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Abstract

Afew months ago, a phone call from Robert Krulwich, a well-known radio and television reporter who has won a number of awards for science journalism, sent me on a search through the entomological literature to find out just how high insects can get. The query was for a story idea he had, not on insect hallucinogen abuse (although such a query would have been equally probable from Robert Krulwich, who has a knack for finding science stories with broad public appeal) but rather on insect traffic patterns. As it happens, the question has intrigued entomologists for the better part of the past century and has generated a diverse array of publications. Until heavier-than-air aircraft were invented, nobody really had a clear idea of how high insects could fly. Early on, there was general recognition of the ability of some species to take advantage of passive transport by wind. Because wind is slowed down near the earth's surface by friction, the layer of air nearest the ground (the boundary layer) moves relatively more slowly. In the boundary layer, insect flight speed can exceed air speed, and as a consequence, insects can control their movements. But beyond the boundary layer, active flight presents a real challenge to small creatures, and insects that escape the boundary layer are at the mercy of the wind. Intrepid mountaineers scaling Himalayan peaks often found arthropods awaiting them on the snowfields; the Mount Everest expeditions of 1921 and 1924 reported flies at 4,900 m, butterflies and moths at 6,400 m, and spiders as high as 6,700 m (Holzapfel and Harrell 1968). On the other side of the world, hover flies were found occupying the summit of Tacana, on the border between Mexico and Guatemala, at 4,090 m (Spalding 1979). L.W. Swan (1963) even identified a high-altitude life zone populated largely by