Described by the Guardian as “the UK’s most elusive butterfly”, the Large Tortoiseshell was once fairly common in the UK. However, the species suffered a severe decline in numbers throughout the 20th Century. Part of the reason for its decline was that its caterpillars feed mainly on elm trees, millions of which were wiped out by Dutch Elm Disease. It is now generally thought to be nationally extinct.
However, Large Tortoiseshells have been spotted a few times this year on Portland.
You can find more information on the Large Tortoiseshell from the Butterfly Conservation here. Also, the Butterfly Conservation gives this tip for identifying the Large Tortoiseshell:
As well as being larger than the Small Tortoiseshell, the Large Tortoiseshell is it is generally duller, does not have the white marks on the leading edge of the forewings and almost never has blue spots along the edge of the forewings.
And there’s a lovely photo of a Large Tortoiseshell here.