<unk> s (order Ixodida) are parasitic arachnids that are part of the mite superorder Parasitiformes. Adult<unk> s are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness".<unk> s are external parasites, living by feeding on the blood of mammals, birds, and sometimes reptiles and amphibians. The timing of the origin of<unk> s is uncertain, though the oldest known<unk>  fossils are from the Cretaceous period, around 100 million years old.<unk> s are widely distributed around the world, especially in warm, humid climates.<unk> s belong to two major families, the Ixodidae or hard<unk> s, and the Argasidae, or soft<unk> s. Nuttalliella, a genus of<unk>  from southern Africa is the only member of the family Nuttalliellidae, and represents the most primitive living lineage of<unk> s. Adults have ovoid/pear-shaped bodies (idiosomas) which become engorged with blood when they feed, and eight legs. Their cephalothorax and abdomen are completely fused. In addition to having a hard shield on their dorsal surfaces, known as the scutum, hard<unk> s have a beak-like structure at the front containing the mouthparts, whereas soft<unk> s have their mouthparts on the underside of their bodies.<unk> s locate potential hosts by sensing odor, body heat, moisture, and/or vibrations in the environment.<unk> s have four stages to their lifecycle, namely egg, larva, nymph, and adult.<unk> s belonging to the Ixodidae family undergo either a one-host, two-host, or three-host lifestyle. Argasid<unk> s have up to seven nymphal stages (instars), each one requiring blood ingestion, Argasid<unk> s undergo a multihost lifestyle. Because of their hematophagous (blood-ingesting) diets,<unk> s act as vectors of many serious diseases that affect humans and other animals.
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