The Microscopic Universe That Thrives in Our Sky

How to see the tiny creatures that blow around in the earth's aeolian zones

| September-October 1999

Air is invisible, but far from empty—it is its own dense brew of life. Consider that the atmosphere over a single square mile of the earth’s surface contains 25 million airborne insects. Consider that the fraction of organic detritus that falls out of orbit can become thick enough to plow. Consider that within the space of a very tall imaginary top hat, 50 to 100 million microorganisms swirl above your head.

Airborne life-forms—tens of thousands of species collectively called aerial plankton—form a web of ecological relationships that researchers barely understand. Not only are the sheer numbers astonishing, so is the variety: caterpillars, spiders, aphids, butterflies, moths, beetles, mites, and many other invertebrates, plus countless seeds, spores, and pollen grains of fungi, algae, mosses, liverworts, and flowering plants.

A series of collections, made first with a net on a kite in the 1930s and later on the wings of biplanes, determined that the band of fairly stable air located between 100 and 15,000 feet above the ground literally swims with life. Early observers and a contingent of skeptics wondered if aerial plankton simply represented the passive, accidental transport of lightweight objects. At best, they said, these objects were early life stages of plants and animals merely dispersing to new sites and only temporarily aloft. But evidence now suggests that the aeolian zone—named in the 1960s by San Francisco State College’s Lawrence Swan, after the Greek wind god Aeolus—is more than a temporary shelter for homeless organisms. It is an aerial community of both intentional and unintentional participants.

The life-forms found here rise and fall on atmospheric tides, circling the globe, sometimes to great heights. No one knows for certain just how high life travels in the atmosphere, but in 1974, Russian rockets collected air samples that contained six species of common opportunistic microbes at heights of 35 to 50 miles. Likewise, no one knows how long individual organisms choose to stay aloft, though it is known that tiny aphids control their journeys to the extent that they can descend, against adverse winds, to reach favored food patches.

While the majority of aeolian zone travelers are either too small or too distant to be observed by humans, we occasionally glimpse the dazzling fecundity overhead. Macroinsects, especially migratory forms of locusts, butterflies, and dragonflies, provide good examples. One cloud of locusts observed in the Middle East in 1926 contained more than 10 billion individuals. In California, a continuous stream of 3 billion painted lady butterflies once passed an observer during a three-day period. A recent tally near Chicago counted 1.2 million dragonflies during fall migration.

Ballooning spiders are another conspicuous component of the aeolian zone. I have stood on high, barren peaks with spiders appearing out of thin air to crawl on my head and shoulders. These spiders, typically young ones, are dispersing in search of new homes. Each spider rides on its own silken balloon (technically, a parachute) constructed by reeling out filaments until the wind fills it. This is effective travel—airborne spiders have been reported at heights of seven miles and frequently land on ships hundreds of miles at sea. At times, the air becomes so dense with ballooning spiders that their strands completely encrust exposed surfaces and form sheets of silk known as gossamer.