Like crayfish, shrimp, and sowbugs, scuds are Crustaceans. Amphipoda which contains three families. Gammarus and Hyalella are the families of greatest importance to flyfishers. Like insects, scuds periodically shed their exoskeleton but unlike insects, scuds don't have nymph or pupal stages. Baby scuds look like their adult counterpart, only cuter.
Scuds require a relatively large amount calcium to support their molts.
Scuds are found almost exclusively in alkaline waters and where ever they are found, trout actively seek out these nutrient rich packets of energy. In scud-rich waters such as Eagle Lake, I've seen trout bellies distended by hundreds of scuds. Scuds populations have been measured as dense as 10,000 creatures per cubic meter of water. A single pair can spawn half a dozen times in a year and produce 20,000 young. The single largest threat to scud populations is the rapid drawdown of tailwaters below dams.