Eeyore, the reason the 13-year locusts, which
are really cicadas, not the big grasshoppers called locusts, emerge only
every 13 years is much too complicated for me and for you and even for Owl
because it involves arithmetic, which is beyond us. Nevertheless, I'm going to
tell it to you because arithmetic that's too complicated is really
the point.

There are a lot of predators that like to eat
13-cicadas. Suppose the cicadas emerged every year. The predators might increase
their numbers and eat more and more of the cicadas each year as the years
went on. Also, fungi and other diseases might evolve to infect the cicadas
more and more as the years went on. Pretty soon, the cicadas would have a
very hard time and there would be a lot less of them around.

Could they avoid an increase of their enemies by
coming out only every two years? I'm afraid not. The enemies might
evolve to increase their numbers every other year and again make things go badly
for the cicadas.

The same is true for every three years, every
four years and even every five years. Also for six, eight, nine and ten years
because those numbers are divisible by two, three, four and five. How about
seven or eleven? Could enemies evolve to
time a big increase in their numbers to coincide with the emergence of lots of
cicadas every seven or eleven years? Maybe yes. Maybe no. But one thing's for
sure. 13 years or 17 years are not divisible by anything except 13 or 17. Many
scientists think that's too complicated for cicada enemies. Therefore, they
believe seven species of "periodical cicadas" have evolved to spend 13 or
17 years as nymphs, burrowing around under ground, emerging in enormous numbers
only after 13 or 17 years.

The Wild Turkeys will feast on 13-year cicadas
in 2011, but there will be so many of them (i.e. up to 1.5 million per acre) in
just this year that lots of them will reproduce. Next year, and for eleven
more years afterwards, the turkeys, shrikes and other cicada gluttons, not to
mention the fungi and the diseases, will have to make do without 13-year cicadas
on the menu.

Now I've got you confused, I can see it. Don't
we hear cicadas every summer? Yes, but those are annual cicadas, not periodical
cicadas, and they have found other ways to survive and to
reproduce.

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