Two Queen butterflies posing on a Rubiaceae?
What species is the plant?
Picture taken at Sonora Desert Museum on October 23, 2010.
Click "original" in sizes below for the highest resolution picture,
then "large" or other to get back. Next picture is a zoomed crop of flower and fruit.

Zoomed crop of plant in previous picture showing flower, leaves, and fruit.

Zoomed crop of same plant, showing flower buds and spent flowers.