Our walks through the woods takes us across a small bridge over a stream, down a wooden walkway built over a wet area, and then on a flat, gently winding path that ends in a view of the lake. Because this path is just at end of our road, we walk it often, even more so now that our social lives have been involuntarily simplified by a pandemic, and so we can watch how it changes from day to day and through the different seasons. Last week we had a really good snow storm which left over a foot of snow on the ground. Before the storm, the ground was a mix of dark green fern leaves and patches of browns or reds from fallen needles and leaves, but now we walked through pure white on a narrow path packed down by the feet of other walkers.
On a closer look, perhaps not pure white. The path is very popular with dog walkers, and in prime locations, tree trunks, bends in the path, stumps poking out above the snow, the white has turned to patches of yellow, which my kids were quite happy to point out. In some places the snow was flecked with tiny black dots, scattered thinly in some places, but strangely concentrated in footprints and pockmarks in the snow around tree trunks. At first I thought maybe these were bits of seeds or bark fragments scattered by squirrels or birds in the tree branches, but the way they clustered didn’t seem like how debris would fall from higher up in the tree, so I took a closer look… and I saw one of the little specks jump right off the snow! All those little black specks were tiny living things, sitting on top of at least 10 inches of snow!
I am always curious, and especially so about living things that surprise me. Tiny critters hanging out in winter on top of snow, jumping like teeny little popcorn kernels exploding… that surprised me! How are these animals surviving in the cold, and what could they be eating in freshly fallen snow?? More surprising, the wasn’t the first time I’d seen insects right in/on snow. Last winter, on the same path, I saw small black insects in the snow piled on the railings of the bridge over the stream. I dug through my image files to find my pictures of the bugs from last winter ….
NOT the same critters at all! Both small, both black, both on snow… but the one from the bridge was larger, maybe 2-3mm in length, with obvious legs and more narrow body, while the ones from this week are tiny, maybe 1mm long, with a wider body and tiny little legs. I looked up “snow bugs” online, and quickly found my tiny little hopping animals, which turned out to in fact be a type of springtail. Springtails are famous for being able to handle really extreme environments and are one of the very few animals that can survive on the Antarctic continent.
I’ve been trying to avoid using the word ‘insect’ in this post, because it turns out that springtails are not insects. While they have six legs like insects (they are hexapods), they are actually a separate group of animals with their own unique traits not shared with insects. I learned a new word as I read about them – they are cryptozoa (not to be confused with ‘cryptids’ like Bigfoot or the chupacabra), tiny animals that live in soils and dark places and are therefore rarely noticed by us. In fact, springtails might be the most abundant six legged critter on the planet, with up to 75,000 per square meter, or roughly 8,000 per square foot) in the soils of some habitats (!).
A springtail’s claim to fame, other than their super high abundance and ability to handle really challenging environments, is that they basically have a spring loaded tail (hence “springtail”). This forked tail (or “furcula”, which oddly enough also refers to the wishbone found in birds) normally curls under their body, flat along their abdomen. The springtail can explosively pop out tail, ejecting themselves into the air seemingly instantly into the air, giving their movement a flea-like quality (hence their other name, “snow fleas”). For a fantastic video on how they move, follow this link to a clip from the BBC’s “Life in the Undergrowth” series, narrated by the wonderful David Attenborough.
While they might remind us of fleas, springtails do not bite. In fact, they really don’t interact with humans at all, preferring to feed on bits and pieces of organic debris in the leaf litter in soil. Why then do they show up in such large numbers on the surface of snow? Unsurprisingly, springtail behavior is not an intensely studied topic. Some sources say they are looking for new food sources, which I don’t find a very satisfying explanation, given that organic material in the soil should be just as available to them whether covered by snow or not. They do seem to like the bases of trees, though. Perhaps they are finding some tiny little bits of food in the snow that have dripped or fallen off the tree trunks above.
What about my other snow insects, then? The more I read about springtails, the less like a springtail my bridge-railing bug looked. Its body shape was just wrong, especially with that last pair of legs so far back on its body. But through the wonders of the Google image search engine, I soon found a match, a type of insect called a stonefly. Stoneflies are prehistoric-looking insects, and in fact are similar to fossils of insects as much as 300 million years old. Their generally creepy looking appearance could be do to them being fairly closely related to a number of insects we humans generally do not like, such as cockroaches, lice and earwigs.
Like their somewhat more distant relatives, the dragonflies and damselflies, stoneflies spend a good portion of their lives living in water, not as their adult form (which is what I saw), but in a stage called a ‘nymph’. In the insect world, a nymph is an immature form of an insect that looks quite similar to the adult form, but smaller. This is in contrast to “larvae”, which are immature forms of an insect that differ extensively from the adult form, like caterpillars from butterflies. Stonefly nymphs live in actively running water, like in streams or rivers, where they feed on bits and pieces of debris caught on the streambed. They quickly die if trapped in stagnant water or if the water becomes too warm, both of which reduce the amount of oxygen in the water, and are very sensitive to a variety of pollutants. Because of this sensitivity, stoneflies are often used as an ‘indicator species’ for assessing water quality. While we might not find them as cute as a fox, or as impressive as a bald eagle, their presence on our walking path bridge means our stream is clean and free of pollutants, so I’m super happy I spotted them!
Follow this link to see a video with winter stoneflies next to a stream (not my video): https://youtu.be/y8lCQqXEJKw
Small winter stoneflies, the kind I probably saw on the bridge, live almost their entire lives in the water, emerging only to mate and lay eggs. Like the springtails, they are cold tolerant, and like springtails, these particular insects don’t fly (at least, not much). Adults emerge from the water between midwinter and early spring and walk up and away from the water to find a mate. The males apparently drum their abdomens on the snow, using the vibrations to attract females. After mating, females head back to the water to lay their eggs so that the generation of nymphs can hatch into their aquatic habitat. Why do winter stoneflies emerge and mate in such cold weather? The most likely answer is to avoid being eaten. What better time of year for a non-flying insect to crawl out of the water and stand around looking for a mate?
Science in Translation 