VERY good guys. They will become http://en.wikipedia.org/wiki/Papilio_polyxenes aka Black Swallowtail Butterfly They might strip the plant clean of leaves but not too many gardeners really care about that, they love having the caterpillars and butterflies more than having an uneaten plant. That is the last instar in their development, so they should be leaving to pupate soon.
The ones I had earlier this year really did a number on the Fennel! I'm planning on plant more next year just for them.
Oh, definitely soon-to-be-butterflies! They love parsley, fennel, dill, and rue. I plant loads of dill just for the caterpillars (instars) to chomp on. Good photo, and good gardening if you have swallowtail caterpillars!
When my kids were little, we looked for these chewing on the Dill. Then we would put one in a 2 liter bottle with a branch and some munchies and watch it turn into a cocoon, and then a pretty Swallowtail butterfly. Ahhh, memories...
We just kept the bottle on the potting bench, and it was only a matter of a day or so before they were in their cocoon. I can not remember how long before the butterfly emerged. I know it wasn't long, because it kept the kid's attention. They loved to set them free
What kind of branch and munchies did you supply for the caterpillar? I watched and watched but never saw where one built it's cocoon. Sure would be neat to watch it's metamorphosis.
