The www.popularscience.co.uk website has been sent for review a book called Shapes by Philip Ball . It's an interesting book about shapes in nature - everything from why shells have a particular form to why a zebra's pattern looks like it does. Something the reviewer drew to my attention, was a fascinating observation about a creature for whom the idea of prime numbers isn't just a bit of abstract maths, but a matter of life and death. Apparently there are some cicacada species whose life cycle tends to operate on prime numbers - say a 13 year or 17 year period. Most of their life they are tucked away safe underground, but once every 13 or 17 years they pop up to breed and are vulnerable. It has been suggested this is a self preservation thing. Typically the number of predators around go through regular peaks. Imagine your life cycle was 12 years rather than 13. Then you'd be particularly susceptible to predators with a 2, 3, 4 or 6 year peak, if that peak synchroni