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Isonychia bicolor (mayfly hatch)Isonychia bicolor is commonly called the Mahogany Dun. In the past, many people also referred to Isonychia sadleri and Isonychia harperi, which entomologists now have placed under Isonychia bicolor. This is one of the major hatches in the Midwest on many freestone streams and rivers. Fly fisherman should carry a selection of fly imitations to match the Mahogany Duns throughout the summer months. The nymphs are excellent swimmers and at emergence time most nymphs will crawl out of the water to shuck their nymphal case and become a dun. Look on rocks and logs that are partially submerged in the water along the shallows for shucked Isonychia bicolor nymph cases. At these times dun patterns are not important unless the nymphs are emerging in mid-stream which is usually not the case. In evening look for the returning Isonychia spinners over the riffled waters. They offer the trout a choice meal. See photos below:
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