These are pretty much always called Crayfish, Scuds, and Sowbugs.
Crustaceans are classified as one of those inconvenient intermediate taxonomic levels, the subphylum. The varieties of interest to trout anglers are in the class Malacostraca.
It contains the orders of scuds (Amphipoda), sowbugs (Isopoda), opossum or mysis shrimp (Mysida), and crayish (Decapoda). Crayfish and scuds are both extremely important trout prey.