Winter Stoneflies

2013 December 7

In March of 2013, we had been sledding at the Nara Nature Center and Trails at the east end of town, and had happened to spot a couple of small black insects walking slowly over the surface of the snow. These were winter stoneflies, and they had come from the Pilgrim River that flows through the park. Since the last time I photographed them was in 2008 and the pictures weren’t very good, I’d been wanting to rephotograph them. So, I went back to the river on March 30 to collect some more specimens. I collected two different kinds that day, one with wings that just barely covered its abdomen (and could probably fly);

And one that was smaller, and had only stubby wings (and most likely could not fly).

Both of them were hard to photograph indoors, because they are heavily cold-adapted and in the warm conditions they overheated and ran like crazy all the time. They were easier to photograph if I put them back in the snow. They kept walking around in the snow, but did it more sedately, so I could keep up.

But, my first batch of pictures of black insects on white snow didn’t work out very well, so we all went back to Pilgrim River again on April 8 to get more specimens to try again. This time, we actually walked all along the trail beside the river (even though it was still under a couple of feet of snow). There were still the smaller, stubby-winged stoneflies running around;

and there were also several much larger ones, with full-sized wings longer than their abdomens.

How much bigger were the big ones than the small ones? Well, here’s a comparison picture. The big ones were about an inch long, nearly the size of average-sized crane flies.

So, anyway, I would judge that we have at least three kinds of winter stoneflies here – small with stubby wings, medium-sized with wings that just cover the abdomen, and large with wings that extend well past the abdomen.

The big one with the long wings looks like something in the genus Taeniopteryx, and in fact looks practically identical to this specimen of Taeniopteryx burksi.

The smaller ones with the rudimentary wings are probably male Small Winter Stoneflies in the family Capniidae. The medium-sized winged one in the first picture may be a female of the same species as the little ones. These look most like one of the 42 species in the genus Allocapnia.

One of the big reasons that winter stoneflies are of interest, is that their nymphs are aquatic and breathe through gills, and their gills aren’t very large. Which means that they need really well-oxygenated water in order to get enough oxygen to live. And the things that make water well-oxygenated are when it is:

Cold (because gases like oxygen are more soluble in cold liquids than hot liquids)[1];
Fast-flowing and turbulent (because the oxygen has to travel from the surface of the water, and turbulence both make more surface area, and quickly carries dissolved oxygen deeper into the water); and
Relatively free of rotting material and organic pollutants (because the organisms that cause decomposition consume oxygen, and so if there are a lot of them growing in the water they quickly use up all the oxygen).

So, the fact that there are so many stoneflies in the Pilgrim River shows that it is clean and well-oxygenated, and lots of things can live in it, from fish on down.

The winter stonefly adults are also well-adapted to being out in the winter. The black color probably helps them warm up quickly in the sunlight, and they are cold-adapted enough that they don’t need to be warmed up much to function. Sure, it makes them stand out against the snow, but most of the insect-eating birds are migratory, and don’t arrive until a month or more later than the time when the winter snowfly adults are emerging. So they’re pretty safe.

[1] If you were to just go by everyday experience dissolving things in water in the kitchen, where things like salt and sugar both dissolve better in hot water than in cold water, you’d expect that hot water would also dissolve more oxygen. But, it turns out that oxygen, nitrogen, carbon dioxide, and other gases are actually much less soluble in hot water than in cold water[2]. This is due to the thermodynamics of the system, specifically the entropy change. See, one of the factors that determines whether a reaction (like dissolution) will happen, is whether it will increase the entropy (a measure of how randomized the system is). Solids are very highly ordered with immobile atoms and have low entropy; liquids have more mobile atoms associating randomly with each other and are a bit higher entropy; things dissolved in liquids are even more disordered and have even higher entropy; and gases are mainly just rocketing around bouncing off each other, and have dramatically higher entropy than all the other states. And, reactions that increase entropy become more favored as the temperature increases, because hot atoms move around faster and so are more disordered than cold atoms. So, the upshot is that dissolution of high-entropy gases to produce lower-entropy solutions is more favored when the liquid it is dissolving in is colder.

[2] At least, gases are more soluble in cold water right up until it freezes. But, ice has very little room in its crystal structure to include other kinds of atoms, and so the dissolved gases get forced out again as the ice crystals grow. This is why the ice cubes in your freezer are cloudy with gas bubbles, and why making clear ice requires that the water be deaerated first. This is also part of the reason why, if you put a can of soda pop into the freezer, it is likely to explode[3]

[3] Speaking of exploding soda cans, here’s something I found in my old computer files that I wrote some years ago (sometime in the late 1990s), considering the burning, critical question of “Why does a soda can explode messily when it gets left in the freezer?”

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