According to this page, there are two general groups of cicadas in Alabama—the periodicals and the annuals. These are remnants of annuals. The next periodicals in northern Alabama will emerge in 2011. They’re on a 13-year cycle. (I thought there were only 17-year cycles, but there are 13-year ones too.)
I learned something else interesting today. Cicadas are on 13- and 17-year cycles because those are prime numbers. They therefore escape synchronization with the life cycles of predators, like praying mantises and cicada killers. If a cicada had a 15-year cycle, eventually it’d be consistently screwed with predators that have three- or five-year cycles, right? I thought that was nifty.