Early black or brown stones can hatch in Jan. or Feb. in the East on warmer days. They develop into adults in the stream and walk out along the bottom. The so called "hatch" are really the females returning by flight to the stream surface as ovipositors. This generally takes place in early afternoon on sunny days.This slowly gets earlier as the day time lengthens so that it is about 11:00 AM by the end of March.