I mentioned sawflies the other day but I just noticed some more at the weekend. The larvae are munching their way through the leaves of one of my Betula albo-sinenensis – no common silver birch for them. I am fairly sure the species is Nematus septentrionalis. These have two or more generations a year and when the larvae are full they drop to the ground to pupate and hatch as adults. Like other sawflies they are gregarious and like to munch away in groups.
And when you get too close or you startle them they arch their backs, presumably to make the group look bigger and frighten off predators.