The stonefly is a relatively large aquatic insect commonly found in cool, clean trout water throughout North America. Even though these bugs are less common than mayflies and caddis because of environmental factors, stoneflies can be quite important to trout and steelhead anglers in the western United States (especially in the Pacific Northwest) and they can make a well-prepared mid-western or eastern trout fly fisher’s day during a prolific spring, summer, or fall hatch in ultra-clean and higher elevation lakes, rivers, and creeks.
Stoneflies display easily recognizable characteristics throughout their typical life cycle and are most often identified as nymphs by their long matched tails and antennae. As adults, stoneflies display two sets of wings which do not work in unison during flight, rendering this insect rather awkward when airborne. Biologically and like mayflies, all stoneflies undergo incomplete metamorphosis, experiencing only three major stages within their typical life cycle.
The first of these three stages is the larval stage where these bugs are commonly referred to as stonefly nymphs. The nymph stage is spent entirely beneath the surface of the water. Stoneflies, unlike mayflies and caddis have very stringent survival requirements like clean, cool, and well-oxygenated water so their populations tend to be strong only in northern environments, alpine environments, and otherwise clean rivers and lakes.
Stonefly nymphs broadly resemble mayfly nymphs, but are generally a bit larger, have only two tails, and do not display visible gills along their abdomens. Stonefly nymphs can occur in a range of colors with blacks, browns, yellows, and rusty oranges being the most common. The stonefly nymph’s set of six large, powerful legs allow it to hug its often rocky environment for up to three years before it finds itself ready to hatch to a winged adult.
Unlike other common aquatic insects, stoneflies are a type of trout food that does not hatch beneath, within, or on top of the water’s surface film. Stoneflies undergo their final molt to winged adults on land, generally finding a log, rock, or sandy area near shore to initiate their adult life stage. For stoneflies, the trip from the river or lake bottom to land is relatively short and hurried; if they dawdle or get hung up by a significant roadblock, they quickly become breakfast, lunch, or dinner
for hungry and opportunistic trout.
Adult stoneflies, for all intents and purposes, look just like their nymph form with the addition of 4 wings which generally lay flat along the fly’s back while it rests. The tails an antennae of fully mature stoneflies may also appear to be less pronounced. Male stoneflies, which tend to be considerably smaller than females of the species will typically find their mates along the same rocks, vegetation, and sandy areas on which they hatched. Fertilized females will return to the water to deposit their eggs. Females fly low above the water, “dipping” their egg balls into the water. They are also often found skating or skitting across the water’s surface, depositing their eggs with great effort — when female stoneflies are hard at work laying their eggs, trout can’t resist these large and vulnerable morsels.
Stoneflies follow three stages in their typical life cycle of incomplete metamorphosis: nymph, adult, and sexually mature adult. As far as fly fishing goes, there are really two main points during this life cycle that require distinct artificial fly imitations. These imitation points are: nymph and adult. A nymph imitation is fished entirely in the subsurface and is designed to imitate the stonefly during the major portion of its pre-adult stage and just prior to its final, on-land molt to adulthood. An adult stonefly is fished dry (on the water’s surface) and generally imitates a sexually mature female in the midst of her strenuous egg laying process.
When packing your fly box with stonefly imitations, it’s important to research the species of stonefly most commonly found in the waters of your targeted destination. Similar to fishing a local caddis hatch, matching both size and color of the local stonefly population can be the difference between a so-so day of fly fishing and an epic adventure on the water. Because stoneflies spend the proportionally greatest time in the nymph stage and are most vulnerable to trout during their trip from water to land, never leave for a fly fishing destination without a solid selection of stonefly nymphs on hand. Don’t underestimate the importance of having a good selection of adult stoneflies in lots of colors and sizes either. These poor fliers are often blown from their landward perches into the drift, where they are helpless in the face of a waiting trout.
Due to its relatively large size (when compared to other aquatic insects), the stonefly is a fun pattern to fish. When fished like a dry fly, on the surface of the water, the resulting strikes are aggressive and memorable. Typically during a stonefly hatch the largest trout in the water come to feast. When fished sub surface as a nymph, few trout will pass up the larger meal ticket being offered. Having the right equipment, especially fly rod and fly line, makes all the difference when casting the larger stonefly pattern. Leland recommends the below fly rods for the best results.