Nectar robbers

If you are a short-tongued bumblebee faced with a long flower you have a problem.  You can't reach the nectaries which are cunningly placed at the base of the flower to make you squeeze past the anthers and stigma(s).  Unless, of course, you cheat.  Comfrey is a very enticing flower but its sweet nectar is out of reach of bees such as this male early bumblebee (Bombus pratorum).  His solution is to use a hole bitten in the side of the flower to steal the nectar without helping with pollination.

If you look carefully you will see that on some comfrey plants every flower has a tiny hole bitten in the side.

Other short-tongued bees that rob nectar include the tree bumblebee (Bombus hypnorum)

and the common carder bee (Bombus pascuorum).

Worker bumblebees vary in side and the smaller ones can get right inside using the correct entrance, like this common carder bee.  It is interesting that there were no holes in the flowers on this plant.

Long-tongued bees like the garden bumblebee (Bombus hortorum) don't have a problem.

Surprisingly the robbing doesn't seem to have much effect on a plant's reproductive success.  You can read more about the biology of nectar robbing here.