I snapped this butterfly today in central Spain. It seems to be a dark green fritillary.
The upper wing patterns suggest aglaja or adippe or possibly niobe. However, the underwing is nothing like niobe and differs from adippe in lacking the 4 brown spots, which leaves me with aglaja as the identification . However, my book describes the underwing as "base green with large silver spots" whereas I would describe this butterfly's underwings as yellow with green/silvery spots. I found some sub-species described on another website but none of those (UK variants) matched this specimen. Can anyone tell me if this is a know sub-species?
It was not my intention to answer my own question but I will. I was photographing a dark green fritillary (Argynnis aglaja) recently and caught the following two images just 5 seconds apart.
The first image shows a mainly yellowish underwing with silvery spots while the second shows the green underside with silver spots that is described in my field guide. So I conclude that the colour must be variable - probably the result of a phosphorescence-like phenomenon. You can see that the butterfly has slightly turned between the two images and that is enough to change yellow to green!