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I'm not very confident distinguishing brown argus from female common blue.

When they're like this it's easy! I don't recall seeing them this blue before:



But what about this (taken at the same site)? There's a blueish sheen visible on the left wing and thorax, but barely noticeable. Is this another blue, or brown argus?



Thanks for any help.

I'd say Brown Argus based on:

1. The shape of the abdomen, which would suggest a male

2. A complete lack of any blue scales whatsoever

Cheers,

- Pete

Hi dilettante,
Of course Pete is correct - the second shot is male Brown Argus. It's important to remember that Brown Argus can have a blue (or purple, bronze, green, yellow) lustre or sheen to the wings, but it doesn't have any blue scales. Also, male Brown Argus can have blue hair on the thorax, abdomen and even blue hairs over the wing bases. This can give a sometimes strong blue tinge to the body and wing bases which often causes confusion, as it seems to contradict the overly simplified rule "no blue at all on a Brown Argus". As Pete points out - the shape and length of the abdomen (reaches out as far as the hind wing rear margins) signifies 'male'. Look at the relative length of the abdomen on your female Common Blue for comparison. If a superficially similar butterfly has blue scales near the wing bases (or anywhere else on the upper wing surface) then it's a female Common Blue. Hope this helps.
Neil

That's great information. Many thanks to you both. It's good to know I have both on my local patch of scrub.