Snow flea circus

Friday, that lovely mild afternoon, was a good day for blog fodder. I don’t know if it was that my eyes were more open than usual, or if I was just particularly serendipitous in where I walked and looked. I had gone over to check out a snag a short distance from the trail (hooray for snowshoes!) because of what looked like interesting wood-borer patterns. I forgot all about the borers, though, once I got closer. The snag was covered in little black bugs. Though I’d never seen them before, I knew immediately what they were.

These are “snow fleas”, a species of springtail (order Collembola). I first learned about them two winters ago, the same winter that I started this blog. They were mentioned in the Stokes Guide to Observing Insect Lives that my mom had. That winter, last winter, and now this winter, I’ve watched for these so-called “snow fleas”, but without having ever encountered any, or at least not to my knowledge. So I was beyond delighted to finally find some.

Naturally I hadn’t brought along my macro lens, not expecting to encounter anything in the dead of winter that would require its magnification power. It was a long hike back to the house to get it, so I wanted to bring some home with me so I could get some better pictures of the critters. For a while in the fall I’d hiked around with one of my pill bottles (that I use for containing and chilling moths for photos) in my pocket, but had taken it out once the snow fell. I rooted about in my jacket pockets, and finally found the plastic wrapper bag from the 3D glasses used when I’d gone to see Avatar last weekend (loved it, btw), which I’d for some reason folded up neatly and tucked into my pocket. Good enough! I knocked a few individuals into the plastic sleeve and headed home.

I dumped my catch into a white mixing bowl with tall sides and set up my camera on a tripod to try to get some close-up photos. It wasn’t easy. In the warmth of the house, these critters were especially active, and they were tiny, most only 2-3mm (1/10th of an inch), making it hard to find them in the viewfinder. Added to that, they jump – the origin of the latter part of their name, “snow fleas”. They do look a little like fleas when you find them all together, small and dark and leaping about.

In fact, however, not only are they not fleas, but there’s been debate on whether they’re even insects. The general consensus now seems to be that they are in the Class Entognatha, which is a sister Class to insects (Insecta), both of which are contained within the Subphylum Hexapoda (translating to six-footed). Interestingly, six-leggedness seems to have evolved separately in insects and springtails, suggesting that the common Hexapoda ancestor was not itself six-legged.

The leaping is achieved through a spring-like mechanism, two long prongs at the end of their abdomen that are folded underneath and hook into place behind small catches. I gather the actual propulsion is very similar to how a grasshopper’s leg works, which I explained back in the fall. With the prong locked in place, the springtail contracts its abdominal muscles tightly, building up potential kinetic energy. Then they release the catch and the prongs spring out rapidly and push off against the ground, launching the bug into the air. I happened to catch a couple of individuals just as they were flexing their abdomen, moments before the disappeared out of my viewfinder. Note how their antennae curl, too, with the tension. :) Of course, beyond pointing itself in the general direction it wants to go, the springtail doesn’t have much control over its flight.

Here’s another one, which I think has just released the catch; I think the two “tails” you see at the end of the abdomen are actually the prongs, right before the bug leaves the scene. Too bad about the image quality; even with the aperture cranked wide open, in order to catch these guys in sufficient detail, and without them being a complete blur, I had to bump up the ISO setting to an unfortunately grainy 1600.

And even then, their little legs worked too fast to be captured unless they were standing still. I think I must have damaged this one when I knocked it off the tree – that looks like it might be one of the prongs, sticking out from the side of the abdomen. They’re just little structures, relative to the size of the critter, but they’re strong, and the kinetic energy the springtail can store is powerful.

And so what about the first part of their name, “snow fleas”? There seem to be two species of springtail, Hypogastrura nivicola and H. harveyi, that are most commonly encountered during the winter. Their life cycle is such that the adults are primarily active and observed in winter. In late winter or early spring they mate and lay their eggs in the leaf litter. The larvae feed on detritus during the warm months, finally maturing into adults in the fall. It’s unclear to me how long the adults live, whether they’re just around during the winter months, or if they exist all year long. If they are around in the summer, they would be rarely seen, blending in to the forest floor.

It’s also not really clear what the critters are doing out and about in the winter, wandering around on the snow. One hypothesis supposes that they are feeding on algae that’s growing at the snow’s surface, or on trees or snags (such as the one I found them on), but there’s been no evidence to definitively prove that. Another source said they feed on detritus, like their larvae, though that wouldn’t explain what they’re doing on the snow. Regardless of their purpose, the fact is that they’re regularly encountered as black specks on the snow surface, resembling (depending on the description you read) soot, gunpowder or pepper (or just random black specks, if you’re not the imaginative sort) scattered across the surface of the snow. You might not even realize they’re alive until you take a really close, good look.

Springtails are found on every continent. Yes, that means they’re also found on Antarctica, and, aside from potentially a few bird parasites, are the only Hexapods to live there. The reason they can survive in Antarctica, and on the snow in our northern winters, is because their blood contains a unique type of antifreeze protein not found in other insects. This antifreeze protects against ice-crystal formation within the organism to -6°C (21°F), several degrees below freezing. They still prefer to come out on warm sunny days, of course, and like to be on or near trees which gather and radiate warmth, but this is why they’re perfectly happy crawling around on the snow.

There’s been some research into using this protein to, among other things, help prolong the “shelf-life” of transplant organs while they’re between donor and recipient. Organs kept at colder temperatures will last longer, but currently can’t be brought below freezing because ice crystal formation in the tissues will puncture and damage cell walls. The most interesting thing about the springtails’ antifreeze is that it breaks down at warmer temperatures, unlike the antifreeze of other insects, so once the organ was transplanted into the new recipient the proteins would be flushed very quickly from the body.

Speaking of snow fleas on snow… My mom was up to visit this afternoon, and suggested we go over to the Perth Wildlife Refuge for a hike (I agreed – it was one of the places on my list of ten places I wanted to get to this year). I’m not sure what made me look, but I stooped to check out some black specks underneath a tree at one point – and they turned out to be springtails!

In this photo, the black speck I’m pointing to is a springtail. None of the other specks in the photo are, however, they’re simply bits of dirt and debris. Now I know why I’d never seen one! They’ve got great “camouflage”. The way the Stokes book – and also the websites I’ve read – described it, it sounded like I should be looking for a gray patch that looked like someone took the lid off a pepper shaker and sprinkled it in a small area. Actually, even though nearly every tree I checked the base of had a few springtails, they were always scattered, never in a dense enough formation to cause any noticeable darkening of the snow surface. Or, at least, not any more than the naturally-occurring debris did.

That said, they will, apparently, come together in groups like that when they’re on the move. My mom read me the section from the Stokes Guide when she got home this evening. In the winter they’ll sometimes take a notion to move from one place to another (perhaps to look for richer food sources?), and will aggregate into patches of up to a million individuals, which then, very slowly, move up to 25 m (82 feet) through the forest. If there’s not much snow, half the group may move along under the leaf litter. If there is snow, when they make camp for the night the ones on the snow surface will make their way to the base of trees, where the snow has melted away to expose the ground, and return down to the leaf litter to spend the night. Presumably if they’re not moving from point A to B they scatter out across the snow as I found them today.