This little guy is a black swallowtail caterpillar. Isn't it so pretty?
It was found and identified by our good friend Emma, who is Stella's big sister and who is nine. (The grown ups didn't know what it was, or thought it was a monarch caterpillar. I thought it was pretty cool that Emma was the one who knew.) It was eating some fennel, along with many others, at Nancy's house. Nancy graciously allowed us to take it home so we could watch it turn into a butterfly, and with help from Emma (and her mom), we have some fennel and parsley to feed it.
It has a defense mechanism that Eric learned about very quickly - when you squeeze it, these orange things come out of its head. He lost holding privileges shortly after that.