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.
some biological populations do have built in variation in the timing of their development. this is a topic that is getting some academic attention.
A local guide once told me a tale about "winter sulphurs" on the Letort, and I believed him, though the whole thing sounded apocalyptic to me at the time. It would be very, very interesting to know what this bug is, though. Perhaps it's a very early invaria or dorthea. If the opportunity presents itself to capture one of these at some point, I will drop everything and to quote Spence, "run its ass down."