Joanne, it could still be a Papilio canadensis (Canadian Tiger Swallowtail) caterpillar, because they ARE green. The different tiger caterpillar start out dark, then look somewhat like bird dropping, then they turn green. The day or so before pupation, they darken and are brownish or dark brownish. I googled Papilio canadensis images and this is what turned up. BTW, the key to ID on the tiger caterpillars is mainly the eyespots, once they develop eyespots.
http://www.google.com/search?q...
I've only raised two species of tiger...Eastern Tiger Swallowtail and Two-tailed Tiger Swallowtail.
