A Staff Report from the Straight Dope Science Advisory Board

How do the spiders outside an upper-story window in a high-rise get there?

July 8, 2003

Dear Straight Dope:

As I am writing this, I am sitting on the 28th floor of a high rise in downtown Chicago. There are about 10 spiders with webs outside the office windows. How did they get there? I can't imagine that they climbed up 280 feet on the outside of the building. What made then stop at the 28th floor? Why not continue to other 12 floors? How do they even know that there are bugs up there? This is all very confusing and no one in my office seems to know. One guy suggested that spiders make little "web parachutes" and let the wind carry them up to the heights. But if that is the case why don't we ever see thousands of spiders flying through the air?

The guy who commented about "parachutes" was very nearly correct. Many tiny spiders perform a feat called "ballooning," which involves letting a long dragline of silk out into the wind. When the pull on the silk exceeds the weight of the spider, then off it goes, riding the wind currents. The phenomenon figured in E.B. White's children's book Charlotte's Web and Gary Larson once did a "Far Side" parody of it, about how buffalo disperse by ballooning. The spiders have no control once they're airborne, of course, and most of them die--either from being blown too high and freezing, staying aloft too long and starving, or landing in an inhospitable spot. But a few lucky individuals land somewhere they can survive, like your high-rise. 

The air is full of tiny critters like this, mostly flying insects, but also a fair number of spiders--often they're referred to as "aerial plankton."  You don't see them because the density is low and the critters are small; mostly less than a millimeter or two. But put up a fine mesh screen with a trap at the top, and you'll rapidly accumulate specimens. This is, in fact, the principle behind the most widely-used types of traps in a professional field entomologist's kit (known as a Malaise trap, or flight-intercept trap).

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