My daughter sent me this photograph of a caterpillar that hitched a ride on her skirt when she was on her way to the grocery store. It is the larva of a Red-spotted Purple butterfly (Limenitis anthemis). It has a striking color pattern because it tries to fool predators by mimicking a bird dropping. These caterpillars will eat the leaves of many trees including wild cherry and willow.
Here is a photograph of an adult Red-spotted Purple butterfly that I saw on our back deck last summer. The blue iridescence on its wings is very striking. Red-spotted Purples are thought to gain some protection from birds by mimicking the Pipe Vine Swallowtail (Battus philenor), which is very distasteful to birds.
This photograph shows the underwing pattern of the Red-spotted Purple. The two orange bars near the front of the forewing and the three orange spots near the base of the hindwing are diagnostic.