The unprecedented 2010 flooding in Pakistan resulted in more than a fifth of the country becoming submerged.  As a result, millions of spiders climbed into trees to escape the rising floodwaters. The water took so long to recede that all the vegetation became covered in a thick mass of webbing.

On the bright side, these areas have seen far fewer malaria-spreading mosquitos than would normally be present in areas with so much stagnant water. 

http://www.wired.com/wiredscience/2011/03/pakistan-tree-spiders/#