The nymph of a dragonfly n of the family Corduliidae or Libellulidae, often used as bait.
1967 Racine Jrl.–Times (WI) 16 June sec C 3/4, Fond du Lac County . . rock flies and May flies good for perch. 1991 Reporter (Fond du Lac WI) 26 June sec D 1/1, Vern Lovejoy of Vern’s Bait Shop in Waupun reported taking 17 nice perch Tuesday morning using rock flies and tiny jigs tipped with waxworms. 2010 in 2020 DARE File—Internet WI, [YouTube video; description:] Catching helgramites in a pond in Wisconsin for fishing. [Transcription:] I’m out dipping hellgrammites. That’s what they are [showing a pair of them in his hand], they’re a mayfly larva. . . They’re double winged, bluewinged mayflies basically or dragonflies. And there’s also rock flies in here too and they’re the smaller version, the smaller dragonfly. 2014 Ibid seWI, As I know it the rounder ones [=dragon fly larvae] I call rock Flies and the longer ones are helgramites. 2020 DARE File—Internet seWI, [Advt:] Hellgrammites. . . Spider Hellgrammites AKA—Rockflies, Thunderbugs. . . Long Green Hellgrammites AKA—Dragonfly Larva.