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Air

1.

On August 10, 1926, a Stinson Detroiter SM-1 six-seater monoplane took off from the rudimentary airstrip at Tallulah, Louisiana. The Detroiter was the first airplane built with an electric starter motor, wheel brakes, and a heated cabin, but it was not a good climber, so the pilot leveled off quickly, circled the airstrip and surrounding landscape, held open the specially fitted sticky trap beneath the plane's wing for the designated ten minutes, and soon returned to land. As he touched down, P. A. Glick and his colleagues at the Division of Cotton Insect Investigations of the U.S. Bureau of Entomology and Plant Quarantine ran out to meet him.

It was a historic flight: the first attempt to collect insects by airplane. Glick and his associates, as well as researchers at the Department of Agriculture and at regional organizations such as the New York State Museum, were trying to discover the migration secrets of gypsy moths, cotton bollworm moths, and other insects that were munching their way through the nation's natural resources. They wanted to predict infestations, to know what might happen next. How could they contain these insect enemies if they didn't know where, when, and how they traveled?

2.

Before Tallulah, high-altitude entomology had barely got off the ground. Researchers sent up balloons and kites fitted with hanging nets, climbed up pylons, and pestered lighthouse keepers and mountaineers. But armed now with the new airplane technology, Glick went down to Tlahualilo in Durango, Mexico. There, 3,000 feet above the valley plain, his pilots trapped the pink bollworm moth, a feared invader of the U.S. cotton crop. Face-to-face with the unanticipated scale of his task, Glick wrote tersely that "the pink bollworm moths are carried in the upper air currents for considerable distances."

There were only a few flies and wasps in that first trap at Tallulah. But over the next five years, the researchers flew more than 1,300 sorties from the Louisiana airstrip and captured tens of thousands more insects at altitudes ranging from 20 to 15,000 feet. They generated a long series of charts and tables, cataloguing individual insects of 700 named species according to the height at which they were collected, time of day, wind speed and direction, temperature, barometric pressure, humidity, dew point, and many other physical variables. They already knew something about long-distance dispersal. They had heard about the butterflies, gnats, water striders, leaf bugs, booklice, and katydids sighted hundreds of miles out on the open ocean; about the aphids that Captain William Parry had encountered on ice floes during his polar expedition of 1828; and about those other aphids that, in 1925, made the 800-mile journey across the frigid, windswept Barents Sea between the Kola Peninsula, in Russia, and Spitsbergen, off Norway, in just twenty-four hours. Still, they were taken aback by the enormous quantities of animals they were discovering in the air above Louisiana and unashamedly astonished by the heights at which they found them. All of a sudden, it seemed, the heavens had opened.

Unmoored, they turned to the ocean, began talking about the "aeroplankton" drifting in the vastness of the open skies. They told each other about tiny insects, some of them wingless, all with large surface-area-to-weight ratios, plucked from their earthly tethers by a sharp gust of wind, picked up on air currents and thrust high into the convection streams without volition or capacity for resistance, some terrible accident, carried great distances across oceans and continents, then dropped with the same fateful arbitrariness in a downdraft on some distant mountaintop or valley plain. They estimated that at any given time on any given day throughout the year, the air column rising from 50 to 14,000 feet above one square mile of Louisiana countryside contained an average of 25 million insects and perhaps as many as 36 million. They found ladybugs at 6,000 feet during the daytime, striped cucumber beetles at 3,000 feet during the night. They collected three scorpion flies at 5,000 feet, thirty-one fruit flies between 200 and 3,000, a fungus gnat at 7,000 and another at 10,000. They trapped an anthrax- transmitting horsefly at 200 feet and another at 1,000. They caught wingless worker ants as high as 4,000 feet and sixteen species of parasitic ichneumon wasps at altitudes up to 5,000 feet. At 15,000 feet, "probably the highest elevation at which any specimen has ever been taken above the surface of the earth," they trapped a ballooning spider, a feat that reminded Glick of spiders thought to have circumnavigated the globe on the trade winds and led him to write that "the young of most spiders are more or less addicted to this mode of transportation," an image of excited little animals packing their luggage that opened a small rupture in the consensus around the passivity of all this airborne movement and led to Glick's subsequent observation that ballooning spiders not only climb up to an exposed site (a twig or a flower, for instance), stand on tiptoe, raise their abdomen, test the atmosphere, throw out silk filaments, and launch themselves into the blue, all free legs spread-eagled, but that they also use their bodies and their silk to control their descent and the location of their landing. Thirty-six million little animals flying unseen above one square mile of countryside? The heavens opened. The air column was "a vault of insect-laden air" from which fell " a continuous rain."

