Tick Systematics and Identification

Ticks are obligate ectoparasites of terrestrial vertebrates (amphibians, reptiles, birds, and mammals). They belong to the class Arachnida, which as a group are distinguished from the class Insecta by having four pairs of legs as nymphs and adults, lacking both antennae and wings, and having two pairs of appendages associated with their mouthparts, i.e., chelicerae and pedipalps. All ticks and some mites are the only members of the Arachnida that are parasitic. In terms of size, ticks are the largest acarines and can be recognized by their dorsoventrally flattened appearance and by one component of the mouthparts, a hypostome with recurved teeth that acts as a holdfast organ, anchoring the tick to its host. They also possess a unique sensory apparatus, called Haller's organ on the tarsus of each foreleg. Ticks can be divided into three families, the Argasidae (soft ticks), Ixodidae (hard ticks), and Nuttalliellidae. The last-named family includes a single species from Africa that is infrequently collected, but the other two families are widespread and include many species, some of which may bite humans. While several species of ticks may ocionally attach to humans, relatively few species commonly bite humans, and some of these are vectors of disease-causing pathogens. This chapter considers ticks that are frequently recorded as biting humans. It lists identification guides for ticks that occur in the world's different zoogeographic regions.