Mayflies live underwater for a year or two as nymphs, then emerge for a single day as winged adults to mate and die. That emergence is the hatch — a ten-minute to three-hour window where trout feed selectively.
Mayfly families anglers care about: BWO (Blue-Winged Olive), PMD (Pale Morning Dun), Trico, Hendrickson, Sulphur, Green Drake.