I assume that when you say stone nymphs, you mean Stone Flies? There are species of Stone Flies that hatch throughout the year. Right now and through the summer you have Yellow and Lime Sallies, yellow a # 14 and Lime a # 16. Later in the summer and into the fall you'll find there are few Stone Flies hatching, but because they typically remain in the streams for up to 3 years Stone Fly Nymphs are available to trout year round. You'll have to study up on them beause there are quite a few and it depnds on where you are fishing as to what species you'll find.