The stone fly is an important year round source of food for the trout. Unlike other aquatic insects which hatch on a yearly cycle, stoneflies remain nymphs for up to 3 years before they leave the water to metamorphose as an adult. Stoneflies undergo incomplete metamorphosis. Like the mayflies, there is no pupal stage.
The two largest and most important stoneflies found on western waters are the Giant Black Stonefly, (Pteronarcys californica) and the Golden Stonefly, (Acroneuria pacifica). Locally we have a smaller golden and black stone fly. But the larger stoneflies occur on the Colorado.
The female stone fly lays the eggs on the water surface where they fall to the bottom of the river. There they will hatch in the spring and a nymph will emerge. As the tiny nymphs grow they shed their skin or outer shell (exoskeleton) to accommodate their growth. This is referred to as moulting. Essentially the exoskeleton splits open and the new, larger nymph emerges with a new exoskeleton. These growth stages are called larval instars. The nymphs go through many instars before emerging as adults.
Stoneflies require unpolluted, fast flowing waters with bottoms paved with large rocks. They also like overhanging trees. Their emerging environment requires an optimal temperature which varies with each species and region. Consequently, as the water temperature rises the stonefly hatches progress upstream over the subject body of water. Day by day, stoneflies will hatch progressively farther upstream until the hatch is over for the year. Sudden changes in the weather can alter the predictability of the hatch slowing it down or in fact stopping it for a time until conditions improve.
As the emergence of stoneflies requires the water temperature to rise to the level which triggers it, the emergence will coincide with the end of winter and the passing of spring. The hatch therefore occurs from mid-spring, usually late May - early June, but as noted earlier this depends upon the weather patterns for the year and the local conditions. Unlike the mayflies, caddis flies and midges, when the stonefly nymphs are ready to hatch they don't swim to the surface. Instead they crawl to the river bank and onto nearby rocks or reeds. Consequently , when the nymph starts its lateral movement to the shore to finally metamorphose, it will also become more vulnerable to the trout.
Emergence generally begins at night and lasts into the morning hours. Once out of the river, stoneflies seek shelter in the branches of willows, reeds, rocks or trees. There they lie in the sun and split their exoskeletons open along the back. They extricate themselves from their shucks and emerge as sexually mature stonefly adults. Once their wings are dry enough the adults fly to the bushes where they finish drying their wings.
The stonefly is a clumsy erratic flying creature. The adults are flat and long with great wings that lay down on their body when not in use. They can range in color from brown to black to golden or orange.
Mating and laying eggs.
Upon maturing the stoneflies mate and the females return to the water surface to lay their eggs. In the course of laying its eggs, the female makes quite a disturbance, s kipping across the water, dipping her abdomen into the surface. The eggs fall to the bottom to await next spring to hatch. Female stones will typically make several journeys over the river surface. Many of the insects return to the water at the same time and this constitutes the hatch. Clumsy fliers under the best conditions, stoneflies frequently miscalculate their sweeps to the water surface and fall into the film. Their wings become too wet to allow them to fly free and their bodies become water-logged. They become an excellent target for the hungry trout. Since the stonefly females are very active on the water one needs a very buoyant fly to imitate them well. The Stimulator is a great pattern to use during the stonefly hatch.