The waters are so healthy that certain October mayflies are fat laden (that French Cooking) on French streams. Slippery When Fat!!! Or pleasingly healthy depending upon your tastes.
I can’t speak to all mayfly species, but those that still exist in October are smaller, like size 18’s heading into Winter. Small patterns, lighter tippets. Reverse the size come Spring for many mayfly species.
SwittersB & Exploring 