It can be found in a slow moving water or lakes. Nymph swims in a distinctive style. Unlike a mayfly nymph, it twists its abdomen horizontally just like a snake.
When it hatches, it moves to shallow and crawls up anything from grass, tree to even a fisherman. It is not unusual that all your wader are covered with crawling damsel nymph if they are hatching in a big number.
When it matures, male becomes red and female green. It is quite hard to fish when trouts are aiming for oviposing couple which hits a surface with its tail. Some trout does not respond well to a fly that sit on the water.