Springtails (Collembola)

Swarming Snow Fleas

Hiking up Saddleback Mountain in the Adirondacks, I was startled to see a pink puddle in the middle of a trail that was otherwise white snow and ice and black mud. In the puddle and on the snow surrounding it were tens of thousands of tiny reddish creatures.

I had heard of such things, and so I suspected I was seeing a swarm of springtails. I assumed they were mating. According to Ken Christiansen, however, the springtails weren't having a good time. They were trapped.

There are few, if any, people in the world, who know more about springtails than Dr. Christiansen, professor emeritus of biology at Grinnell College in Iowa. Despite the fact that springtails are one of the earth's most numerous and successful groups of animals, most people don't even know what they are, never mind what they might be doing in a winter puddle, 4,000 feet up the side of a mountain.

Part of the reason people don't know much about springtails is that most are smaller than this letter "s." Many are far smaller. Under a magnifying glass, you can see that springtails are vaugely insect-like. They have six legs, for example.

Evidence about the evolution of arthropods makes it fairly certain that springtails and insects share a common, six-legged ancestor. But the two groups split off from each other so long ago that most experts now consider the springtails to be a lineage quite separate from insects.

springtailUnlike most insects, springtails don’t have wings. What they do have is an appendage called a "furcula" near the end of the abdomen. The furcula is held like a spring against the bottom of the abdomen by a kind of latch.

When a springtail wants to move fast to escape a predator, the furcula springs downward, propelling the tiny not-quite-an-insect into the air. Springtails that live under the ground often have a greatly reduced furcula, but those that live above ground are famous, among those few who know about them, for the power of their leaps. A typical springtail leap would be the equivalent of a man jumping over a 12-story building.

"If springtails were the size of cats," Dr. Christiansen imagines wistfully, "there would be whole zoos dedicated to them. They are so colorful and interesting!" Imagine a zoo filled with cat-sized animals that could leap 3- or 4-story buildings.

Dr. Christiansen's frustration stems from the fact that because springtails are so small and innocuous, few people ever get to see how beautifully decorated they can be. About the only time people ever notice springtails is when they see them jumping around on top of snow. It is this phenomenon which has earned them the nickname "snow fleas."

But despite the fact that we only notice them on snow, springtails are everywhere.

There are more than 6,000 species, found on every continent, including Antarctica. They live in forests, in swamps, high in the Himalayas. They can live several feet down into the soil, at the top of trees, and everywhere in between.

Most springtails, however, live in the top layers of soil or in the leaf litter on top of the soil. It has been estimated that there are as many as 3 trillion springtails in a single acre of temperate forest.

In fact, Stephen Hopkin, at the University of Reading in England, who recently authored one of the few major books on springtails, believes springtails are the most abundant insect-like creatures on earth. Dr. Hopkin says, "There are many more species of beetles, but springtails have many more individuals. There are more springtails than even ants."

With the exception of a few species that have adapted to deserts, springtails like very damp places. On the relatively rare occasions when springtails get inside houses, they don't usually last long because our home environments just aren't wet enough for them. Most springtails feed on fungi and decaying vegetation and play a major role in the natural process of recycling nutrients.

With the exception of a single species in Australia, springtails aren’t known to feed on crops. They don't infest our houses. They don't bite. It appears that, unlike every other major group of arthropods, springtails have never evolved parasitism.

No one really knows why not.

It's possible that springtails, early on in their history, latched on to such a good formula that they just haven't had enough evolutionary incentive to change. As Ken Christiansen points out, all the types of springtails that have been found trapped in amber fossils from 45 million years ago are still around today.

One thing springtails do extremely well is live in cold conditions. Right now, in New England, when almost all insects are hibernating or surviving as eggs waiting for spring, there are springtails by the millions of trillions feeding, growing, and multiplying under the snow.

In our area, some species have both a summer form and a winter form. The summer form is better suited to withstand heat and drying, and the winter form is better adapted for cold weather and wetter conditions.

It's not that any one individual switches back and forth between the summer and winter forms. What changes is the kind of adult that develops from eggs laid at different times of year.

The winter form of some springtails can be very well-adapted to living under snow. And with so few of their usual predators active this time of year, their numbers can grow explosively. Sometimes they come bubbling up to cover the surface of the snow.

Why they do this is a mystery. Ken Christiansen believes the overpopulated springtails simply crowd each other out.

He suggests that the pink puddles I saw in the Adirondacks were the result of springtails erupting nearby. They were jumping around on the snow and some landed in the puddle by mistake. Once there, they were trapped.

It's not clear that those that avoided the puddle fared much better. Ken Christiansen believes that most "snow fleas" aren't going anywhere in particular. They just dry up and blow away.

But who knows? One of the other amazing tricks some springtails have mastered is the ability to dry up completely, and then revive when rewetted. Instant arthropods.

Given the incredible adaptability of these tiny creatures, it wouldn't surprise me if some of the springtails that get trapped on top of the snow simply dry up and blow away, hoping to settle in a damp spot where they can start over. That pink puddle of snow fleas I saw in the Adirondacks may just have been the ones who found a little more moisture than they could handle.  




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