Snow fleas: Now You See Them, Now You Don’t

Snow fleas: Now You See Them, Now You Don’t

Illustration by Adelaide Tyrol

The other day I noticed snow fleas dusting new snow around a maple trunk. These minuscule creatures — broadly known as springtails — are out early this year, and I didn’t hear them coming. Not that I could. But I like to imagine the thunder of millions of tiny footfalls as they migrate upwards from the snow cover, pushing and jostling as they climb. Just a short while ago, they were scattered below, on the ground, quietly eating organic tidbits. 
Six thousand species of springtail populate the world, and the snow fleas we see in New Hampshire and Vermont are but one variety. They have cousins living afar in leaf litter, tree canopies and on rocky shores. They can be found in dark caves and in sunny meadows, and on ice in Antarctica and warm lava on a Pacific island.  They are practically everywhere. They have been around a long time, too, but have changed little over the ages. Fossils reveal springtails existed 400 million years ago.
Snow fleas are elusive but staggeringly populous. More than 100,000 of them can live comfortably in one square meter of soil. Here in Northern New England, it’s their sudden emergence that catches our attention.
Sometimes so many snow fleas appear that the snow seems carpeted with ashes. Approach them, and the entire mass rises in a dark cloud, swirling chaotically before descending to coat the surface again. When there are fewer snow fleas, you might see only tiny black dots that suddenly vanish in your presence.
The scene can be so strange you might think you’re hallucinating.
Catch one if you can, and check it out. Under strong magnification, you will see a lumpy and segmented body with bluish reflective scales. The snow flea, only one-sixteenth of an inch long, lacks wings, but it has three pairs of legs. It also has two antennae and two clusters of eyes, 16 in each.
Turn the snow flea over, and you will see something really unusual. Beneath its abdomen is one of the most extraordinary devices found in the animal world: a lever-like appendage held under tension by a two-hook clasp. It’s this appendage that gives the snow flea its incredible jumping ability.
To understand how it works, think of a common spring-loaded mousetrap. Touch the trigger holdings the peanut butter, and the wire bar slams shut. Now gently place the cocked mousetrap upside down on the floor. Tap it with a pencil, and the trap snaps and springs into the air. Finally, imagine tens of thousands of mousetraps flying in the air. That approximates what happens when you approach a bunch of snow fleas.
Undisturbed, the snow flea walks on snow or ground pleasantly feeding and going about its business. When you arrive, it leaps hundreds of times its own body length (think of a human jumping over the Empire State Building), followed by just about every other snow flea in the area. 
Your arrival was just detected by a snow flea’s eyes or antennae, and a nerve impulse was sent to the clasp, which instantly released the appendage that catapulted the little creature out of danger.
After landing, the clasp will quickly grab and hold the appendage while internal (hydrostatic) fluid pressure builds up until the lever again becomes a cocked spring.
The spring-like mechanism is not the only unusual feature on the underside of a snow flea. The tiny creature has a tubular peg used for “drinking” and taking in oxygen. Rapid absorption of moisture is necessary to rebuild that hydrostatic pressure in the lever.
For snow fleas during much of the year it’s a relatively uneventful scene down at ground level. Like other springtails, snow fleas spend most of their time feeding on soil bacteria, fungi, algae, pollen and organic material. Sometimes they devour microscopic animals like rotifers and nematode roundworms. (During their brief appearances on snow, they mostly eat pollen and algal cells that have settled out of the air.)
In spring, like many creatures, they mate. The reproductive process isn’t dramatic: Males and females are seemingly oblivious of each other. Males deposit stalked droplets of sperm on the ground, and a wandering female picks up a droplet by chance and whisks it into her genital opening. Fertilization occurs, and eggs are laid in the soil.
In a matter of days, tiny nymphs appear as simpler, smaller versions of the adults, and they begin their generally unnoticed lives – until winter when they impress a human visitor with their circus acts in the snow.

Bill Amos of St. Johnsbury, Vermont, is a retired biologist and author.


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