I fish mayfly hatches often enough that I like to have nymphs so convincing that I feel they can’t miss, though the term “can’t miss” is always relative in fly fishing. If I’m fairly certain the fish are taking, for instance, Pale Morning Dun nymphs, and especially if the fish are wise to anglers and their tricks, I’ll reach for an Anatomical Pmd/Baetis. I often refer to the Anatomical Series as my “confidence nymphs.”

When I fish an Anatomical, I never blame failure on the fly, but I always feel the fly’s realism contributes when I succeed. Anatomicals have gills. They have appropriately splayed legs. They have the correct number of tails rather than a brush of fibers. But Anatomicals don’t have the crawl-away perfection of an hours-to-construct, realistic pattern by Bill Blackstone or tying pioneer Bill Blades. I think Anatomicals are as detailed as they need be to convince careful trout--more detail would be for fishermen, not fish. Consequently, tying them is neither overlong nor unmanageable for average tiers.

Here are the three current Anatomical nymphs and the four major Western mayfly species they imitate.

Anatomical Pmd/Baetis
Blue-winged Olives (BWOs) and Pale Morning Duns (PMDs) are year-round staples of Western river mayfly fishing. Small, dark brown, slender, and lightly gilled, the nymphs of these two mayflies look so similar that an imitation of one imitates the other.

BWOs hatch year-round but are most important from fall through early spring when few other mayfly, stonefly, or caddis species are hatching to compete with them. BWOs prefer cloudy, even foul, weather (though they will hatch in sunshine) and typically appear between 11 A.M. and 4 P.M. To imitate BWOs I fish Anatomical PMD/Baetis Nymphs in two ways: right along the bottom in the hour before the hatch, when the trout know to look for the active nymphs; and near the surface, dangling from the bend of a dry fly’s hook on 8 or 10 inches of tippet, to rising trout that seem to want neither an emerger nor a dry fly.

PMDs are generally the longest- and strongest-hatching summer mayflies on Western rivers. Like BWOs, they prefer cloud cover but also hatch briefly in the sunshine. At first they hatch from around 11 A.M. to 1 P.M., but later in the season they hatch in the earlier and later hours, lying quiet during the hot part of the day. They return to midday hatching in the fall. When there is no hatch in good PMD water, I often fish my Anatomical PMD/Baetis as a dropper above a heavy stonefly nymph in riffles and other lively currents. But an Anatomical PMD/Baetis fished a few inches below a dry fly can be just the thing when duns are hatching and trout are up feeding on them.

This article originally appeared in the May 2007 issue of Fly Fisherman.