Allan,
The original Bumbles were fished on rivers, and were tied on much larger hooks - #4 and #6.
These days Bumbles are mostly fished on still waters (loughs and lochs, in Ireland and Scotland respectively) - as the top fly, or "bob" fly, on a team of three. It skims the waves, while the other two flies are fished sub-surface, in essence providing an anchor point for the top fly.
Cheers,
Hans W