Demon defoliators.

I thought these were caterpillars decimating the leave of my dog rose and also a rambling rose. When I got up close to photograph them they all reared up and stood to attention. I think perhaps they are pretending to be twigs to hide from predators.
An internet search came up with rose sawfly larvae which was confirmed by someone from the local wildlife trust. Apparently they are much loved by the birds, wish some would come and find them.