Written By Kris Brown.

Posted on October 10th, 2019.

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What if you were an aquatic insect preparing to spread your wings for the first time and saw this?

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It was early Sept. 2019 and Stefni Krutz and I were conducting our sediment breakthroughs research on a recently logged woodlot along the East Kill near Jewett, NY. We decided to stop for lunch by the creek. The low streamflow exposed slabs of bedrock, which were perfect for sitting.

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What a great spot for lunch!

While I tucked into my triple-stack PB&J sandwich, I noticed dozens of aquatic macroinvertebrate nymphal casings resting on the rocks. These casings are evidence of the macroinvertebrates’ transition from nymphs to adults. They left behind their old skin so to speak.

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Stonefly nymphal casing.

I found casings belonging to stoneflies and dragonflies, which indicate that this stretch of East Kill has good water quality. As a side, you can use aquatic macroinvertebrates to evaluate your stream’s water quality. Just flip over some rocks in the stream bed and see what is living there. Different macroinvertebrates have different pollutant tolerances, so the presence or absence of certain stream bugs is telling about water quality.

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Assortment of nymphal casings. Stoneflies are on the left. The dragonfly is the big one on the right.

I found two dragonfly casings tangled up in a spider web. Then I found a spider that was looking pretty large and it got me thinking. If I were a big spider, I’d hang out and wait for these tasty macroinvertebrates to hatch from the rocks. Heck, maybe I wouldn’t even wait for them to hatch. I learned from this University of Kentucky Cooperative Extension Service publication that spiders do prey on stoneflies and that stoneflies actually don’t fly very well.

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Dragonfly nymphal casing caught up in a spider web.

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The crafty predator.

What do you think? Are streamside spiders picking off these nymphs as they hatch into adults?
I’ve included a list of resources that I used to write this blog. Thanks for reading.


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