Of the approximately 130 species of crab spiders (Araneae: Thomisidae) found in North America, only about 20 are known to regularly inhabit flowers. These ambush predators wait patiently for pollinating insects to move close and then grab them with their long front legs. They use their fangs to bite small holes in their prey, inject digestive fluid, and then suck out the liquefied organs and muscle.
I came across the individual above as it was lying in wait on a common tansy (Tanacetum vulgare, Asteraceae) in northern Washington. This plant is native to Eurasia but was introduced to North America by early settlers who valued its use in folk remedies. Long after humans abandoned the cultivation of this plant it continued to spread on its own, and today can be found across much of the continent.