Collectively the Ixodidae and Argasidae may be distinguished from other Acari by the presence of a median probe, armed with recurved teeth, which project forwards beneath the mouth and between the palpi, and of a conspicuous spiracular area above and usually behind the base of the fourth leg on each side.
The two families Argasidae and Ixodidae may be distinguished as follows.
In the Ixodidae the capitulum is not overlapped by a forward extension of the dorsal area, which is smooth and firmly chitinized either in front or all over; the palpi are usually modified, that is to say, their second and third segments are usually excavated internally to form a sheath for the hypostome; there is a distinct sucker beneath the claws and the difference between the sexes is well marked, the males having the dorsal integument thickly and continuously chitinized, whereas in the females only its anterior portion bears a chitinous plate, the rest of the integument being soft to admit of its distension by the blood which is imbibed in quantity by members of this sex.
The common sheep-tick (Ixodes vicinus) of England, for example, infects cattle and dogs as well as sheep; and the pathogenetic Ixodidae above mentioned occur parasitically upon other mammals than those to which they convey the diseases specified.
Reptiles are infested as well as mammals, and it is no uncommon thing to find specimens of Ixodidae of various kinds adherent to tortoises, snakes and lizards.