There were many caterpillars clustered on a small Siberian elm (Ulmus pumila) tree not far from the Ottawa River. They turn into a beautiful butterfly (inset photo), which is also known as the Camberwell Beauty in England. The caterpillars are often called Spiny Elm Caterpillars. Thanks to Ross Layberry (co-author, Butterflies of Canada) for confirming the ID and letting me know that these are the 3rd instar.