Any of numerous small, parasitic arachnids of the suborder Ixodida that feed on the blood of animals. Like their close relatives the mites and unlike spiders, ticks have no division between cephalothorax and abdomen. Ticks differ from mites by being generally larger and having a sensory pit at the end of their first pair of legs. Many ticks transmit febrile diseases, such as Rocky Mountain spotted fever and Lyme disease.