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 almost unique to this group of mayflies (also occurring in the rare family Oligoneuriidae). They are important in the trout and fly fishing community as "Slate Drakes" due to their frequent but periodic hatches (emergence of subimagos or "duns") and return to the water surface as spent adults (imagoes or "spinners") in June–October.
Upper Midwest: 2
0 = least tolerant, 10 = most tolerant
Collector / Filterer
Engulfer / Predator
Single Tarsal Claw
Three Pairs of Legs on Thorax
Usually 3 Tails
Forelegs With Long Hair
+ Expanded Character List
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 dorsallamella and shorter fibrilliform portion. Nymphs with streamlined, minnow-like body shape. Isonychia is the only genus in the family. Widespread and common.
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