Arnside and beyond

By gladders

A question of identity

I'm sitting here on a foul November evening, dreaming of the butterflies of Spring and Summer. They are fading in the memory. Time to bring out a backblip or two from my time out from blipworld earlier in the year.

This is a recently emerged butterfly perched on the flower bud of a rough hawkbit (Leontodon hispidus) plant that was growing in the rich meadow at Heathwaite on Arnside Knott. Here is what it looks like with its wings open. The question is: is it a Brown Argus (Aricia agestis) or a Northern Brown Argus (Aricia artaxerxes)? If you aren't interested in butterflies, stop reading now.

Had I been in southern England, I would have identified this without hesitation as a Brown Argus. I have one butterfly guidebook that says that the two species are never found in the same place, nor flying at the same time (though its diagrams of flight times show there is an overlap at this time in June). The Millennium Atlas of Butterflies, published in 2000, confirmed that the Brown Argus was not found in Cumbria. The Lakeland Wildlife website shows no records for Cumbria of the Brown Argus, although it has to be said, the website does need updating.

Yet it looks rather like a Brown Argus, particularly in this view of the underside of the wings with the pattern of white spots with black centres. The Northern Brown has the white spots but often lacks the black centres. The NBA also often has two conspicuous white spots on the upperside of the forewing, whereas this one has the black mark with a tiny dot of white.

The National Biodiversity Network distribution map shows something quite different to the above sources. Here there are records shown for the Arnside area, the Brown Argus is known to be a species that is expanding its range. It occupies the same sort of limestone grassland habitats and has the same larval foodplant (common rock-rose) as the Northern Brown. Could it be that at Heathwaite, we now have both species (and I did see some examples that had all the typical features of Northern Brown)? Maybe so, which then raises questions as to whether these once geographically isolated species are capable of hybridising in the window of a couple of weeks when they both may be on the wing.

So, that's why I am sorting out these photos on a wet evening. They need to go off to the Cumbria Biological Record Centre for identification.

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