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.
Thyridium: A small, hardened sensory spot lying within the thyridial cell of the wings of some caddisfly species, similar to the nygma but in a different location.
This other word references the same concept: thyridium.