Haller's Organ in the Tick Family Argasidae (Acari: Parasitiformes: Ixodida)

The structure of Haller's organ in all major groups of soft ticks, family Argasidae, is reexamined, and hypotheses of setal homology are proposed based on a comparative study of developmental patterns. The larvae of most taxa have 7 setae in the anterior pit (1 serrate, 1 conical, 2 fine, 2 porose, 1 grooved) and 4 porose setae in the capsule. The postlarval stages add up to 3 setae to the anterior pit (1 serrate, 1 porose, 1 grooved) and up to 3 porose setae to the capsule. Preliminary hypotheses of polarity, distinguishing ancestral from derived character states, are generated for most characters examined by comparisons with results obtained for the other major family of ticks, Ixodidae, and (where possible) other members of Parasitiformes. Most of the variability in the setal complement of the anterior pit and capsule results from paedomorphosis, and nearly all the observed changes in the setal complement (both modifications of addition patterns and changes in fundamental numbers) are restricted to single species or small species groups. A correlation between changes in Haller's organ structure and changes in life history patterns is suggested but cannot be confirmed based on presently available data.