Four-coloured Cuckoo Bee Bombus sylvestris
This is a more common cuckoo bee, seen regularly all over Europe, including the British Isles. It is not a big species and a parasite to the Early Bumblebee mainly, to which it bears a good resemblance. But it is known using other hosts as well, especially the Common Carder Bee. In the pictures on this page you see a drone. Both females and males reach a length of approximately 15 mm. Strangely enough the females have a bigger wingspan: males reach 30 mm., females 35 mm. There's evidence females find the nest of their host by scent. Once inside their host's nest they often sit still for hours on end, picking up the scent of the nest before depositing the eggs. Once accepted by the host she'll leave the nest to feed, returning frequently to deposit even more eggs.
This species is also known by its former scientific names Psithyrus sylvestris and Fernaldaepsithyrus sylvestris.