Nematus pavidus larvae twinsies, Netherlands
Assuming I have the species right, meet an obsessive feeder. These larvae feed in groups and often in an S shape as seen on the photo. Supposedly it is a defensive pose to deflect attacks, but I'm unsure of how it works in detail.
As this leaf was half-ruined, I cut it off and put it on a larger leaf for some closeups. During all this handling, they didn't care and just kept feeding, no matter what I did.
This observation is also a lesson in the importance of knowledge. Our region is currently plagued by the notorious buxus moth, an invasive species that shreds plants to bits in the blink of an eye. Henriette assumed this was such a case, and proceeded to cut of all affected leafs and dispose of the larvae. Only afterwards did I find out that despite a superficial resemblance, this is not a buxus moth larvae.
It's still a plague though that had eaten half a tall plant in a day or so, so I don't disagree with the disposal. Note that this species is also referred to as the birch sawfly, yet that common name is ambiguous and can refer to multiple species.