These photos supplement pages 77-86 of the Guide to Aquatic Invertebrates of the Upper Midwest. Information on the biology of Perlidae can be found on page 85. Perlidae can be mistaken for Heptageniidae Mayflies. Heptageniid Mayflies have gills projecting laterally from their abdomen, while Perlidae Stoneflies have gills on the ventral side of their thorax. Click to compare with Heptageniidae.