More astounding than the variety of insects found in the air, of which I have given only a hint, is that some completely wingless species were taken. Forty bristletails were collected at elevations up to 8,000 feet, 26 springtails at altitudes as high as 11,000 feet, a flea at 2,000 feet, and worker ants and immature stages of a half-dozen different orders at many altitudes.

Flower thrips.

We have a more specific relationship of wind, temperatures, and insect movements. On July 1, 1938, when the best crops in a decade were in prospect in eastern Montana, a dramatic invasion of lesser migratory grasshoppers occurred. The insects, apparently originating at about the center of the border between the Dakotas, flew northwest into Montana and reached the Saskatchewan border by July 17, causing great destruction to crops.

The general direction of movement was from southeast to northwest. Although the insects had a "flight psychology," they seldom moved in numbers until air temperatures approached 80 F. J. A. Munro and Stanley Saugstad, of the North Dakota Agricultural Experiment Station, noted in connection with this flight that "the winds from the south and southeast, being warmer than those from other directions, were more effective in promoting sustained flights of the insects." They ascertained that in the 29 days following July 17 winds from the south and southeast prevailed and brought with them an average daily maximum of 88.9 . For 11 days the winds were from the north and northwest, and the corresponding temperature average was 79 . Thus there was a differential of nearly 10 in favor of southerly winds, and the daily maxima fell well within the temperature range that would release flights, while northerly winds hardly reached that point.

Very likely the flight direction toward the northwest was governed largely by the warmer winds, which would have assisted in carrying the grasshoppers in that direction. M. B. Freeburg, of Northwest Airlines, reported that during that flight grasshoppers were encountered at 7,000 feet on July 27 and at 11,000 feet on July 26 and that in other years they had been noted up to 13,000 feet.

With proper temperatures, it is obvious that the air could be filled with migratory grasshoppers to a great height above the ground, the direction of movement largely governed by prevailing winds. Winds may not represent the only factor in directional distribution, however, for on some days with northerly winds a few of these insects were seen to rise and unsuccessfully attempt to fly directly into the air currents and in the direction which they had previously followed.

We have a number of instances of the transportation of insects long distances each season through the air apparently a normal occurrence with some pests. The corn earworm usually cannot withstand northern winters and passes the cold season in the Southern States, whence generally come the moths that produce the larvae that attack corn and tomatoes in the north.

The cotton leafworm is transported long distances through the air. A tropical insect, it winters almost entirely outside the United States. As the season progresses it moves farther and farther north, with successive generations on cotton. In the fall the adults may appear in the Northern States and even in Canada, far beyond the range of their host plant. Winter destroys all stages of the insect in the north, and the country is annually repopulated from the south.

Air currents have many indirect effects on insects. Heavy winds may assist in reducing numbers. Eggs of the corn earworm and the European corn borer are dislodged by wind from the host plants and drop to the ground. Wind movements affect other factors in the weather, changing temperatures, bringing in moisture, and the like.

Charles Macnamara, of Arnprior, Ontario, Canada, has said: "Apparently it is only their lack of chlorophyll and consequent inability to assimilate mineral matter that keeps them [insects] from eating holes in the universe."

The food habits of insects are as diverse as they are important, but what they eat, when they eat, where they eat, and how much they eat is largely controlled by the weather and climate in which they find themselves situated.

So responsive are insects to their meteorological environment that we must have knowledge of its effects if we are to understand, extend, or refine their control. We must know the direct effects of all weather factors and the indirect effects, such as those on hosts, parasites, and predators.

We can consider the climate of an area and fit certain crop practices into the expected climatic performance in order to reduce depredations of pests. We can measure weather and give short-range warnings of things to come. But until we can do something to control weather or can predict it accurately long into the future, we are limited to activities of the kind I have cited in any attempt to manipulate populations of pests by employing the assistance of the many factors that make weather and climate.

HARLOW B. MILLS, a native of Iowa and a graduate of Iowa State College, is chief of the Illinois State Natural History Survey. Dr. Mills has worked on cotton insects in Louisiana and Texas, cereal insects in Iowa and Montana, and insects affecting livestock in Montana. For 10 years he was head of the department of zoology and entomology in Montana State College.

Clover leafhopper.