|The spines are toxic and cause skin rash.|
|As always, you can click on the picture to embiggen.|
|The popular name for this caterpillar is "Spiny Elm Caterpillar". The scientific name is Nymphalis antiopa|
|This is what the adult looks like. They have a wingspan of 3". Picture from Wikipedia.|
A quick check of the internet leads me to believe that they are the larvae of the Morning Cloak Butterfly.
This butterfly even has its own song. Don't bother listening if you dislike Bluegrass music.
Looking at the adult, I am more than willing to lose a few leaves on my willows.