Living in a Caddisfly Case: Part Three

 By Cassie Sevigny

We were measuring rocks (and water chemistry, and the width of the river) because Stream Team was tasked with collecting data on how the 2014 Lolo fires affected the watershed. The size of rocks in pebble count matters because it tells us the overall composition of the streambed. Are they mostly large, easy for critters to live under? Or does the river bed consist of small, ground down, powdery material? Fine grain sand contributes to turbidity, or cloudiness.

Turbidity has serious implications for aquatic creatures. Fish and insects breathe the same form of oxygen we do, O₂. They pass water over their gills and filter out the oxygen dissolved in it. This oxygen enters the water from the air as the river burbles over rocks and moves through the landscape. After a fire, wind and rain loosen soil that tree roots and foliage previously protected. This loose soil, along with ash particles, washes into the river. These particles dissolve into the spaces between water molecules that would normally hold oxygen. The fish struggle to breathe the suspended soil underwater as we struggle on land to breathe the smoke.

Some insects, like caddisflies, are particularly sensitive to these changes. When picking up rocks for Pebble Count, or wading in a stream, finding caddisfly nymphs clinging to the underneaths of things indicates that the stream is relatively healthy.

When a caddisfly nymph hatches on a streambed, its instincts tell it all the ways a predator could crush its soft body. As a nymph, only its head, thorax, and legs have a protective exoskeleton, while its squishy abdomen stretches out behind it. Some species spin spider-like web-lairs for safety. Some choose to do nothing, content with their river maggot appearance. A few are gem-green, more akin to an aqueous inchworm. My favorite ones are like the hermit crabs of the river world: case-building caddisflies.

“Look for walking rocks,” I tell kids on WEN field trips, when we have emptied out a net of bugs into a tub of water. A case-building nymph uses its saliva-silk to bind pebbles together around its body. It uses whatever it can find to create its portable house. It might use grains of sand, pebbles of varying size artfully arranged, or perhaps even bits of twigs and stems for a boxy, striped look. Every so often I find lighter toned cases with glimmering rocks, as if the caddisflies were as enraptured with the sparkle as I was. If you don’t have pockets, incorporate the sparkle into your house!

The sparkly cases remind me of the French artist, Hubert Duprat, who kept caddisflies in captivity. He gave them only pearls, gold flakes, and turquoise. Those were the caddisflies of high fashion, crawling around with precious stone adornments.

Once the case is made, only the head and legs stick out, like black eyelashes, so it can crawl around and snag food. The caddisfly eats bits of plant and animal debris that collect on the bottom of the stream, like dust in our houses. Because of the black bodies, you can easily spot the half-inch-long caddisflies in a white tub. It’s the tiny ones that are tricky. That’s when my advice comes in handy. If you are patient enough to let the water settle, only the live insects will move. If you see tiny pebbles or grains of sand moving independently along the bottom, you’ve found a caddisfly!

When the caddisfly is old enough, they grow wings and move out, leaving the case behind. If you find one of these empty cases, you’ll see a tiny air hole on the back end. This tiny air hole is a perfect spot to string up the case on a bracelet or necklace. One WEN volunteer, Al, works at an insect taxonomy lab and brings us leftover cases to do just that.

“Where are these caddisfly cases from?” Deb asked during one of our jewelry-making sessions.

“King County,” Al said.

Seattle is in King County. They were from home.