queen of that colony, and uses pheromones and physical attacks to force the workers of that colony to feed her and her young. Usually, cuckoo bumblebees can be described as queen-intolerant inquilines, since the host queen is often killed to enable the parasite to produce more offspring, though some species, such as B. bohemicus, actually enjoy increased success when they leave the host queen alive.

The female Psithyrus has a number of morphological adaptations for combat, such as larger mandibles, a tough cuticle and a larger venom sac that increase her chances of taking over a nest. Upon emerging from their cocoons, the Psithyrus males and females disperse and mate. The males do not survive the winter but, like nonparasitic bumblebee queens, Psithyrus females find suitable locations to spend the winter and enter diapause after mating. They usually emerge from hibernation later than their host species. Each species of cuckoo bumblebee has a specific host species, which it may physically resemble. In the case of the parasitism of B. terrestris by B. (Psithyrus) vestalis, genetic analysis of individuals captured in the wild showed that about 42% of the host species' nests at a single location had " their fight against their parasite".