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.

Bob Fisher
Independence, MO
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