The nymphs are sprawling forms with bodies, legs and head that are strongly flattened. The head is large in relation to the body. Eyes are also large and are located on the dorsal surface of the head. Gills are present on abdominal segments 1 to 7. Adults have two tails and five freely moveable joints in the hind tarsus. Two pairs of cubital intercalary veins are present in the fore wings, while both fore and hind wings display heavy veination. The males have large spherical eyes that are normally light in coloration during the day and black at night.
Members of this genus are more important to Western anglers than those of the East and Midwest. Species are of variable size, ranging from 4 to 12 mm. Nymphs have three tails and platelike gills on abdominal segments 1 to 7, which do not extend beneath the body.