Having moved from the countryside to a house with a very overgrown garden in the city, I have found black jumping insects living in the soil in my greenhouse and under pots outside. They look like shrimps or fleas and are about 5mm long . When disturbed they wriggle and jump and quickly dissapper into the soil. A search of Google images suggests they may be Terrestrial amphipods. I thought these were only found in warmer climates. I am in Exeter in Devon. Anyone any ideas about what they are and more importantly will they harm plants or can they be left in peace?