Patrick:

I believe that what you have there is an Io moth catrerpiller. Like all caterpillers, they will eat their fill of your leaves until they are fat enough to begin the metamorphisis process. On my larger trees, I just let them do their thing. On a smaller tree with no leaves to spare, I would remove them unless you want a free leaf pruning.

Harry

Thanks Harry! They have been relocated to a branch in a pickle jar in my daughter's bedroom!