![Image](http://farm6.staticflickr.com/5350/9497345973_222d30985f_z.jpg)
Hoary Comma butterfly, Polygonia Gracilis by ke9tv, on Flickr
The explanation of that behaviour is fascinating (to me, at least). If I've captured your interest, head on over to the article on my blog to hear how it all relates to butterfly mating behaviour and alkali-metal ions.