These are lappet moth caterpillars of the Metanastria species.

These are lappet moth caterpillars of the Metanastria species. They can be tricky to tell apart, but these on the right are probably M. gemella. They have very interesting behaviour; you may find a large group (which can be referred to as an army of caterpillars) all bunched together on a tree trunk. In the daytime, they have this habit of aggregation that allows them to collectively defend themselves against the attacks of predators and parasitoids. Come night time, off they go to feed. You may find them in a long line following each other as they move to find their food, all nicely in single file, just like obedient little soldiers. Beware, as they can leave a nasty rash or sting if you brush against their hairs. SEE THEM Look for the caterpillars grouped together on tree trunks around May to June, or feeding at night in large groups in low branches of trees.