Stoneflies are the largest of the trout stream insects, and also among the easiest to identify.  For the nymphs, the two large antennae, two tails, and three tarsal segments are a dead giveaway.  The adults sport two sets of wings that fold flat on top of the body when the insect is at rest. 


For more details check out these websites: 



Here are some photos of specimens I have collected from various streams in Colorado: 

             STONE FLY NYMPHS