The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
In angling books, Leptophlebia mayfly nymphs have a reputation as poor swimmers. In reality, they're very adept swimmers, much moreso than most other mayfly nymphs categorized as "crawlers."
The clumsy-looking mayfly nymphs of the genus Baetisca are surprisingly good swimmers.
Water boatmen are excellent swimmers, and you can see here how they use their oars to push themselves through the water, a motion easily imitated by the fly fisher.
Damselfly nymphs are among the most effective invertebrate predators in freshwater. You have to see one in motion to get a feel for their slow, deliberate style of stalking.
These nymphs may be the best swimmers of all North American mayflies.
Dragonfly nymphs propel themselves through the water with a miniature jet engine, taking water in below their mouths and shooting it out their back ends. You can see the ripples from the jet out this one's back in this video.
This little Ephemera nymph swam around for a while and tried to burrow into the sand in my photography tank.