Pteronarcys (pronounced tear-a-nar-sis) dorsata is the largest stonefly found in the Upper Midwest trout streams. Its common name is the Giant Stonefly and out West it is called the American Salmonfly. The nymphs take 2-4 years to fully develop. So, these large morsels are trout food 24/7, all season long.
In the cold-water trout streams of northern Wisconsin, you can find these big nymphs underneath decaying logs and branches in the shallows. Pteronarcys stonefly nymphs feed on detritus and diatoms. Also, can be found in the fast rocky runs, but it is much easier to find the nymphs in the shallows living on woody debris.
This past September I captured and released several Pteronarcys spp. nymphs from the spot in the above photo. The nymphs were approximately 1-3/8″ long from their head to the end of their abdomen. They were all at least one year old and will eventually crawl out of the river next year. These large stonefly nymphs ranged in color from dark brownish to black.
Turning the big stonefly nymphs over, you will see their gill tufts between their legs and neck. Placing the dorsata stonefly nymphs in water you can really see their gill tufts.
When I set the nymphs back in the water they immediately flipped over on to their back. Their bodies were always slightly curled as the nymphs slowly sank to the bottom of the river. There was no swimming or struggling movements as they drifted towards the bottom.
Note: I did not confirm that they are Pteronarcys dorsata nymphs. Pteronarcys pictetii was also documented by Hilsenhoff. He found both stonefly species in about equal numbers in his Pine-Popple research study.