For mayfly nymphs and stonefly nymphs to develop from an egg to a mature nymph they go through several instars. Each instar period ends when the nymph sheds their existing exoskeleton. Then over 2-3 days a new exoskeleton hardens, which marks the beginning of the next instar period.
Depending on the mayfly nymph species they go through 11-13 instars as they develop from an egg to a mature nymph. Most mayfly nymphs in our trout streams develop over a 1-year period.
Whereas many of our stonefly nymphs take 2-3 years to fully develop going through 12-23 instars.
Being an aquatic bug nut I spend a lot of time turning over rocks and sticks in our trout streams all season long. Finding in our trout rivers nymphs that have recently molted its exoskeleton is amazing to see.
When the nymph first sheds its exoskeleton, their body is pale and soft until the new exoskeleton hardens. This marks the beginning of the next instar period. Once the nymph’s new exoskeleton hardens it color changes to what we are used to seeing nymphs look like in the river.
About 10 years ago, while I was turning over rocks in a riffle and I spotted a Stenonema mayfly nymph that had recently molted.
The other day I turned over a log in the river while looking for Pteronarcys dorsata nymphs. To my surprise I found a stonefly nymph that had recently molted, and the new exoskeleton had not hardened yet.
You can see how vulnerable the nymphs are after they first molt their exoskeleton. Luckily, they stay hidden underneath the logs and rocks to avoid being an easy meal for trout and chubs in the river.