COMPUTING SCIENCE
Bugs That Count
Brian Hayes
A Clockwork Insect
Cicadas spend almost all of their lives underground, as
"nymphs" feeding on xylem sucked out of tree roots; they
come to the surface only to mate. Most species have a life cycle
lasting a few years, but individuals are not synchronized; all age
groups are present at all times, and each year a fraction of the
population emerges to breed. True periodicity, where an entire
population moves through the various stages of life in synchrony, is
extremely rare. Of 1,500 cicada species worldwide, only a handful in
the genus Magicicada are known to be periodical; all of
them live in North America east of the Great Plains.
The taxonomy of the Magicicada group is somewhat
controversial and more than a little confusing. If you sort a
collection of specimens by appearance or mating call or molecular
markers, they fall into three sets; but it turns out that each of
these sets includes both 13–year and 17–year forms. Are
there six species, or only three? Complicating matters further, John
R. Cooley, David C. Marshall and Chris Simon of the University of
Connecticut have recently identified a seventh variety that has a
13–year period but shows genetic affinities to a 17–year group.
Then there is the division into broods. A brood is a synchronized
population, in which all individuals are the same age. Generation
after generation, they go through life in lockstep. One might expect
that a brood would consist of a single species, but that's generally
not the case. Brood X, for example, includes the 17–year forms
of all three species. Geographically, adjacent broods tend to have
sharp boundaries, with little overlap. Where two broods do share the
same real estate, they are chronologically isolated, typically with
four years between their emergences.
If you are a cicada trying to emerge in synchrony with all your
broodmates, there are two problems you need to solve: First you must
choose the right year, and then the right day (or night, rather)
within that year. The latter task is easier. Cicadas synchronize the
night of their emergence by waiting for an external cue: They crawl
out of their burrows when the soil warms to a certain temperature,
about 64 degrees Fahrenheit.
Keeping track of the years is more challenging. First you need an
oscillator of some kind—a device that goes
tick–tick–tick at a steady pace. The cicada oscillator
presumably ticks once per year. Second, you need to tally the
successive ticks, like a prisoner scratching marks on the wall of a
cell. Finally you have to recognize when the tick count has reached
the target value of 13 or 17.
The cicada oscillator is probably an annual variation in some
property of the xylem the insects consume, reflecting a deciduous
tree's yearly cycle of growing and shedding leaves. Support for this
hypothesis comes from an ingenious experiment conducted by Richard
Karban, Carrie A. Black and Steven A. Weinbaum of the University of
California, Davis. They reared cicadas on orchard trees that can be
forced to go through two foliage cycles in a single year. Most of
the cicadas matured after 17 of the artificially induced cycles,
regardless of calendar time.
The cicada's tally mechanism remains unknown. One example of a
biological counting device is the telomere, a distinctive segment of
DNA found near the tips of chromosomes in eukaryotic cells. Each
time a cell divides, a bit of the telomere is snipped off; when
there's none left, the cell ceases to replicate. Thus the telomere
counts generations and brings the cell line to an end after a
predetermined number of divisions. Perhaps the cicada employs some
conceptually similar countdown mechanism, although the biochemical
details are surely different.
There is no reason to suppose that cicadas count strictly by ones.
Indeed, the coexistence of 13–year and 17–year periods
suggests other possibilities. For example, the two life cycles might
be broken down as (3x4)+1=13 and (4x4)+1=17. In other words, there
might be a four–year subcycle, which could be repeated either
three or four times, followed by a single additional year. An
appealing idea is to identify such subcycles with the stages, or
instars, in the development of the juvenile cicada. And it's notable
that what distinguishes 17–year from 13–year forms is a
four–year prolongation of the second instar. Unfortunately for
the hypothesis, the rest of the nymphal stages are not uniform,
four–year subcycles. Nymphs pass through them at different
paces. Only at the end of the cycle do the members of a brood
get back in synch.
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