A spider tiptoes before heading to the sky. (University of Bristol). It’s been a longstanding mystery: Spiders have been seen hundreds of miles out to sea and thousands of feet up in the air. They are amazing creatures, but flight per se isn’t one of their skills. And yet: A spider climbs to the top of a branch, releases a fan of silk, and off it goes — It’s called spider “ballooning.”
Charles Darwin found the phenomenon puzzling when he saw a swarm land on the H.M.S. Beagle some 60 miles offshore.
“A beautiful day: but the wind has been steadily against us.— In the evening all the ropes were coated & fringed with Gossamer web — I caught some of the Aeronaut spiders which must have come at least 60 miles. How inexplicable is the cause which induces these small insects, as it now appears in both hemispheres, to undertake their aerial excursions.”
The obvious explanation might be that their web catches the breeze and carries them away, but this notion doesn’t quite fly. Spiders balloon only in very light winds of less than three meters per second and wind couldn’t pull out enough silk to explain how they travel so far and get up so high anyway. A 1939 aerial collection expedition of the U.S. Department of Agriculture sampled high-altitude insects — calling them “plankton of the air” — and found substantial numbers of Araneida as high as 5,000 feet above Tallulah, LA.
Another idea was that they were somehow leveraging the earth’s electromagnetic charge, but that idea had never been studiously tested. Now two scientists from the University of Bristol, Erica L. Morley and Daniel Robert, have shown that spiders balloon by taking advantage of the earth’s electrical field.
The electric field around a tree
It’s been known since early in the last century that there is always an active atmospheric potential gradient (APG). On days with unsettled weather, the APG may strengthen.