Crab Spiders (Thomisidae) typically conceal themselves within the petals, seeds, or leaves of a flowering plant, waiting in ambush for insects to come along. They quickly lurch out to capture any unsuspecting insect that happens by. Prey rarely escape the clutches of these bold little spiders. The spiders get their name from their superficial resemblance to crabs.
The action in the above photo took place on the seed head of a Purple Coneflower (Echinacea purpurea) in the very fine garden on either side of the steps leading down to the Butterfly House.