Important Info about Red-Spotted Purples and Their Kin
When red-spotted purples, viceroys, and white admirals (and their hybrids) lay their eggs, they lay it on the very end of the leaf. When the egg hatches, the tiny hatchling makes a string, or 'perch' if you will. The hatchling will object to being removed from the leaf it started off life on, and if you force it, you'll undoubtedly kill it. You'd be surprised how much they can grow on what looks like a bad leaf.
Please, everybody, leave these caterpillars on their original leaf, no matter how bad it looks, at least until the caterpillar gets some size on it. I put a fresh, clean stem of the host plant in the hole in the leftovers container next to the stem the original leaf is in, and the cat will eventually, on its own time, move over to the better leaf.
These caterpillars, at the end of the summer/early fall, might make a hibernaculum, so it's a good idea to leave any tree limbs alone if there are leaves on there still.
The Goatweed Leafwing also starts out with a frass string. And then as it gets bigger it makes a rolled leaf nest.
Here's what hatchling or very early instar RSP cats look like on their 'string' --