Taxonomy

Ticks are members of the same phylum (Arthropoda) of the animal kingdom as insects, but are in a different class.
The subphylum Chelicerata includes the class Arachnida, which again contains several subclasses. The subclass Acari (syn. Acaria, Acarina, Acarida) includes ticks.
The main difference is the body of a tick is composed of only two sections while insect bodies have three sections.

There are two well established families of ticks, the Ixodidae (hard ticks), and Argasidae (soft ticks).

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Evolution

Ticks are blood feeding external parasites of mammals, birds, and reptiles throughout the world.
The evolutionary thesis suggests that both ixodid and argasid ticks have been in existence since the late Paleozoic to early Mesozoic eras. Antecedent forms evolved as obligate external parasites of smooth-skinned reptiles during the late Paleozoic era.

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Distribution

Approximately 850 tick species have been described worldwide.
The family Ixodidae is by far the largest and economically most important family with 13 genera and approximately 650 species.

The most important species of hard ticks in North America as parasites of companion animals are:
Ixodes pacificus, Ixodes scapularis (dammini) (Black-legged (Deer) tick), Dermacentor variabilis (American dog tick), Rhipicephalus sanguineus (Brown dog tick) and Amblyomma americanum (Lone star tick), Haemaphysalis leporispalustris, Boophilus annulatus and Boophilus microplus.

Established and reported distribution of the Lyme disease vectors Ixodes scapularis (I. dammini) and I. pacificus, by county, United States. 1907-1996.

I. ricinus (Castor bean tick, sheep tick) a prostriate tick, is the most common tick in Northern Europe and is an important vector of both animal and human diseases.