The group Bernald and I led in La Selva collected aquatic insects from several stream sites that varied in total discharge and water velocity (essentially a riffle-pool macroinvertebrate community study).   One of the coolest finds was this aquatic Lepidopteran larva in the genus Petrophila (Pyralidae), particularly since I had never seen any aquatic insect larva in this insect Order.  Interestingly, many aquatic lepidopteran larva are shredders/herbivores of Typha.