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.
Size - (excluding tails) - 12 mm
Status at time of photo - preserved, but photographed within days of capture.
W. C. Day, Aquatic Insects of California differentiates morrisoni nymphs by submedian pale dots on tergites 8 & 9, which look to be in evidence. This is one of the "super-hatches" on this river that can be prolific March through April.