Common Blue and Brown Argus underwings

Common Blue butterfly, Polyommatus icarus, and Brown Argus, Aricia agestis.

A short post showing the difference in the underwings of two very similar butterflies, the Common Blue and the Brown Argus.  The male Common Blue is easy to tell, because it is always bright blue from above.  The female is variable and can be quite brown, though usually with a hint of blue about the body, and the Brown Argus is always brown. 

There are two features of the underwings that are easy to see in photos; rather less so on live and active creatures. 

The Brown Argus has no spots on the lower half of the underside of its forewings.  The Common Blue does. 

The Brown Argus has two spots on the leading edge of the underside of its hind wings that are aligned together, like a colon.  On the Common Blue, the equivalent spots are aligned so as to form part of an arch of spots following the line of the edge of the wing.

So now you can easily tell which of these two is which, yes?