I have noticed that my speedo (and other brand) swimming costumes
always 'wear out' around the middle of your backside (ahem!) area -
resulting in frequent purchase of new costumes, or a bad view for
other swimmera (maybe that's just me).

Does any one know why its this area that runs out first?

Thanks 

insh

"The general rule of thumb is that if you're using the suit in
chlorinated water, you should get about 60 hours out of it. At that
point, the chlorine will likely have stripped the fabric of its
structure and it will collapse - the fabric will seem to give way,
particularly in the butt area."

http://web.archive.org/web/20041014074828/http://divemar.com/waterline/docs/faq_bsuits.html

It seems reasonable to assume that the fabric in this area of the suit
is subjected to more stress than other areas.

Sitting in a wet suit will also where out the backside.

Being a ex-lifeguard... i can tell you 60 hrs is an extremely low
estimate... The amount of chemicals in the water will affect the time
significantly.  Most importantly the lifetimes are mainly reduced
because of lack of care... When one is done swimming if they do not
shower (and rinse the bathing suit) thoroughly the chemicals will sit
and the bathing suit will not last as long

The wear your refering to has nothing to do with chemicals though..
Its a fabric thing... the fabric is being stretched out and the
strands in the fabric will break.  I know this because most
lifeguarding (outside of teaching) at indoor pools occur on dry
land... yet the wear and tear still occurs.

Id like to make some notes... 

Girls bathing suits will not last as long as mens... 
likewise speedos will not last as long as shorts