The Salmonfly is a type of stonefly and stoneflies are an insect order separate from mayflies.
You might think that this Salmonfly larva looks like a Mayfly larva but the difference is they have two long fellers on the end of its abdomen whereas a Mayfly has three. Also, the gills on the Salmonfly larvae are located in their leg pits.
Trout love eating Salmonfly larvae, where they both live in fast flowing clear streams. With powerful legs the Salmonfly larvae are able to hold onto underwater rocks and eat algae. They are about 50mm in length.