The ticks are the largest of the Acarina. They are characterized by the fact that the hypostome, or "tongue" ([fig. 45]) is large and file-like, roughened by sharp teeth. They possess a breathing pore on each side of the body, above the third or fourth coxæ ([fig. 45b]).
There are two distinct families of the Ixodoidea, differing greatly in structure, life-history and habits. These are the Argasidæ and the Ixodidæ. We shall follow Nuttall (1908) in characterizing these two families and in pointing out their biological differences, and shall discuss briefly the more important species which attack man. The consideration of the ticks as carriers of disease will be reserved for a later chapter.
Argasidæ
In the ticks belonging to the family Argasidæ, there is comparatively little sexual dimorphism, while this is very marked in the Ixodidæ. The capitulum, or so-called "head" is ventral, instead of terminal; the palpi are leg-like, with the segments subequal; the scutum, or dorsal shield, is absent; eyes, when present, are lateral, on supracoxal folds. The spiracles are very small; coxæ unarmed; tarsi without ventral spurs, and the pulvilli are absent or rudimentary.
In habits and life history the Argasidæ present striking characteristics. In the first place, they are long-lived, a factor which counts for much in the maintenance of the species. They are intermittent feeders, being comparable with the bed-bug in this respect. There are two or more nymphal stages, and they may molt after attaining maturity. The female lays comparatively few eggs in several small batches.
Nuttall (1911) concludes that "The Argasidæ represent the relatively primitive type of ticks because they are less constantly parasitic than are the Ixodidæ. Their nymphs and adults are rapid feeders and chiefly infest the habitat of their hosts. * * * Owing to the Argasidæ infesting the habitats of their hosts, their resistance to prolonged starvation and their rapid feeding habits, they do not need to bring forth a large progeny, because there is less loss of life in the various stages, as compared with the Ixodidæ, prior to their attaining maturity."