www.ukbutterflies.co.uk

Does anyone have any tips for identifying Common Blue v Brown Argus? I'm struggling a bit!

I think this might be a female Common Blue:


and this might be a Brown Argus:


but it wouldn't surprise me if I'm completely wrong. Would appreciate some advice!

Thanks,

Katie

Just getting the dreaded red X for number 2

Martin.

I don't mind being wrong but I want to know why!

According to my new book...you are correct on both counts.

Martin.

Some notes recently sent to me by Adrian Hoskins, who has written many of the species descriptions on the UK Butterflies website:

Behaviour
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Brown Argus always has a silvery appearance in flight. Close observation shows how the dark upperside and silvery underside alternate as the wings flap, giving a very distictive appearance in flight. Size is not totally reliable, as small specimens of Common Blue can be as small as Brown Argus.

Upperside
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Brown Argus ( both sexes ) NEVER has any trace of blue scales on the upperside. Female Common Blue ALWAYS has a trace of blue scales around the base of the forewings, which is apparent even in very old specimens.

Underside
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Brown Argus underside ground colour ( both sexes ) is a uniform pale greyish brown. Ground colour of female Common Blue underside is pale brown, and there is a flush of metallic green scales towards the base of the hindwing. The black spots on the outer margins of Common Blue female immediately abutt the orange spots. The outer black spots and orange spots on Brown Argus are separated by a narrow white area.

There are differences in the arrangement and number of black spots on the underside forewings of each species - these are awkward to see as the forewings are usually hidden by the hindwings, but you should be able to see that the median row of black spots forms a tighter arc on Brown Argus, but is a bit more linear in Common Blue. Bown Argus has only one black spots in the forewing discal cell ( at the outer end ), but Common Blue has an additional spot close to the basal end of the forewing discal cell, though this may be hidden behind the hindwing when the butterfly is at rest. The median black spots on the hindwing of Common Blue are evenly spaced, and form an undulating line. In the Brown Argus they are more staggered, and the 2 spots nearest the upper edge of the hindwing are paired like a colon.

These differences can be seen easily in the Lewington illustrations in the Collins Field Guide ( Tolman ), and appear to be constant in all specimens.

Cheers,

Pete (& Adrian!)

Thanks for your helpful advice. I'll go away and swot up on my butterfly anatomy now...

A few more photos are here on my blog:
http://bogbumper.blogspot.com/2006/07/b ... -blue.html

Katie

In the field you can tell the difference immediately because as Pete has pointed out the brown argus appears silvery when flying. As regards size, I've never seen a common blue as small as a brown argus. The difference is quite marked.
But once you have a photo and there's no way of judging size it's harder. If you have an open wing shot then the brown is uniform and although you get some irridescence and the body can sometimes appear blueish there are no sprinkling of blue scales on the wings that the female blue has. For closed wings where the forewings are visible, it's a cynch. The inner ring of spots run from the rear to the forewing of the female common with two/three spots on the forewing. These spots are missing from the forewing of the brown argus. Even though the forewing is tucked away on your top photo you can still see the first spot marking it as a blue, and it's missing on your lower picture making it a brown argus.

OK, I think I have these two worked out, but how do you differentiate the female Chalkhill Blue? I rarely venture into Chalkhill territory but a male has recently turned up on one of my regular walks, in what just might be suitable breeding territory - it will be interesting to see if there are more.

Check out the regular UK Butterflies info pages. But from my limited (and recent) experience, female chalkhills are brown with no blue scales like adonis/common, but like the adonis they have the dark bands going out into the white border which distinguishes them from brown argus.