So that's why....

I could not figure out what a pair of flies were doing wandering around our solitary open sunflower, taking on the might of the bumblers. Here's the answer.

Just prior to this I photographed a scene that just had to end in tears. A small spider approached this fly from the flank. As it raised it's front legs about to pounce the fly just raised an antennae and waved it. The tiny spider decided that retreat was the sensible option.

For a really good look hit the magnifying glass in the picture.