This Skwala nymph still has a couple months left to go before hatching, but it's still a good representative of its species, which was extremely abundant in my sample for a stonefly of this size. It's obvious why the Yakima is known for its Skwala hatch.
Collembola are actually a subclass of a class called Entognatha. They are the most common of the only six-legged arthropods (Hexapods) that aren't insects. Many Collembolans are terrestrial, but some live on the surface of their water, and they can be a supplemental food source for trout, especially young-of-the-year trout.
They rarely grow longer than 6mm or occur in trout streams in such numbers that an imitation with a fly is warranted, but they are interesting creatures.