I took this shot of a male Bombus pratorum nectar robbing a few days ago: the bee's short tongue is inserted into a hole previously pierced at the base of the flower, where it can easily reach the nectaries. The other flower on the right has also these "drinking holes", which once present, are used by short or long tongued bees alike. The first B. pratorum workers of the year, which are very small, do appear to be able to feed through the corolla opening, and often visit intact comfrey flowers. A possibility is that the small workers are actually just collecting pollen - not nectar - using a "buzzing" technique. This consists on holding onto the flower and then vibrating its wings - and making a characteristic high pitch sound. At certain vibratory frequencies, the pollen becomes dislodged from anthers and streams out of the flower, where is gathered by the bee.
B. pratorum worker collecting comfrey pollen, note the white pollen baskets
You might have seen bumblebees buzzing on poppies, as that way they dislogde much pollen, which covers their hairy bodies and that they then collect into their pollen baskets. These buzzing visits by bumblebees are common in several species, including comfrey, borage and kiwifruit and some of these flowers are actually pollinated this way. Not only B. pratorum, but other bumblebee species gather pollen in this way. Despite being relatively common, this behaviour is easily overlooked, for example, if you spot the bees casually, or from some distance, or there is background noise, but you can see the difference in sound between a bee collecting nectar (the first bee in the video, a B. pascuorum) and bee collecting pollen (a B. pratorum) in Comfrey in this YouTube video by pixiebaggins:
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