Mayfly larvae have long filamentous cerci. Typical insect cerci function as sensory organs. However, the mayfly, Ephemerella subvaria when confronted with a stonefly or other predator will raise its cerci and its abdomen in a “scorpion” pose. This pose has a deterrent effect on stoneflies. Stoneflies, back off, go around or otherwise avoid the mayfly. Stoneflies do not readily attack and eat these mayflies, suggesting that they are defended in some way against predation and their scorpion pose is a warning display. Warning displays by terrestrial insects often use colorful patterns. In the murky water where colors can appear dingy, a physical stance may be more effective warning display.
*Barbara L. Peckarsky. Mayfly Cerci as Defense against Stonefly Predation: Deflection and Detection. Oikos, Vol. 48, No. 2 (Feb., 1987), pp. 161-170.