Acronicta rumicis.

This striking catterpillar protects itself from birds by having dense clumps of hair (setae) and alarming colouration. This hair acts as a defence against birds and predatory insects such as parasitoid wasps which find it difficult to penetrate beyond the hairs to lay eggs beneath the caterpillar’s skin.

Status: widely distributed and quite common in most of Britain.

In Cornwall and other mild-climated areas in the south the Knot Grass moth flies in two generations in May and June and again from August to September.

The caterpillars feed on a wide variety of low herbaceous plants including plantain, dock and knotgrass, as well as bramble, sallows and hawthorn.

More info here: http://www.ukmoths.org.uk/species/acronicta-rumicis/