Window flowers
David Ehrlich (Sat, 06 Nov 2010 10:38:40 PDT)

Alberto Castillo wrote >  The bottom of the cup is more clear in several species
of Cypella and may be seen as a circle of light  by the insect flying downwards
to the flower.                        

Or, I guess, by beetles underneath.

Thank you very much Alberto.  This explanation makes a great deal of sense.

David E.