The common groundhopper (Tetrix undulata) is a relative of grasshoppers and bush-crickets; the order Orthoptera. It is not considered a common species but it is thought that it is probably under recorded given its relatively small size and choice of habitat.
This insect could not be considered a 'looker'! At first sight I thought it was a bird dropping on a sedge leaf but then it moved and I took a closer look, Having never seen anything like it before I had to take a photograph and then thumb through the books when I got home. It is quite small, about half an inch long, is a dull mottled brown and has quite warty or undulating skin (undulata).
It is one of three species of groundhopper found in Britain and all three are found in Dorset too. This is the most likely to be found although, as I said, it is not common. It favours damp habitat around rivers, water seepages, gravel pits and so on. I found this one next to the Longham reservoirs which fits with this habitat exactly!