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Hardy Winter Stoneflies Thrive in Cold Conditions

Other than the spiders diligently eating tiny bugs in our homes all winter long, we usually associate bugs with warmer weather and give them little thought in the winter months. 


A winter stonefly on snow.
A winter stonefly. (Photo via Shutterstock)

Bugs are a diverse group of animals, and how they handle winter is just as diverse. A few will migrate to warmer weather. Some leave only eggs hidden somewhere waiting to hatch in spring. Many will enter diapause, a hibernation-like state where they stop all activity till the weather warms up. But the winter stonefly completes its life cycle, entering adulthood, mating and laying eggs, all in the winter months.


The stoneflies


Winter stoneflies are part of a large group of bugs called, you guessed it, stoneflies. They belong to their own order, called Plecoptera or “straight wing,” named for how their wings lay straight behind them. Even though adults have wings, they are not great at flying and spend most of their time crawling instead.


 

Words to know

Crevice: A narrow opening, especially in a rock or wall.

Diapause: A period of suspended development in an insect.

Diverse: Showing a great deal of variety.

Hardy: Capable of enduring difficult conditions.

Nymph: An immature form of an insect.

Pristine: Clean and fresh as if new.

 

All stoneflies have similar lifecycles, living much of their lives as nymphs under water. Then, like dragonflies, they crawl up from the depths and pop out of their “skin” (exoskeleton). They emerge as adults with wings, leaving an empty nymph shell behind. 


Whether in the nymph or adult phase, stoneflies are drab dark brown, dark gray or black in color. They are a smaller bug, only slightly larger than a half-inch long, about the size of a Cheerio. They are considered “shredders,” which means they tear their food up into smaller pieces to swallow. Their diet consists of dead organic matter or living aquatic plants and algae. 


Winter residents


There are 65 types of stoneflies native to Illinois, and 20 of these species are winter stoneflies. Winter stoneflies are unusual because at the time they choose to emerge it is very cold outside. In fact, it is during the hot summers that these nymphs will burrow into the ground to wait out the warm weather. When they emerge between the months of November and March, the empty cases of stonefly nymphs can be found on rocks or bridges near the water. 


A stonefly on soil.
A stonefly. (Photo via Shutterstock)

So how do winter stoneflies avoid freezing in colder months? They have a few adaptations and habits that can help them. Their dark color allows them to absorb warmth from the sunlight. Walking on their tippy toes keeps their tiny bodies from touching cold surfaces. Hanging out in little crevices or pockets in rocks and riverbanks helps them avoid strong winter winds. And finally, they can swap out fluids in their body for glycerol. Glycerol is a naturally occurring alcohol, and alcohol does not freeze in this climate. While other insects that are waiting out winter also replace the water in their bodies with anti-freezing glycerol, adult winter stoneflies can increase their amount of antifreeze if need be. 


The adult’s purpose in emerging in the snowy, icy cold of winter is it to find a mate. Males begin by making a drumming rhythm by tapping their butts to the ground. Females respond by drumming back. Listening to the music, they find each other and mate. The female will then lay eggs on the surface of the water, whether liquid or ice. Eventually the eggs will settle on bottom of the river. Then little nymphs will hatch, and the cycle begins again. 


Wandering to Will County?


Stoneflies can only live in pristine (pure and unpolluted) waterways. They are common in southern Illinois and in the middle fork of the Vermillion River less than two hours south of Will County. They can also be found in snow-covered waterways in Wisconsin and Michigan. Because animals are not truly restricted to the ranges we give them, it is not impossible to imagine seeing one of these hardy little bugs in our neck of the woods. And what a great sign that would be!


Stoneflies are bioindicators. That means they are a living thing that tells us something. They can tell us about the quality of the water because they only live in environments where the water is very clean. There are four separate categories of tolerance to pollution, and stoneflies are in the group requiring the best quality. 


So as you enjoy winter walks and pass over bridges or walk along river banks, pay attention for winter stoneflies crawling along the icy waters. Or look on hard structures for the empty nymph cases they leave behind. If you see one, cheer. It means the water you are near is super clean!

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