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Can someone tell me if this is a Holly Blue or a Common Blue please..? How do you tell which is which..?

Hi Pixie. That's a male holly blue. The giveaways are the slightly broader dark margin towards the apex of the forewing, the chequered fringe of the forewing and the shade of blue, though that last is dodgy as light conditions and cameras can change everything. If you saw the underside you will have seen it is basically white, with black spots but no orange markings.

Guy

Thanks. I thought it was a Holly Blue but wasn't sure as I have one of the white underside with the little black spots too. It seems that the underside of these two is the easiest way to tell them apart as most of the upper side pics I've seen they look very similar...

One clue can come from the location.

Common Blues normally stay near the ground (there’s always an exception to the rule).

Holly Blues normally prefer to be on bushes (there’s always an exception to the rule).

Jack

The only other Blue with a similar underside in the UK is the Small Blue, which the upperside is dark brown. Ive seen Holly Blues the same size as Small Blue, but opened up to the Blue upperside. Holly Blue is a treetop species that does come down to lay on shrubs like Dogwood or feed on flowers.

Holly Blue is a treetop species that does come down to lay on shrubs like Dogwood or feed on flowers.

I added the "excdep;tion" in my earlier comment. I have seen Holly Blues on the ground on for example mud but they do normally stay much higher.

Jack