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Bugs That Count

Brian Hayes

Along the East Coast of the United States, a cohort of periodical cicadas known as Brood X occupies the prime turf from New York City down to Washington, D.C. Brood Xers are the hip, urban cicadas, the inside–the–Beltway cicadas, the media–savvy celebrity cicadas. When they emerge from their underground existence every 17 years, they face predatory flocks of science writers and television crews, hungry for a story.

This year was a Brood X year. Back in May and June, all along the Metroliner corridor, the air was abuzz with cicada calls, echoed and amplified by the attentive journalists. Then, in just a few weeks, it was all over. The cicadas paired off, fell silent, laid eggs and died. The press moved on to the next sensation. Perhaps a few straggler cicadas showed up days or weeks late, but no one was there to notice, and their prospects cannot have been bright. I worry that the same fate may befall an article about cicadas appearing weeks after the great emergence, at the very moment when most of us want to hear not another word about red–eyed sap–sucking insects for at least 17 years. I beg my readers' indulgence for one long, last, lonely stridulation as the summer comes to a close.

Periodical cicadas are remarkable in many ways, but I want to focus on one of the simplest aspects of their life cycle: the mere fact that these insects can count as high as 17. Some of them count to 13 instead—and it has not escaped notice that both of these numbers are primes. Of course no one believes that a cicada forms a mental representation of the number 17 or 13, much less that it understands the concept of a prime; but evidently it has some reliable mechanism for marking the passage of the years and keeping an accurate tally. That's wonder enough.

The physiological details of how cicadas count will have to be worked out by biologists in the lab and the field, but in the meantime computer simulations may help to determine how precise the timekeeping mechanism needs to be. Computer models also offer some hints about which factors in the cicada's ecological circumstances are most important in maintaining its synchronized way of life. On the other hand, the models do nothing to dispel the sense of mystery about these organisms; bugs that count are deeply odd.

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