Having not seen many of these caterpillars for some years (except for a dead one last year), I was happy to find four of them within about an hour, all on elms, which are their only larval food source. Apparently the serrated humps on the body are meant to resemble the edge of an elm leaf. The adult (moth) is a very handsome creature: https://pbase.com/laroseforest/image/167791250