Isonychia: Faster than a speeding bullet

I then explain how the nymph uses its front legs to filter its food from the water current. Facing upstream, the nymph grips the substrate firmly with its middle and hind legs. The front legs are held up in front of its head, and fine particles of organic matter--tiny drifting insect larvae, minute pieces of broken leaves and other plant remains, and even some of the feces of a million insects upstream--are captured in this comb of setae.

Wiping these food-laden fringes on the front legs over its mouth parts, the nymph removes and eats this melange of fine particles. Over the course of its underwater life, each Isonychia nymph removes hundreds of thousands of these tiny particles from the water and turns them into a single large, lively bit of potential trout food. And, no less importantly, it clears up the water in the stream. Isonychia nymphs, to stream ecologists, function as "filtering collectors," much like the net-making caddis flies; that is, they feed by catching and removing fine particulate organic material from the water current. And if its feeding place on a rock becomes overcrowded, the Isonychia nymph can easily swim elsewhere. It folds its legs along its body and undulates rapidly up and down ("Like a dolphin!," remember?). Incidentally, all mayfly nymphs swim in this manner (if not as well as Isonychia), and are hence easily differentiated from stonefly nymphs in a water sample just by the very different way each group swims. Stoneflies swim like a fish--by wiggling from side to side--and no clumsy stonefly nymph has anywhere near the swimming ability of many mayflies.

By Dean Hansen

"Wow!" the third-grader exclaimed. "It's faster than a speeding bullet!"

"Who said Superman was dead?" I thought.

I really enjoy bringing live aquatic insects into grade school classrooms for hands-on learning about the insect life in our streams and rivers. Elementary school kids have a wonderful enthusiasm and sense of wonder when seeing, for the first time, some of the creatures trout fishers know so well.

In this case, the "speeding bullet" was a mature nymph of the swimming mayfly known as Isonychia, a fairly common inhabitant of the rocky fast water of Midwest rivers and streams. Its ability to dart through the water like a scared minnow has to be seen to be appreciated.

"OK," I asked the student, "how does it swim? Like a fish, wiggling from side to side, or like a whale or dolphin, wiggling up and down?" Her eyes dived back to the nymph in the pan of water on her desk.

"Like a dolphin!" she answered triumphantly.

Another kid, I hoped, gently steered away from the all-too-common I-hate-bugs attitude I see in older students. Thank you, Isonychia, I thought to myself.

One can't help but be impressed by the size, striking dark color, and unrivaled swimming ability of a mature nymph of Isonychia, but most fly anglers are much more likely to be familiar with the adults, rather than the nymphs, of Isonychia. Both the dun and spinner are large. The duns can be up to five eighths of an inch long (not including the two long tails) requiring a size #8 or #10 hook to imitate. The dun's opaque wings have a distinctive gray "smoky" cast, and the middle and hind legs are yellowish, in contrast to the brown front legs. The smoky wings give rise to the common name of Leadwing Coachman or Slate Drake for the dun, and the brown body is the obvious source of the name Mahogany Dun.

The spinner's wings are uncolored and transparent. The front legs remain brown, again in contrast to the yellow middle and hind legs, and the front legs' final segments, the tarsi in entomological terms, are white. Charles Wetzel used this feature in naming his imitation of the Isonychia spinner, the White-Gloved Howdy. The spinner looks as if it is extending its gloved hands in a "Howdy" greeting.

Caucci and Nastasi in their book Hatches II give excellent color photos of the duns and spinners. Entomologists recognize over two dozen species of Isonychia in the United States and Canada. Most of these are confined to the southeast U. S., however, and Midwest fly anglers really need to be familiar with only two species: I. bicolor and sadleri . The nymphs of both species require fast-moving water, and they may be found on rocks and gravel, on submerged logs or large woody debris, and in beds of submerged coontail or other aquatic plants of larger trout streams and rivers.

And Isonychia nymphs have a unique method of feeding.

"Now use your magnifying glasses and tell me what the swimming mayfly has on his front legs," I ask the class. Twenty-some third graders peer closely at their Isonychia nymphs, hoping they will remain still for a moment.

"Long fuzzy things!" "Lots of tiny hairs!" Even most first-graders, pressing a magnifying glass to their eyes and all but putting their noses into the water, can see the fringe of long, fine hairs (setae) lining the inner surface of the front legs of the Isonychia nymph

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The recommended nymph fishing technique with Isonychia reminds one of fishing a stonefly emergence. Isonychia nymphs generally eschew the "let's-all-swim-to-the-water-surface-at once, emerge-as-duns, and-float-helplessly-so-the -trout-can feed" approach most mayflies follow. Clever fellows, these Isonychia. The mature nymph instead swims to the shore or to a protruding rock and climbs out of the water, and transformation to the dun stage occurs in relative safety. So you obviously don't fish an Isonychia emerger, but rather a nymph fished dead drift or with action near the shore, midstream rocks, or logs sticking out of the water. This hatching activity takes place in late afternoon or evening, and the cast nymphal skin or shuck remains on the rock for days. It's easy to see if any Isonychia have been hatching lately--just look for these obvious shucks on rocks an inch or two above the waterline. Because the dun is rarely on the water surface, the nymph and spinner imitations are the most important of the three stages. The male spinners gather in swarms near dusk high above riffle areas, and mating occurs in the air. The egg-laying females and spent males will appear on the water surface shortly thereafter, and imitations such as Caucci and Nastasi's Compara-spinner or Wetzel's White-Gloved Howdy tied on a #8 or #10 hook and fished dead-drift are called for.

In the Midwest, hatches of Isonychia begin in mid-June and continue to mid-July, although smaller numbers of the genus will continue to emerge well into August.

Dean Hansen has a Ph.D. in aquatic entomology. You'll find his column in every issue of Midwest Fly Fishing.
copyright Midwest Fly Fishing Magazine 1999