There are many species of sawflies, all with larvae that resemble caterpillars (although they are anatomically different). The larvae ingest a variety of plant leaves and conifer needles. Several species of sawflies are gregarious, so it isn't uncommon to find large groups of them feeding together on a plant. Others, such as columbine, rose, hollyhock, and hibiscus sawflies, feed solo, skeletonizing leaves as they feed.
Larva: Steven Katovich, USDA Forest Service, Bugwood.org
Adult: John H. Ghent, USDA Forest Service, Bugwood.org