Stoneflies

Various Stoneflies & their Habiat – Turning over rocks in streams for more than three decades has taught me a few things. For a start, I’ve seen insect larvae that are abundant one month and absent totally a month later. Any insect species with a full one-year life span or less displays a similar pattern. I begin to find nymphs of the tiny winter black stoneflies, for example, in the fall. They grow through the fall and winter, emerge by April, and are then absent in any sampling for the next several months.

However, I can count on finding examples of more long-lived aquatic insect larvae anytime I go out with my kick net. One can dredge up half-grown Hexagenia nymphs, for example, in August; these are just over a year old and they will continue growing and emerge the next July.

Hellgrammites are also collectable year-around; in October one can find small hatchlings from that year’s egg-laying activity, larger one-year old larvae and quite good-sized two-year olds. The latter will be even larger the following May, when they leave the stream to pupate and transform to a winged Dobsonfly.

In short, examples of insects with a two- or three-year larval life spans are present in the stream in edible (to game fish) sizes every day of the year. At this time of the year, that’s especially good news to steelhead anglers across the Midwest who already know that their prized steelies can be tricked by imitations of Hex and stonefly nymphs

Probably the most widely distributed, most frequently collected, and largest aquatic nymphs in the Midwest belong to four genera of large stoneflies: the Giant Stones, Pteronarcys sp., and the Big Golden Stones, which include three common species of Acroneuria, Paragnetina, and Phasganophora.

The Salmon Flies or Giant Stones Pteronarcys californica, of the Madison and other western rivers are well known for its large size, large numbers and the large fish that feed on them. But we in the Midwest have two similar and equally large species, Pteronarcys dorsata and P. pictetii.

These two species, with populations throughout the upper Midwest especially, feed largely on decaying leaves and other dead vegetation and they can often be found in numbers in leaf mats that accumulate on sticks and rocks in the current. I also find pictetii locally in kick-net samples from rocky riffles and from dead streamside grass dangling in the current, and in submerged woody debris.

Nymphs of Pteronarcys are recognized by their large size and particularly by the presence of gills on both the underside of the abdomen and the first couple segments of the abdomen.

While the Salmon Fly of the Madison River emerges in late June and early July, in much of the Midwest, P. dorsata emerges in April and May, that is, fairly soon after snow melt. P. pictetii emerges in May and June, at a time when rivers are returning to more normal water levels.

Emergence of our local Giant Stones is very similar to that of the western species. The mature nymphs crawl across the stream bed and then climb out of the water onto streamside rocks or shrubs. The exoskeleton splits along the head and thorax, the adult slowly emerges and then the wings expand and harden over the next 30 to 60 minutes. The empty shucks can remain on stones, alder stems or bridge abutments for weeks.

While Pteronarcys nymphs are vegetarians, the local Large Goldens are all meat eaters. There’s no hiding for days in a leaf mat munching fungal strands and globs of bacteria from a rotting leaf surface for them. Dining consists of actively crawling about on rocks or woody debris and engulfing any tiny mayfly, stonefly or midge larva that doesn’t clear out of the way.

Examples of Acroneuria, Paragnetina, and Phasganophora are found by picking up and checking the undersides of rocks. Just be quick — these are active predators and Acroneuria in particular can run away quite rapidly.

Acroneura lycorias, with its strongly contrasting black and yellow coloration, is certainly one of the more striking insect nymphs in a stream. Just as striking, however, is to see a pale white individual of the species. These aren’t albinos —the nymph has just molted, and some time is needed before the colors develop in the new exoskeleton. These white forms aren’t rare, and some dedicated fly tiers even tie up all-white imitations of this transitory stage.

A. lycorias lives in a variety of flowing waters, from tiny creeks to large rivers. Its emergence period in the Midwest can begin as early as early May and last until mid-July.

The two other examples of Big Golden Stones in the Midwest, Phasganophora capitata and Paragnetina media, look quite similar. Like many stoneflies, they have conspicuous tufted gills on the underside of the thorax. They are not nearly as colorful as A. lycorias, but they are quite large and are just as desirable a morsel for any hungry trout. They can also be found on the underside of rocks, on woody debris or on any other stable substrate.

P. capitata has a fairly short emergence period of mid May to late June, while P. media emerged from mid-May to late June.

Because these species all require two or three years to reach maturity in Midwest streams, examples of two or even three age classes are found year around in our streams. A reasonable imitation is tied on a #2 – #8 hook, depending on the sizes of the naturals you observe. Effective fly patterns include the Golden Stone, Bitch Creek and Kaufmann’s Black Stone. These patterns should be presented dead drift (all these species are clumsy swimmers, at best) along the bottom. During emergences, remember that nymphs are moving towards shore, so cast to cover there.

List of Stone Fly Patterns for Fly Tying