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.
(top photo: the larva on a Liatris, by Hope Abrams)