From: "Ted White" <twhite8 at cox.net>
To: "WSFA members" <WSFAlist at WSFA.org>
Subject: [WSFA] Re: Cicadas and prime numbers
Date: Sat, 15 May 2004 03:38:26 -0400
Reply-To: WSFA members <WSFAlist at WSFA.org>

----- Original Message -----
From: <ronkean at juno.com>
To: <WSFAlist at WSFA.org>
Sent: Saturday, May 15, 2004 2:52 AM
Subject: [WSFA] Cicadas and prime numbers

>
> On Fri, 14 May 2004 12:53:18 -0400 (EDT) "Keith F. Lynch"
> <kfl at KeithLynch.net> writes:
>
> > I too wonder what Steve means by an area being less than 17 years
> > old.
> > There are plenty of places with buildings on now them that had no
> > buildings on them 17 years ago.  But so what?  Cicadas aren't
> > indoor
> > creatures.
> > --
>
> These cicadas spend most of their lives two or three feet beneath the
> surface, sucking on tree roots (according to a recent news article).
> Construction which disturbs the earth likely kills cicadas at the
> affected spots, and in any case paving or building over the places where
> cicadas live prevents those cicadas from emerging, since they can't
> burrow through asphalt or concrete.  Even adding fill dirt might prevent
> them from emerging.
>
> Cicadas will emerge at places where trees or bushes were 17 years ago,
> even if the same trees are not still there now, if they have been able to
> find some roots from which to feed, and if their emergence is not
> otherwise blocked, and if they have not been killed in the meantime.  So
> there are reasons why cicadas would be expected to be less plentiful in
> areas where there has been construction during the past 17 years, but
> some cicadas might still be able to emerge in those places.
>
> Sometimes, when digging, I have encountered creatures which were probably
> cicadas, though at depths of less than two feet.
>
> 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.   This
preserves the brood's integrity; it's an evolutionary survival tactic,
since the brood as a whole survives by emerging en masse and in numbers
which overwhelm preditors.   Anything which decreases the brood strength
threatens its overall survival.    Also the long periods minimize the
chances of a "bad year" weatherwise; this lengthy period underground is
apparently a response to the period immediately following the last ice age.
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.

--Ted White