What does a brimstone butterfly look like?

This brimstone (Gonepteryx rhamni) was so obvious as it fluttered around the woodland understory at Wandlebury with its sulfurous wings. However, as soon as it settled and closed it was suddenly as well camouflaged as any insect might be while feeing on a periwinkle (Vinca minor).

According to Wikipedia, across much of its range, G. rhamni is the only species of its genus, and is therefore simply known locally as the brimstone. I’m dubious about their claim that the word “butterfly” comes from this insects in that its yellow upper wings led early British naturalists to call it the butter-coloured fly. Much more pleasing is the idea that the yellow faecal paste butterflies produce is known in Dutch as boterschijte and that’s where our word butterfly comes from. Sounds ridiculously contrived, almost as contrived as the idea that these insects alight on uncovered butter and eat it. It’s really just that lots of butterflies have a buttery colour and they fly.