Spiny Caterpillar
The caterpillar of an ermine moth, a Spilosoma sp, possibly Spilosoma urticae (Lepidoptera: Arctiidae). It was moving fram a leaf of silverweed (Argentina anserina) to a leaf of creeping buttercup (Ranunculus repens).
Photo taken the field below Chazey Wood, near Caversham, UK, on 2014-06-29.