Whilst I was taking photographs of these beautiful Brimstone butterflies (Gonepteryx rhamni) nectaring on the little pink flower-pots of Bell heather (Erica cinerea), I noticed that they stopped when a cloud passed over.

The day was relatively warm, but the site was on top of a small mountain, so when large clouds obscured the sun, the temperature fell markedly. I was nevertheless, surprised at how soon they stopped feeding and went and sat on a leaf! See below.

The Brimstones were flitting around in a mass of gorse and heather. I wonder how much nectar each of those little pink flowers holds?

When the sun went in, the butterflies retired to a leaf and sat still until the cloud passed. They had presumably given some thought to their position and when the sun came out again, they waited for a few moments, warming up in the sunshine, before flying off and starting to feed again. I think these lovely greenish-yellow individuals were males.

Butterflies are heliothermic. Which means that they rely on the sun to warm up. To get the most from sunlight, they position themselves so that they can receive the most incident radiation. In butterflies that bask with open wings, the best position is to keep the vertical body axis parallel to the incident radiation, but that does not mean that they always follow scientific advice! This basking Peacock butterfly (below) is casting a shadow to the right!

Many species of butterflies do not however, open their wings and bask with their dorsal wing surfaces, like these Brimstones. They keep their wings closed and orientate the ventral surfaces perpendicular to the solar radiation; these are so-called lateral baskers. Although it must vary from species to species, most of the radiant heat is apparently absorbed by the basal regions of the ventral hind wings (Kingsolver, 1988). It is then transferred to the body, elevating the body temperature. The yellow hind wing of the lateral basking Gonepteryx was found to absorb more light, and presumably more heat, than that of the large white of Pieris brassicae (Schmitz, 1994).

Male Brimstone butterflies have ultraviolet patterns on the dorsal surfaces of their wings, but these are not visible through human eyes and will in any case only be visible when the wings are open, during flight (below) or when the males are displaying to the females.

As a final aside, my attention was drawn to the orange, crescent-shaped ‘shoulder’ on the base of the hind wing (below). I have no idea what this structure is for – there are tiny little, orange tufts on the leading edge – but it must be quite prominent from the front. A bit like landing lights! So much we do not (or at least I do not!) know about butterflies!

The photographs of the Brimstone butterflies were taken on 9th September 2018, at a height of about 400m on Monte Panda, Concello de Mañón, Galicia, Spain.

References

Kingsolver, J. G. (1988). Thermoregulation, flight, and the evolution of wing pattern in pierid butterflies: the topography of adaptive landscapes. American Zoologist, 28(3), 899-912.

Schmitz, H. (1994). Thermal characterization of butterfly wings—1. Absorption in relation to different color, surface structure and basking type. Journal of Thermal Biology, 19(6), 403-412.