The caterpillar of the Wavy Emerald Moth (Synchlora aerata), family Geometridae, a species found throughout much of North America. The larvae feed on many plants in the family Asteraceae (like Liatris spp. and Rudbeckia spp.) as well as a variety of other flowering plants.
They are known to pluck the petals from the flowers of their host plants and affix them to their backs using silk. Once the petals begin to wilt and discolor, the caterpillar discards the old petals and picks new petals, which camouflage the animal.The top photo shows the cat next to a host plant blossom. This photo shows one in the classic "looped" position on a stem:
We have lots of asters, Liatris, and Rudbeckia in our garden and our nearby hiking areas. I'm going to look for one of the fascinating critters next summer. And the adult moth, btw, is gorgeous.
Found at Fauna, via A London Salmagundi. Photo credit Hopefoote.
Addendum: Reposted from January to add these photos found at Why Evolution is True, showing the same species of caterpillar decorating/camouflaging itself with different flower components:
There's an even more impressive example at the link, as well as a subtle connection to Lady Chatterley's Lover.