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Genus Overview
Isonychia is the only genus in the family Isonychiidae. There are about 15 known North American species. They have a widespread distribution east of the Rocky Mountains, and tend to be found swimming in fast flowing water. They have a fusiform, minnow-like body shape, making them strong swimmers and clingers. Long setae (hair) on the inner margins of the femora and tibia of forelegs are used for filter feeding, a trait unique to this group of mayflies. They are important in the trout and fly fishing community due to their frequent but periodic hatches (emergence of subimagos) in June–October.    
Characteristics
POLLUTION TOLERANCE
Southeast: 3.8
Upper Midwest: 2
Midwest: 1.9
Mid-Atlantic: 2
0 = least tolerant, 10 = most tolerant
FEEDING HABITS
Collector / Filterer
Engulfer / Predator
MOVEMENT
Clinger
Swimmer
DISTRIBUTION
Widespread (east of the Rocky Mtns.)
HABITAT
Lotic-erosional
Diagnostic Characters
+ Expanded Character List
Order: Wings developing in wing pads. Mouthparts suitable for chewing. Gills present on tops and sides of abdomen. Segmented legs present. One tarsal claw per leg. Usually with 3 tails (sometimes 2).
Family: Femora and tibiae of forelegs with double rows of long setae along inner margins. Abdominal segment 1 with dorsolateral gills, similar in appearance other gills, each consisting of longer dorsal lamella and shorter fibrilliform portion. Nymphs with streamlined, minnow-like body shape. Isonychia is the only genus in the family. Widespread and common.
Genus: Isonychia is the only genus in the family Isonychiidae, therefore the family level and genus level characters are the same.