It is commonly believed that the word “butterfly” is a derived from “butter-coloured fly” which is attributed to the yellow of the male Brimstone butterfly, the female being a much paler whitish-green. The Brimstone has a most exquisite wing shape, perfectly matching a leaf when roosting overnight or hibernating within foliage. This is one of the few species that hibernates as an adult and, as such, spends the majority of its life as an adult butterfly.

This single-brooded butterfly can be found in most months of the year, although peak flight times are in April and May as the hibernating adults emerge, and again in August when their offspring reach adulthood. Autumn is a good time to see this species as the adults are avid nectar-feeders as they build up their fat reserves in preparation for hibernation.

It was one of the very few butterflies I saw on my morning walk round Castor Hanglands. It was cool and still, and felt more like mid-September than late-August. There were remarkably few insects of any sort, and the site seemed strangely silent, with just the occasional harsh screech of a jay and the delicate fluttering of aspen leaves. I was hopping to try and capture an image of one of the many foraging hornets that can always be seen in the autumn, but none stopped for long enough. Still, this female Brimstone is probably more beautiful - I prefer her markings to the more gaudy male, especially the touches of purple and pink just behind her head.