Stonefly Exuvia and Caddisfly Naiads

Subject: Aquatic bug?!
Location: Glacial river, base of mt Rainier
July 31, 2017 8:24 pm
Hello! We were out playing in a very cold glacial river at the base of Mt Rainier in Washington state and came across these guys today. There were hundreds of them on rocks in the water, but only a few this sprawled out and large outside the water.
Signature: Alexa

Stonefly Exuvia

Dear Alexa,
Your images document two different, unrelated aquatic insects.  The image of the one “sprawled out and large outside the water” is actually the exuvia or cast-off exoskeleton of a Stonefly, and the “hundreds of them on rocks in the water” are Caseworms, the larvae or naiads of Caddisflies.  Larval Caddisflies are known as Caseworms and according to BugGuide:  “Most species live in a mobile case constructed from plant material, algae, grains of sand, pieces of snail shells, or entirely of silk. The case is held together with strands of silk secreted by the larva. In some species the case is attached to a rock, log, or other underwater surface; a few species have no case and are free-living.”  The cases on your individuals appear to be constructed using grains of sand or small pebbles.

Caseworms: Caddisfly Naiads
Caseworms: Caddisfly Naiads
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bugman

BugMan aka Daniel Marlos has been identifying bugs since 1999. WhatsThatbug.com is his passion project and it has helped millions of readers identify the bug that has been bugging them for over two decades. You can reach out to him through our Contact Page.

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