These stunning caterpillars were both found near bodies of water within a couple days of each other (separated by 200 miles). This fits the habitat bill, as the caterpillars are found mostly in deciduous and mixedwood forests, feeding on alder, birch, poplar, hawthorn, and willow. After topping off their fat reserves, the caterpillars will soon enter the pupa stage and will overwinter in leaves or debris, protected in their cocoons. According to BugGuide, this is an uncommon moth, but is widely distributed throughout the United States and southern Canada.
Photo by Sarah Fangsrud on 9/29/20 near Rattlesnake Creek, Missoula, MT