Winter stoneflies active on snow in winter because it's mating season for them
This past weekend we had some particularly warm weather. On Saturday, a day that was sunny but not as blisteringly hot as Sunday, I found tiny bugs wandering around on the snow.
These weren’t as small as snow fleas, those tiny hopping arthropods that swarm on warm days and max out at perhaps 1 to 2 millimeters (that’s approximately a 10th of a centimeter!). These were larger, perhaps a centimeter or more long. Instead of hopping like the diminutive springtails, they were trudging around - that’s what it looked like to me – trudging and not enjoying it.
I saw them first on the snow along the floodplain of the little river (closer to a stream than a river) that runs behind the house.
At first, I thought they might be tiny earwigs with their elongated bodies and two trailing appendages (sort of like the pincers on an earwig), but upon closer inspection realized they were some sort of fly. The trailing appendages were slimmer than the earwig’s pincers and their wings were long, with prominent veins, folded over their backs.
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I finally realized that they looked like a small version of stonefly, an insect that spends its larval life as a top predator in our local streams, emerging as a winged adult only to mate, lay eggs and die. The adult part usually happens in summer. What were these up to?
One of my go-to guides for seasonal nature oddities is Mary Hollands’ website “Naturally Curious” and there I found my bug, a winter stonefly, commonly called a snowfly.
Here’s what she had to say: “Stoneflies spend the larval stage of their life in streams, but as adults they are terrestrial. When the larvae mature, they leave the streams they grew up in, split their larval skins and emerge as winged adults, ready to mate. Most species mature in warmer months, but there are two families, referred to as winter stoneflies, that emerge at this time of year, perhaps because of the scarcity of predators. You can often find these cold-hardy stoneflies crawling around on top of the snow near streams.”
They aren’t just found right next to streams. As I was formulating my hypothesis that these might have something to do with my stream, I checked uphill, around the garden, and found them there as well. So they will travel far!
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When my students and I do stream surveys, one of the bugs we always hope to find in streams is the stonefly larva. These insects are incredibly sensitive to the oxygen content (and the pollution levels) of water. High levels of dissolved oxygen are a good indicator of stream health. Find stonefly larvae in your stream and you most likely have a healthy stream. The fact that these were crawling around my woods in winter means that my stream is a good one. But, why are they out and about in winter?
If you think about it, most insects are hibernating or in some state of torpor or suspended animation as they wait out the cold of winter, waiting to hatch out in spring, mating and growing. This is what attracts all those warblers to New England: Food! So, why would an insect be trudging about in the snow in winter? Sex!
Winter stoneflies emerge this time of year, perhaps, as Mary Holland said, because of the scarcity of predators, but with one thing on their minds – finding a mate. The males look for mates by drumming. They tap their abdomens against a surface – like a log or twig – in a specific pattern. Females drum back. This is how they find each other. They survive the cold of winter by having antifreeze in their blood. Their dark coloration also helps absorb the warmth of the winter sun. This is also what made it easy to see them.
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These winter stoneflies have probably been out and about for the past month or so, I just failed to notice. This makes me rethink my signs of spring. Spring is typically the mating season for many local animals (and plants). But we can’t forget those who do things differently like these tiny stoneflies. I was wrong. The stoneflies trudging through the snow were doing so joyfully (I like to think), looking for the perfect log to drum from, looking for mates.
Susan Pike, a researcher and an environmental sciences and biology teacher at Dover High School, welcomes your ideas for future column topics. She is looking for readers to send her the signs of spring they're noticing so she can document them on her website pikes-hikes.com. Send your photos and observations to spike3116@gmail.com. Read more of her Nature News columns online at Seacoastonline.com and pikes-hikes.com, and follow her on Instagram @pikeshikes.
This article originally appeared on Portsmouth Herald: Winter stoneflies active in winter because it's mating season for them
