Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
Diet: Other insects, small fish
This is pure trivia, and non-hellgrammite related, but Brett's comments on the discards of bats and nighthawks reminded me of an experience on the Delaware. I was fishing with a guide and we saw several mayfly wing and thorax combos floating along with missing abdomens. At first we hypothesized that a small trout must have bitten the abdomen right off. Then we saw a dragonfly intercept a sulphur mid flight, chew up the abdomen almost instantly, and let the wings and thorax fall, helicoptering like a winged maple seed. This is probably not news to the entomologists here, but some everyday anglers might not have yet noticed this phenomenon. Gonzo, do we have a wing and thorax tie yet?
I haven't found trout to be that interested in dragonfly discards, but you never know. I am reminded of an evening on Mud Run when my fishing partner's fly mysteriously ascended skyward, towing a good portion of the tip of his fly line. His fly, while having a minimal effect on the trout we were pursuing, was sufficiently convincing to a large dragonfly--a testament to the powerful flight and lifting capabilities of these insects!