Buttonbush (Cephalanthus occidentalis) is a wetland species of shrub. We have several in the soil around the overflow area of our duck pond. The shrubs grow to head-height and produce globular flowers in the summer. The flowers are rich nectar sources, so they attract a number of insects.
Buttonbush being visited by a fly. Note the long styles projecting from each flower in the globe. Unlike the flowers of most other plant species, the area at the top of these styles (the stigma) serves both to release pollen and to gather incoming pollen. To prevent self-fertilization, the buttonbush first releases its own pollen, then when this is complete the plant switches to receiving pollen from other plants.
David George Haskell 