Duke of Burgundy 4 Legs

I've always heard these were fratillaries, but as it turns out they are actually the only UK resident 'metalmark'.
"..A curious characteristic of this subfamily is that the female has 6 fully-functional legs, whereas the male has only 4 - the forelegs being greatly reduced."
From UK Butterflies site.

I did wonder if they had been disabled in war with other butterflies.