Brown Argus vs Common Blue

[ferny]

Hello everyone,

First post but I thought this would be a good place to ask around.

To seperate brown argus and female common blue I've always used the presence of an extra spot on the underside of the forewing close to the body on the common blue and it's absence on a brown argus.

There's a really nice guide available here:
http://www.britishbutterflies.co.uk/...de-low-res.pdf

And some other suggestions here:
Brown Argus & female Common Blue Butterflies

But here's my question, as this feature isn't noted on either of them, but is used seemingly very successfully by one of my very competant volunteers.

Does the brown argus always only have 5 orange lunules top-side hind wing, compared to the common blue always having 6?

It seems like a fairly obvious feature and is useful for a top-side view so it's absence from the ID guide above is conspicuous. Has anyone else tried this feature? Does it hold water? Is it 'usually but not always' true? Or has he discovered a new feature to help us seperate the two species?

Thanks very much,
Ferny

[GuyF]

Hi Ferny, welcome to the site!
I hadn't heard of this before, but checking my photos and other images online it looks like it does hold true most of the time. It seems that some well-marked Brown Argus, particularly females, can show a hint of a 6th spot, as in this image by Fourwings.

And female Common Blues are so variable that I expect that the number of spots must vary occasionally, here's an image by Jon of an individual showing few clear spots.

So overall, I probably wouldn't identify an individual from that feature alone, but it definitely does look like a useful guide to use when trying to separate these two species, well spotted!
Guy

[basquesteve]

Hi
6 roundels are quite common Its the dark spot in front upper wing that helps identify Arguses also the rear roundels in Common Blues contain white