On the Origin of Soft Ticks (Parasitiformes, Ixodoidea; Argasidae)

The origin of the common evolutionary branch of soft and hard ticks is dated to the Late Permian, approximately coinciding with the end-Permian extinction; the divergence of the common stem of Ixodoidea into the ancestral forms of Argasidae and Ixodidae occurred in the Triassic. The ancestors of the common branch of Ixodoidea (Ixodidae, Argasidae, and Nuttalliellidae) were non-bloodsucking arthropods closely resembling the extant Holothyridae. Bloodsucking and, later, temporary ectoparasitism appeared only after divergence of the main branches of Ixodidae, Argasidae, and Nuttalliellidae, independently in each group. At early stages of the soft tick evolution, bloodsucking was a brief process, while prolonged feeding developed later in their larvae and promoted dispersal. Preservation of short-time feeding can be explained by adaptation to dwelling in bird nests or in shelters visited by birds, and later, also in mammal burrows. The shift of the gnathosoma onto the ventral surface of the idiosoma prevented combing of the parasite from the host during bloodsucking. Short-time feeding resulted in preservation and development of coxal glands as the main organs for removal of excess liquid during bloodsucking. Adaptation to living in shelters (nests, burrows, rock crevices, spaces under dead tree bark, etc.) promoted development of polyphagy and the ability to survive long-term starvation.