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Had this sent to me and I can't make up my mind whether it's Idas or Silver Studded Blue.

I'm erring towards idas mainly based on the more pointed forewing apex and the metallic blue spots on the hindwing.

Anyone any opinions?

I would call it idas mainly because of the colour of the basal area

I couldn't call that one with any confidence. If forced, I'd also go with idas, though I've never actually seen idas flying around bramble! The problem is, both species are incredibly variable. Silver-studs in the Alps can have no silver at all or show all the spots infilled with gleaming scales. On average, idas has more complete silver. Did you see others at the same site? Did you see what the likely foodplant was?

Guy

EDIT - on further inspection, I don't think I can go better than 50/50. I can believe it's either. Did you see the upperside, even in flight?

Padfield wrote: ↑Wed Jun 19, 2024 5:34 pm...Did you see the upperside, even in flight?

No. I did ask if any were taken with even a tiny part of the upperside displayed, but this was the only image of the butterfly.

John Vergo wrote: ↑Wed Jun 19, 2024 2:52 pmI would call it idas mainly because of the colour of the basal area

Thanks, John. The light is poor on the image which doesn't help.

I suspect there will be more intermediate photos out there for discussion on this and here is another. The complete row of forewing orange marks indicate Reverdin’s but I indentified it as Idas based on other features. It is from the Italian alps in Piedmont. I did not get an upper wing shot as it was raining and on a steep slope. Any takers ?

Hi Andy,

The orange band is one of those clues where if the band is strong orange right up to the costa, then it is very likely (maybe almost certainly) Reverdin's, but it is quite possible for Reverdin's to have less than convincing orange bands. Yours certainly extends to the costa, but doesn't look quite strong enough to indicate/confirm Reverdin's.

A reason why I think is most likely Idas is that the black chevrons are quite sagittate whereas Reverdin's are usually rounder on the internal edge. This seems to be a consistent clue.

I don't have books here, so I am just going on "feel". Maybe Guy has a view (I also felt the previous one was Idas, but marginally).

Roger

I agree with Roger on this one - I think it's idas, and for the same main reason: the shape of the black crescents on the hindwing. This feature is more constant and reliable than the forewing orange spots. For example, this female idas (photographed in Sion, where argyrognomon doesn't fly, and also recognisable by the hindwing) has very strong orange spots all the way to the costa:



That is unusual, but means the forewing spots, in the female at least, cannot be relied on.

I can't quite work out the legs in Andy's excellent picture, as we can only see three of them (and I wonder where the left front two legs are). Is that the famous tibial spine we see on what appears to be the middle leg? I've never really got to grips with this spine.

Guy

Thank you both for your comments. I find these mini discussions on the forum really useful. It was the only “ silver studded “ type butterfly I saw last week. It is not just the UK where the season is delayed. Hopefully the Alps will have a prolonged season this year.