I find Poplar Sawfly larvae feeding on my poplar trees around my garden every year. Although they feed gregariously and do strip the leaves from some branches it's not enough to be a problem for my wild hawkmoths also feeding on the same trees. So I just leave the Poplar sawflies alone.
When very small resting in this strange position they look a bit like a spider to me as well as the curious resting position of the larvae they can also secrete a fouls smelling liquid to put off predators. A single larvae would be less protected so it safety in numbers. They rest in this position under a leaf until the larvae are full-grown. The Sawfly larvae seem to communicate from the sensitive hairs on their heads. Feeding together in a long line on the edge of a leaf they eat the leaf backwards until their rear ends feel the other side of the leaf edge then decide to move to another leaf one would lead and the others follow. The larvae make a small cocoon in the earth to pupate. There are usually two broods a year and can be found May to August. The flying adult has a yellow-orange coloured abdomen they can be distinguished from wasps and bees by not having a narrow waist between thorax and abdomen.