This specimen resembled several others of around the same size and perhaps the same species, which were pretty common in my February sample from the upper Yakima. Unfortunately, I misplaced the specimen before I could get it under a microscope for a definitive ID.
These widely varied terrestrial insects sometimes accidentally jump into trout streams. I have heard of trout being selective to them, but it is not very common. Anglers often call them jassids.
They are very small and are best imitated on hooks sized 22 or smaller. They can be treated the same, from the angler's point of view, as the Cicadellidae family of tree hoppers.
Start a Discussion of Membracidae
References
Swisher, Doug and Carl Richards. 2000. Selective Trout. The Lyons Press.