Date: Sun, 16 May 2004 17:20:34 -0400
From: "Michael Walsh" <MJW at press.jhu.edu>
To: <WSFAlist at WSFA.org>
Subject: [WSFA] Re: Cicadas and prime numbers
Reply-To: WSFA members <WSFAlist at WSFA.org>

> ronkean at juno.com 5/15/04 10:57:50 PM
>
>On Sat, 15 May 2004 03:38:26 -0400 "Ted White" <twhite8
> at cox.net> writes:
>>
>
>> From: <ronkean at juno.com>
>> > There are 13 year cicadas, and 17 year cicadas.  Why are
>cicada
>> cycles
>> > always prime numbers of years?
>>
>
>> There was a piece in the POST about that, a week or so
>ago.
>> Basically, so
>> they don't emerge in concert with other broods and
>crossbreed.
>
>...
>
>> It's said that a climate line separates the 17-year broods from
>the
>> 13-year
>> broods, which exist further south.   There are also the more
>common
>> cicadas
>> (which we hear in late August until it freezes) which
>apparently
>> have a
>> two-year cycle.
>>
>
>Thanks for the explanation.  I remember Keith having offered
>some
>explanation, but I had forgotten the details.  And the web sites
>I
>googled did not provide an answer as to why 17 would be so
>much better
>than 15, 16, or 18.
>
>Clearly, the 13 year and 17 year varieties will seldom emerge
>the same
>year in the same place, due to the mutual primality.

The science editor here at the JHU Press recalls living in Missouri
when 13 & 17 year cicadas appeared the same year.

mjw