Thanks, Elaine! And you're spot on with the Zebra Longwings. It is indeed, called "roosting". I've read various things about whether butterflies actually "sleep" as we know it, but they definitely have some kind of rest period at night. And the grouping they do is for defense. My bosses just did a blog about roosting; check it out:
http://lepcurious.blogspot.com...
