They're Here

The Brood XIII cicadas are emerging throughout the Midwest in an every-17-year fit of Darwinian survival.  Scientists theorize that their abundance — they emerge by the millions per acre—is an evolutionary advantage: Because of their numbers, predators (primarily birds, insects and fish) cannot possibly eat all of them. 

Because the risk to any individual cicada is so low, the species employs no defense mechanisms other than their numerical superiority.  It is, of course, their numbers that make them such a nuisance to humans.  The solution, according to one scientist, "Wait a month; then they won't be around." (More Brood XIII stories.)