Can't tell you much about the cat, but it makes me want to grow more Meadow Rue. It doesn't look like any butterfly cat I've ever seen, so my guess is it's some kind of moth cat.

It's a cross-striped cabbageworm, Evergestis rimosalis, I believe. Food plants in the cabbage family. The fact that it is on Thalictrum is probably indicative that it is preparing to pupate, and is wandering around to find a suitable location.

Susan

Here is a link that might be useful: cross-striped cabbageworm

It looks like a Canadian owlet moth caterpillar to me.

Sherry

Here is a link that might be useful: Canadian Owlet Caterpillar