A stone fly is an anglers ace in the hole. Stone fly are in most trout rivers and they are almost always an effective pattern to use. While the winged adults only live for a few short weeks, the nymph cycle lasts for years. This means that this nymph stage of this insect is in the water all year round regardless of the season or time of day. Stone fly nymphs are also often large in size so they can be a big meal that many trout cannot pass up.