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Mile-high club

A zone about 25 feet above Earth's surface (depending on atmospheric conditions and species of insect) is known as the flight boundary layer, where wind speed is equivalent to maximum insect flight speed. Because wind speed increases with height, an insect needs to hang out below the flight boundary layer if it wants to be able to fly in any direction in its quest for food, a mate, or shelter.

Nonetheless, insects are commonly found at much greater heights. Mosquitoes have been collected at 1,000 feet. Houseflies can probably get that high, too. Migrating insects such as locusts and butterflies ascend much higher. Ground-based radar has detected insects nearly 2 miles above the surface.

At these heights, insects can maneuver, but the wind is too strong for them to travel upwind. For example, this explains why plagues of locusts lasting several years tend to spread according to the direction of the prevailing winds.

Insects cannot fly if the air is too cold. Temperature decreases with altitude, but in a temperature inversion, a layer of warm air sits atop cooler, denser air. Temperature measurements with kites have shown that migrating insects concentrate in the warm air at the top of temperature inversions.

An exception is passive migrators, including tiny moth larvae and tiny spiders (albeit not insects) that don't need to flap their wings to stay aloft. They migrate on silk threads and can be lifted to great heights by updrafts of air and carried long distances. They are deposited when winds change, or heavy rainfall can wash them out.

Topography—features of a particular area of land—influences movements and layering of air in the lower atmosphere. Therefore, mountain ranges can alter insect flight. Air on the upwind side of a mountain is forced to ascend, creating an updraft that migrating butterflies take advantage of to gain height by gliding in circles.

Because elevation influences average temperatures, it also determines what species live in a particular area. In the past, mountain ranges have limited the spread of diseases carried by insects. But this seems to be changing as annual temperatures rise. For example, mosquito-borne diseases such as malaria and Dengue fever are being reported at increasing elevations in Asia, Africa, and Central and South America.

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