Got the hump

Had a couple of these Burnished Brass in last night's trap and so had them out for a photograph. One of my favourite moths, mainly because I caught them on my first ever moth trapping session back in 1985 on the Isle of Arran. Plus it's the only species I can remember apart from an Old Lady which I think was an island first at the time.

Many species of moth - both macro and micro - have these humps. I've no idea what their function is but I'd love to know. So if anyone out there can elucidate or has any cunning ideas......