The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
This is my understanding over coffee (without checking in a book).
...so-called drying process, post-molt, but it may be more akin to polymerization than to drying proper.
I actually thought about making a video of it this spring.