Two species of importance are T. stygiatus and the somewhat larger T. allectus. Nymphs prefer the silty gravel of slow to medium riffles. Hatches begin in late June or early July, and continue into October. Tricos are early morning hatchers starting as early as 5:00AM. Feeding activity is usually the heaviest between 8:00, and 10:00AM, winding down by Noon. This hatch is a real puzzler on the Upper Delaware, at times large numbers of trout can be found feeding on duns and spinners alike, but on other days when present in large numbers fish fail to raise to them.