In an event that received world-wide coverage a few months ago, the sky was filled with millions of spiders and their silky threads in New South Wales, Australia. The spiders floated around and then down, landing in everything, even in people’s hair and beards. This occasional phenomenon is called a ‘spider rain’ because of the huge numbers of spiders involved, and although the spiders are typically not poisonous, it can cause a real panic.
Young spiders often float up into the sky on silken threads, a dispersal method called ballooning. Ballooning happens all around us from spring through fall, but it seldom gets noticed.
It is thought that conditions have to be just right for a spider rain: large numbers of spiders need to hatch and be ready to balloon, but weather conditions somehow hold them back. Then the weather changes, it is perfect ballooning weather, and lots of spiders become airborne. Once they are airborne the weather shifts again, sending the spiders back to earth in a short time. It is still not well understood how shifts in weather contribute to a spider rain.