Crawlers

Crawler nymphs are much less streamlined than swimmers and have three tails and stout legs. The head is narrower than the thorax. Rick Hafele photo

Adults and spinners have three tails, which makes them easy to identify. Jim Schollmeyer photo

This is the most diverse of the four types of mayflies. The families that makeup the crawler group are Ephemerellidae, Leptophlebiidae, Leptohyphidae, and Caenidae. These four families account for 29 genera and over 160 species. The most important family is Ephemerellidae, which includes such major hatches as sulfurs, Hendrickson, pale morning duns, flavs, and western green drakes.

Given this diversity, not all crawlers look alike, but there are some general features that will help you know a crawler when you see one. Crawler nymphs do not have the streamlined torpedo shape of swimmers. Most tend to be rather rectangular, with stout legs. The exception is the family Leptophlebiidae, which has more slender, delicate bodies than other crawlers. Fortunately, this family has unique "forked" gills that make them relatively easy to recognize. All crawler nymphs have three well-developed tails. Some species have tails fringed with fine hairs (PMDs for example), but most do not. Gills vary from the fork shape of Leptophlebiidae, to oval, plate-like gills on Ephemerellidae, to "operculate" gills on Leptohyphidae and Caenidae. An operculate gill means the first gill is unusually large and covers all the remaining gills. This adaptation keeps the gills free of debris, and allows the nymphs to live in habitats with fine silt that other mayflies avoid. Finally, the head of crawler nymphs is narrower than the thorax.

While recognizing crawler nymphs can test your observation skills, recognizing crawler duns or spinners is a snap. All you have to do is count tails, since all crawler duns and spinners have three tails, while all swimmer and clinger duns and spinners have two. Burrowers can have two or three tails, but they are so much larger that confusion is rarely a problem. Because of the large number of species in the group, the size and color of different crawler duns and spinners varies widely.

The majority of crawlers live in moderate to fast water, although Caenidae live in ponds and lakes. Most species emerge from nymphs to duns in the surface film. Plus, emergence to the dun stage for many species occurs underwater, and the dun floats to the surface. Thus, emerger patterns for this group can be very important. Crawlers also produce excellent surface activity, as both duns and spinners can be abundant and readily available.

Clingers

If crawlers are confusing, then clingers are simple. All clingers belong to a single family, Heptageniidae. This is a diverse family, however, with 15 genera and almost 130 species. Clinger hatches that anglers routinely imitate include the western and eastern March browns, quill Gordons, and pale evening duns.

Clinger nymphs are designed to stick to rocks in fast water, so they are flat, and the head is wider than the thorax. Rick Hafele photo

The duns and spinners have two tails, flat-shaped heads, and very small hind wings. Dave Hughes photo

All clinger nymphs can be quickly recognized by the size and shape of the head. Unlike the other groups, the heads of clingers are extremely flat (squashed from top to bottom) and wider than their thorax or abdomen. In fact, the entire body of clingers is flat. This allows them to cling to are stream dwellers, and most the surface of flat rocks in fast water without getting washed into the current. Well-developed, plate-like gills protrude from each side of their abdomens. And finally, most clinger nymphs have three tails, but species of the genus Epeorus (quill Gordon and yellow quills) have just two tails. Tails are not fringed with fine hairs.

Clinger duns and spinners are best recognized by their two tails (shared with swimmer adults) and flat-shaped heads (not shared with swimmer adults). The wings are well developed, but never as large as the front wings. Most are moderate in size and are often matched with size 14 or 12 hooks.

Clingers are stream dwellers, and most species prefer fast riffle-and-run habitats. Their shape and behavior allow them to live in such places without getting washed away. Because the nymphs cling so effectively to the stream bottom, they are not readily available to feeding trout. Nymphal availability changes, however, at emergence to the dun stage. That's when the nymphs let go of the bottom and swim up to the water's surface for emergence. Duns and spinners can produce good surface activity during emergence and egg-laying.

Burrowers

The burrower group is made up of three main families — Ephemeridae, Polymitarcyidae, and Potamanthidae — that include eight genera and just 26 species. The group includes several well-known and important hatches, such as the Hex or great olive-winged drake, eastern green drake, golden drake, and white fly.

Burrower nymphs are so much larger than other types that it's easy to know when you see one. They have three tails and large, forked gills. Jim Schollmeyer photo

The adults and spinners are larger than any other mayflies you'll see on the water. Jim Schollmeyer photo

The defining characteristic of burrowers is their size. Fully grown nymphs and duns and spinners are over an inch long. Size 6 through 10 hooks are necessary to imitate these giant mayflies. Burrower nymphs have tusk-like mandibles that protrude in front of the head. Besideshelping nymphs burrow into bottom substrates, the mandibles make burrower nymphs easy to recognize. Burrower nymphs also have large forked gills that are fringed with fine hairs. Finally, all nymphs have three tails also fringed with fine hairs.

The size of burrower duns and spinners is usually enough to make them recognizable. Some Hex duns look like small birds emerging off the surface. Besides large bodies, the duns and spinners also have pronounced hind wings roughly half the size of the front wings. Burrower duns and spinners can have two or three tails, depending on the species and whether they are a male or a female.

Burrowers, as the name implies, are the moles of the mayfly world. Nymphs live in underground tunnels in bottom sediments. Some species prefer soft muddy sediments, while others like fine sand and gravel substrates. Nymphs often require two or even three years to reach full size. At that point, the nymphs leave their burrows and swim up to the surface for dun emergence. This often occurs after dark, perhaps as an adaptation to avoid bird predation. Fish, however, have no problem spotting these large mayflies bobbing on the surface in the dark. Blind casting at night with large flies, while big trout slash at mayflies in the dark, is one of the true gifts burrowers have given the angler.

As you observe mayflies throughout the year, try to classify them into one of these four groups. The fish won't care, but it might help you understand what patterns and tactics will work best.