I think these are one of the most colourfully amazing moth caterpillars around! I don't see them often, but was lucky enough early this morning to see this beautiful one on an alder shrub. While many, perhaps most, colourful caterpillars turn into drab moths, this one turns into an amazing vividly patterned moth about the size of a hand! This caterpillar will eventually make a large papery cocoon and emerge in May of next year to begin the cycle again. The moths don't eat (but the caterpillars do), they exist solely to mate, breed and then die.

Here is what the young caterpillars look like, quite different! http://www.pbase.com/laroseforest/image/144479579