I am wondering if any of the museum team have observed this occurance. I came accross this scene when photographing insects within the Wyre forest (Shropshire). This fly was still alive when I touched it (it gave me a fright). Can flies self-impale on plant species such as thistles. My theory with it's positioning is that it's wing beats caused deeper penetration on the thistle thorn rather than escape. However,his perpendicular position to the thorn is curious.