![Wink :wink:](./images/smilies/icon_wink.gif)
It is common to see hoverflies and other Diptera settling on the wings of Brimstones, particularly on mornings in April and May. The flies are probably attracted to the wings because they reflect solar energy very strongly - the heat is reflected back onto the body of the fly, and converted into energy. Brimstones also reflect ultra-violet very strongly, and this is probably detected very easily by passing flies.
![Very Happy :D](./images/smilies/icon_biggrin.gif)
Lots more about butterfly behaviour at :