A female blacklegged, or deer tick, (Ixodes scapularis), about the size of a sesame seed, stumbling its way through hairs on human skin. These ticks are responsible for the transmission of Lyme disease, the most common tick-borne disease in the northern hemisphere and which can be deadly if not properly treated. This tick might, in fact, be infected, but the scientist volunteering his arm has little to fear as the the tick must be attached for at least 24 hours before it can transmit the bacteria responsible for the disease. Blacklegged ticks have saw-like mouthparts with dozens of barbs which make tthem difficult to remove. Ticks belong to an order of arachnids and possess eight multi-segmented legs which are adapted for climbing and clinging.