Thistle Gall

This thistle gall is caused by a 'picture-wing fly' called appropriately, the thistle gall fly! Urophora cardui

I have seen the galls before but never seen the fly that causes it but apparently it is very distinctive with a black felt marker-pen 'm' on each wing. However, they are supposedly quite common here in the south of England.

They parasitise the creeping thistle Cirsium arvense.