Pili-Pala*

I don't know which is Pili and which is Pala but when I saw these two butterflies whirling and twirling over the brambles in what seemed a passionate pas de deux I assumed the dance was a preamble to a sexual union.

However, when they settled on leaves a few inches apart I saw that they were both male specimens of Pyronia tithonus, the gatekeeper butterfly. It's easy to tell from the dark patches on the forewings made by specialized scent cells called androconia.
Androconia are found mainly on male butterflies. They usually exist as slightly raised dark streaks or patches on the forewings, and often have a mealy appearance. At the base of the androconia are tiny sacs containing scent ( pheromones ). The scent is disseminated via tiny hairs or plumes on the edges of the scales, and used to entice females to copulate.
(See here for more fascinating information on butterfly anatomy.)

I did briefly wonder if these might be gay male butterflies attracted to each other in the way that the biologist Bruce Bagemihl documented, in his 1999 book Biological Exuberance, the hundreds of non-human animal species that display homosexual behaviour: The Love That Dare Not Squeak its Name as one newpaper article put it.
But although (of insects) bed bugs, dragonflies and fruit flies are recorded as doing so, butterflies are not and when I turned to the internet I found that the male dance routine is about territorial rivalry not love.

*Pili-pala is the Welsh word for butterfly. I blipped one of these gatekeepers last year and included something about the association with our local West Wales history that its name evokes for me.