A caterpillar of the knotgrass moth, Acronicta rumicis (Lepidoptera: Noctuidae). Similar looking to the caterpillar of the vapourer moth, Orgyia antiqua, but without the four 'shaving brushes' on its back. The above specimen was in ankle-high grass. A couple of weeks earlier I had photographed one on ribwort plantain (Plantago lanceolata):
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-09-25 and 2010-09-10, respectively.