3.

From the mid-1920s through the 1930s, high-altitude researchers in France, England, and the United States were making the same discoveries and coming to the same conclusions. Broadly speaking, they decided, there were two kinds of insect travel. The tiny insects of the aerial plankton occupied the air above 3,000 feet, where they moved involuntarily, unable to resist the fast-moving higher-level currents. Stronger-flying, larger insects kept relatively close to the ground, below the 3,000-feet boundary, harnessing the calmer, low-altitude winds and migrating according to their own routes and schedules. These lower- level migrations could be spectacular. Some, such as those of the monarch butterflies and the Old Testament locusts, were already familiar. Others could take an entomologist by surprise. All were somehow mysterious. In 1900, James William Tutt witnessed millions of noctuid Silver Y moths flying with other insects in a steady east-west line alongside migrating birds. A few years later, William Beebe from the New York Zoological Society-the same William Beebe who pioneered deep-sea exploration in his steel bathysphere-found himself caught in a dense mass of purplish-brown butterflies on the Portachuelo Pass in northern Venezuela. Despite his confusion, Beebe managed to calculate that at least 186,000 insects had swept by him in the first ninety minutes. An hour later, with the torrent now "going full strength," he composed himself enough to pull out his high-power binoculars:

I began about twenty-five feet overhead and then refocussed slowly upward until the limit of vision of the small insects was reached. This, judged by horizontal tests of objects of similar size would be about a half mile zenithwards, and at every fractional turn of the screw, more and more smaller-appearing butterflies fluttered into clarity.

Throughout the entire extent of verticality there was no lessening of denseness of flying insects. . . . For many days this particular phase of migration continued, millions upon millions coming from some unknown source, travelling due south to an equally mysterious destination.

Beebe also reported a different phenomenon: a steady stream of insects of many species-cockchafers, chrysomelid beetles, vespid wasps, bees, moths, butterflies, and "hosts upon hosts of minute winged insect life"-passing together through the migration flyway in a massive motley emigration that apparently took place every year. All that minute insect life was too small to be counted. But aphids, an indistinct haze, will swarm at densities up to 250 times greater than that of butterflies. In fact, these tiny ones-the aphids, the thrips, the microlepidoptera, the smallest beetles, the smallest parasitic wasps, all barely visible to the human eye-form the overwhelming majority of species and individuals of the class Insecta, testimony to the fact that evolution shrank

the insects over the millennia even as it exploded their numbers and differences.

The giant dragonflies of the late Paleozoic, with their thirty-inch wingspans, are no more. As insects miniaturized, they developed their near-endless variety of aerodynamic body shapes and their specialized muscles for super-high-frequency wingbeats. Of the million or so speficies currently described, the average adult body length is at most a mere two tenths of an inch, and the median length is significantly less. Nonetheless, it is the larger, more visible insects, those four tenths of an inch or more in length (that is, at least twenty times larger than the average), that command the attention of researchers. If we subtract the huge volume of genomic studies of the fruit fly Drosophila melanogaster, the literature on tiny insects is scant. It seems clear that the relative abundance of miniature insects that Glick recorded in the air column is less a result of their being so easily carried aloft than a result of the fact that they so outnumber their larger relatives.

