Springtails: a Sign of the Season

On a recent snowshoe through the melting snowpack of the spring woods, Science Forum reader Eric Troyer's eyes darted downward. He saw black dots, as if someone had sprinkled the snow with pepper. Moose tracks and other indentations were speckled almost solid black.

He stopped for a closer look. The dots moved. Some jumped around. The pepper was obviously alive.

The creatures Eric saw were springtails, according to Stephen MacLean, professor of biology at the University of Alaska's Institute of Arctic Biology. Most people would call them insects if they got a close look at a springtail, even though they're not considered to be true insects because of the structure of their heads and mouths. They're about as long as the edge of a quarter is thick; they have six legs; and they have no wings with which to escape predators of the forest floor, where they live.

When threatened by spiders, centipedes, or humans on snowshoe, springtails live up to their name by jumping away, using a rigid, forked tail that folds under their body like a jackknife blade. The springtail releases a catch on the underside of it's body when alarmed, and the tail releases, hopefully catapulting the springtail out of harm's reach.

Springtails, also known as snow fleas, feed on the rich bounty of the forest floor, munching on decaying leaves, wood and other organic fare. MacLean noted their abundance in this column 15 years ago, when he calculated that with every step of his size 11 boots in the forest, he trod on about 2,000 springtails. Most of us never notice them except in spring, when their dark bodies contrast with the snow and the sight of an apparently warm-weather creature leaping about on a cold surface strikes us as being a little bizarre.

What seems even stranger is how MacLean speculates springtails reach the surface of the snow. Rather than crawling up from the soil, which during the winter would have been snuggly warm under a blanket of snow compared to the frigid air temperature above, MacLean said the springtails may spend the winter clinging to spruce trees. With recent temperatures well above freezing, the springtails possibly stirred from their winter dormancy and dropped down to the snow.

Why would anything choose to winter in a spruce tree and be exposed to temperatures such as -50 F? To understand, MacLean said, we first need to shed our "vertebrate homeotherm bias." Conditions that make us comfortable wouldn't suit a springtail, and vice-versa.

Because a springtail probably doesn't eat all winter, it might shut down its body processes altogether to save energy. Warmer temperatures, such as those near the freezing point that occur in the snow-insulated soil during a warm Alaska winter, may actually trigger the springtail's metabolism, which would waste energy. Wintering above the snowpack would keep the springtails' bodies cold enough to stay dormant until spring weather arrived.

But, MacLean added, these are all theories that need to be tested. Researchers have largely overlooked the springtails because of what they aren't.

"We are obsessed (to study) things that are warm and cuddly and furry," MacLean said, adding that springtails also haven't been thoroughly looked at because they're small and tough to identify.

As for the theory of springtails climbing up through the snowpack rather than dropping from spruce trees, MacLean doesn't see why springtails would waste energy climbing a mountain of snow. If they do indeed drop from spruce trees when warm temperatures return, maybe we should celebrate another sign of spring: while waiting for the geese to return, we can also tilt our heads downward to salute the leap of the springtail.