Natural History Mystery

What are these motionless creatures doing in a rocky stream?

These are the cast-off exoskeletons, or exuviae, of stoneflies! Most stonefly nymphs spend close to a year under water, shredding leaves or hunting prey. The nymphs molt from ten to twenty-two times, depending on the species. After each molt their skin is soft and white before hardening and darkening. With each molt they get slightly larger and look slightly more like an adult stonefly, their two pairs of wing pads gradually becoming more visible. 

Stoneflies go through simple metamorphosis – egg to nymph to a winged adult. In spring or early summer (or winter for some stonefly species), the nymphs crawl out of the water, secure their claws to a solid surface and begin to shrug off their nymphal skin for the final time. Squeezing through a split in the head and thorax, the winged adult wriggles to emergence. The adults live only one to four weeks hiding among streamside vegetation and mating – most species don’t even feed at all as adults. 

Stonefly exuvia can serve as a clue in assessing water quality. Stoneflies are one of the most pollution sensitive orders of aquatic insects and their abundance is commonly used as an index of aquatic ecosystem health, along with caddisflies and mayflies.