A group of scientists studied pollination in the Flame Azalea, Rhododendron calendulaceum, with flower parts arranged such that only large insects, such as butterflies can contact both the male and female parts when nectaring. They caged flowers and compared the successful fruit set of flowers in cages that excluded large butterflies to flowers in cages that allowed access of all pollinators. Only 2 of 78 flowers with cages that allowed access to small pollinators such as bees but excluded butterflies set fruit. When butterflies were allowed access, the rate of fruit set was 12 times higher. Examination of tiger swallowtails showed that pollen grains from Flame Azalea stuck to the wings and that during nectaring, over 80 percent of the butterflies made contact with both stamens and pistels. The scientists concluded that the unusual structure of the Flame Azalea is an adaptation to pollination by butterfly wings.
Mary Jane Epps, Suzanne E. Allison and Lorne M. Wolfe. 2015. Reproduction in Flame Azalea ( Rhododendron calendulaceum , Ericaceae): A Rare Case of Insect Wing Pollination. The American Naturalist. Vol. 186, no. 2
DOI: 10.1086/682006 ; http://www.jstor.org/stable/10.1086/682006