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Sheep tick. Coloured scanning electron micrograph (SEM) of the underside of the head of a sheep tick (Ixodes ricinus) showing its mouthparts (centre right). Sheep ticks are vectors for Lyme disease and tick-borne meningoencephalitis in humans, and louping-ill in sheep. Magnification: x72 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/POWER AND SYRED)