Note from the author: A gall is a plant deformity caused by the insertion of an invertebrate's eggs or larvae into a part of its living tissue, later to be used by that larva as food or refuge. Wasps, aphids, sawflies, moths, and midges are all known gall-makers. Some characteristic plant deformities are also caused by fungi but are not technically galls. There are thousands of species of gall-makers in the United States; the few recorded here are a guide to get the beginner started. The intermediate student of galls might take one home to see what comes out, with surprising results. Other tenants of gall housing that may emerge include spiders and parasitic wasps. This guide is organized by host species. Sure, I would also like to acknowledge Carl F. Gronemann who wrote the guide that got me started (and to Cassi Saari for digitizing it): http://www.cassisaari.com/j/wp-content/uploads/2012/03/Gronemann-1930-FiftyCommonPlantGalls.pdf