Which blue?

 

This wonderful butterfly sent me down a bit of a frustrating path tonight.  It's a "tailed blue" — can you see the "tails" trailing off the back edge of each hind wing?  They're hard to see in the field, so you have to look carefully.

Even though I haven't seen one in California yet, my first thought was, "Okay, this is going to be a Western Tailed Blue (Cupido amyntula)."

But then when I started doing some research, I learned that Eastern Tailed Blues (Cupido comyntas) are also present in some parts of California.

So now the question is — which of the tailed blues is this?

Unfortunately, I don't have my butterfly field guides with me right now.  And when I tried to find out how to separate the two species online, I had a hard time tracking down useful information.  Then I reviewed pictures of the two species.  I've seen lots of Eastern Tailed Blues (on the East Coast), but not many Western Tailed Blues. 

I'll also reveal that I didn't get very good looks at the underside of this butterfly (although it looked quite pale).

After all of this, I still couldn't decide which one this was — a Western Tailed Blue or an Eastern Tailed Blue.

So...I'll either have to ask around for help from people with more experience, or wait until I have access to some butterfly books that perhaps can help point me to characters that might separate the two species.  Can you help?

Here's a slightly different view:

 

Photographed at Crane Creek Regional Park in Santa Rosa on 21 February 2015.