Bidens never ceases to amaze me...
The far-away white/yellow guide for insects is completely invisible for us. Then the blue UV reflection of the "husk" protecting the stamen + pollen just before it opens up to reveal the pollen, yet at the same time acting as a guide for the insects when they are close enough (insects only see sharp close-up).[click on image to see a larger one]
Visible light image
Simulated butterfly vision (UV=blue)
These are my personal assumptions btw. and I would think it would be worth working on that scientifically...maybe I should. Isn't nature simply amazing....?!
Stay tuned, more will follow on that fascinating subject...
More info on this very interesting field may be found on my site http://www.pbase.com/kds315/uv_photos