Finding primes in signals is seen as a sign of some kind of intelligence - see e.g. the role of primes in the search for extraterrestrial life (see e.g. [here](http://www.math.dartmouth.edu/~carlp/PDF/extraterrestrial.pdf)).  
 This is because there are relatively few examples of numbers that appear in nature because they are prime.
One example of the use of prime numbers in nature is as an evolutionary strategy used by cicadas of the genus Magicicada (see e.g. [here](http://en.wikipedia.org/wiki/Prime_number#Prime_numbers_in_nature) or here: [1])


**My question:**  

Do you know of any other instances where prime numbers occur in nature? Could you please also give a source/link - and perhaps some background. Thank you.
  
  

[1] Goles, E., Schulz, O. and M. Markus (2001). "Prime number selection of cycles in a predator-prey model", Complexity 6(4): 33-38
  
  
  

**Edit**: Obviously many people misunderstood me. I didn't mean the occurrence of prime numbers just by coincidence - but *because they are prime*. The cicadas example - although being controversial - at least hints at some kind of evolutionary strategy.
  




Examples of prime numbers in nature
Somewhat longish to be a comment, so here goes:


How about examples like:


* Polygons in nature? Unfortunately, the most famous one of them is a hexagon ;-)


But I am certain there are several chemical compounds, physical structures such as crystals, and so on that exhibit polygonal structures with prime number of edges / facets --- perhaps because the "primeness" there leads to a physically / chemically more stable configuration. 


Is this an acceptable kind of primeness? Perhaps not. It seems to me that the question you pose is almost at a "meta" level!


