COMPUTING SCIENCE
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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