Glick himself reported strong-flying dragonflies at 7,000 feet over Tallulah, large insects flying well above the 3,000-foot boundary and flying so comfortably that they shifted direction to avoid his plane. Other researchers, including Beebe, recorded minute weak-flying insects-the supposedly involuntary dispersers-close to the ground, well below the proposed threshold. Researchers of insect flight now talk about the boundary layer in more fluid terms, as a variable region near the earth's surface in which wind speed is less than the speed at which a particular insect is capable of flying, a zone that varies with the strength of the wind and the capacity of the insect. Within the boundary layer, the insect is able to orient itself actively. Above the boundary layer, its direction of flight is strongly influenced by the prevailing winds, and the animal adapts to, rather than overcomes, atmospheric conditions. Given that only about 40 percent of known insects fly at airspeeds greater than three feet per second and that such timid winds-so gentle that a human can barely sense them-are generally found only close to the ground, most insects exercise full control over their directionality only at an altitude of three to six feet.

Yet beyond the boundary layer, thousands of feet into the troposphere, it's likely that only a small proportion of these animals-those without wings (such as spiders and mites), those that become too cold, and those suffering from exhaustion-are passively carried. From the tiniest to the largest, migrating insects are out there actively flying, flapping their wings, maintaining or varying their altitude and direction despite the strength of the winds around them. Sometimes they hover, sometimes they glide, sometimes they free-fall, sometimes they soar. They do what they can to evade birds in the daytime and bats at night. Rarely do they drift along like pollen in a breeze. Or plankton in the ocean.

No, aerial plankton is not a good name for these animals. They don't live in this medium; they occupy it temporarily. And their residency is full of calculation and action. Their exodus is triggered by the impulse to find new habitats and to encounter new hosts. Sometimes their flights are short, repeated dispersals; sometimes they are vast migrations from which the traveler may or may not return. In either case, there is little passivity. Takeoff is oriented to wind and light. If the animal is strong enough, flight is often against or across the wind. Butterflies and locusts streaming in formation may suddenly interrupt a low-level journey with a dramatic collective rise to catch a current at thousands of feet. Even tiny insects appear to seek out thermal drafts. In the upper reaches of the air column, the minute ones take paths strongly determined by the wind, but inside the airstream they hold steady, beating their wings, adjusting their direction and altitude. And then they alight, often prompted by scent or reflecting light, using their bodies to bring themselves to earth.

Forty years ago, Cecil Johnson, the author of a classic text on insect migration and dispersal, pointed out that many, perhaps most, individual insects die on these voyages, but "this is the price such species pay for finding their habitats." Johnson conjured an image of a planet under surveillance, "the surface of the Earth is thus scanned very effectively as millions of individuals, flying on air currents, continuously encounter suitable and unsuitable situations." When the situation does not suit, they soon take off again in search of a better location for feeding or breeding (or some other activity obscure to us), following "a direction determined either by the wind or themselves." It is a fact of planetary life, a great "diffusion system" that transports immense populations of animals "day after day, year after year, century after century."12 What happens to the notion of an invasive species in the face of this continuous and irrepressible traffic of short- and long-range travel, dispersal, and migration? What is left of a notion that everything has its own place, that everything belongs somewhere and nowhere else, that boundaries are inviolable, that with vigilance and chemicals this hyperabundance of willful and random life can be brought under control? Perhaps this was what Glick glimpsed 3,000 feet above Durango, face-to-face with the pink bollworm moth, its flapping wings gleaming in the high- altitude sunshine.

4.

Stop. If you're inside, go to a window. Throw it open and turn your face to the sky. All that empty space, the deep vastness of the air, the heavens wide above you. The sky is full of insects, and all of them are going somewhere. Every day, above and around us, the collective voyage of billions of beings.

That's the letter A: the first thing not to forget. There are other worlds around us. Too often, we pass through them unknowing, seeing but blind, hearing but deaf, touching but not feeling, contained by the limits of our senses, the banality of our imaginations, our Ptolemaic certitudes.