Male.
Female is more impressive and scary. It resembles a host species - Bombus lapidarius.
A clear feature to distinguish a cuckoo bumblebee is a somewhat bald abdomen in both genders (shining black cuticle is clearly visible through the fur).
Also females don't have pollen baskets. And their sting is much more toxic than in true bumblebees, been reported to feel almost like hornet's (better not to check by yourself!)
I didn't knew the fact about the venom - thanks for that!
About the baldness I think that is not a reliable feature as often old specimens of "true" Bombus have bald zones in thorax or abdomen due to age.
Overall impression shows that cuckoo bumblebees are more sparsely-haired.
This can be explained bay the fact that cuckoos aren't exposed to low temperatures of early spring as true bumblebees are - and they wake up just when the host species already have reliable colonies.
On the hairless true bumblebees...
I collected a mating pair of B. maculidorsis in August 2012. The female was certainly young queen born late summer, but the fur on upper abdomen looked quite worn-out, as well as the wings. Maybe this was the fault of males?
Old workers can also be bald, though it's quite easy to tell them from cuckoos by size and pollen baskets.
