Passage IV

NATURAL SCIENCE: This passage is adapted from the book Wind by Jan DeBlieu(©1998 by Jan DeBlieu). Convection currents in the atmosphere, which are responsible for breezes, begin when air is heated over warm land masses or seas and then expands and rises. Thermals are columns of rising warm air.

No other class of animals lives so starkly at the mercy of wind as the insects. In its benign, convective form, wind plucks up bugs by the millions, the trillions, every day and scatters them widely. (This is not to say they are dispersed at random. Biologists have often seen insects choose a precise moment to set out—say, just before the onset of a light rainstorm with west-bearing winds—presumably so that they are most likely to arrive in protected, food-rich habitat.) Once aloft they depend on horizontal currents to ferry them onward. Often they drift all night. In the atmosphere's upper strata they encounter waves of turbulence and layers of different densities that sort and compress them into compact swarms. They may drift as a unit through honeycombs of thermals and land lightly in a rogue's paradise—a flower garden, a newly cultivated field of vegetables. Or they may be accosted by a freshening gale, tossed out to sea, and slammed rudely into the water.

Many insects are wingless, at least in their larval forms, and must depend entirely on wind for transit. Those that can fly have neither the mass nor the strength to fight headwinds. So they ride the breeze, perhaps passively, perhaps with some precise collective plan. In certain species, individuals bunch together like flocks of birds, turning as one organism. In studies conducted during the 1950s, aphids carried by west winds from the irrigated oasis of southern California into the arid basin of the Colorado desert showed an uncanny knack for finding potted alfalfa plants left out for them. Some investigators speculate that spiders and other balloon-riding creatures may control their altitude by trimming the lengths of silk that lift them. At present, however, scientists do not know whether insect wayfarers have anything but the crudest ability to navigate from one habitat to another.

Until the 1920s researchers were unaware that insects can survive well up into the troposphere (the lowest part of Earth's atmosphere). It had been known for a century that the smallest insects could be easily transported by wind. In 1827 and again in 1924 British Arctic explorers discovered live spruce aphids and syrphid flies on the Norwegian island of Spitsbergen. Apparently they had been carried by wind from Russia, 800 miles away. Monarch butterflies had been observed 500 miles from land over the South Pacific, and once a swarm of migratory Iocusts was seen over the North Atlantic, 1,500 miles from land. With the spread of tree-destroying European gypsy moth larvae through the eastern United States during the early 1920s, research into the dispersal of insects took on special urgency.

In 1926 Perry Glick, an entomologist in Louisiana, made a series of flights in which he sampled the fauna of the air with traps made of wire screen mounted on the wings of a small biplane. Few scientists had tried to examine life at such heights; and Glick's study is regarded as a pioneering work. At 200 feet he found a great number and diversity of insects. He caught by far the most when wind velocities were between five and eight miles an hour. Lesser winds seemed to discourage insect flight; greater winds made it nearly impossible.

Above 300 feet the insect population rapidly thinned. Nevertheless, Glick found a few spiders and flies as high as 5,000 feet. He never devised an accurate means of measuring the flow of convection currents in the atmosphere. But on days when the plane encountered turbulent air, he caught many more bugs at 1,000 feet, as if strong thermals had tossed them to unusual heights. Over five years Glick trapped 700 identifiable species. In the end he calculated that on any midsummer day the air above each square mile of Louisiana contained more than 14 million insects.

Many insects travel at night, presumably to avoid the upper-air turbulence caused by convection. Most individuals migrate no more than five miles over the course of their lives. Only a few species embark on lengthy seasonal movements, and then largely because of their proximity to favorable avenues of wind. Monarch butterflies, migrating from the: eastern United States to their overwintering grounds in Mexico, follow routes that parallel the major flyways used by birds. In the North American interior and in northern China and Japan (all regions where winters tend to be particularly severe), certain butterfly species ride jets of warm air north with the coming of spring.