Comparative Morphology of Argasid Larvae (Acari: Ixodida: Argasidae), with Notes on Phylogenetic Relationships

A comparative analysis of larval morphology in the Argasidae was conducted, emphasizing the setation of the appendages. Larvae of 76 argasid species, including one or more representatives of all nominal genera and subgenera, were examined and compared with larval and nymphal Ixodidae, Holothyrida, and Mesostigmata (Parasitiformes). Between-species variation in the setation pattern of the appendages is considerable, whereas within-species variability is low. All of the observed variation in setal patterns can be explained as the result of ontogenetic shifts, including both accelerations and delayed additions. Hypotheses of homology for most palpal and leg setae are established based on ontogenetic and comparative evidence. A preliminary analysis of relationships, based on larval characters only, suggests that all bat-associated taxa in the genera Argas and Ornithodoros are part of a monophyletic lineage including the genus Antricola (and possibly Nothoaspis), the subgenus Argas (Microargas) is more closely related to the subgenus Ornithodoros (s.s.) than to any of the subgenera in Argas, and the subgenera Ornithodoros (Alveonasus), Ornithodoros (Proknekalia), Argas (Ogadenus), and Argas (Secretargas) form a monophyletic group (resembling the genus Alveonasus sensu Filippova 1966). The established genera Argas and Ornithodoros do not appear to be natural groups, but modifications of existing classifications of the Argasidae are delayed until characters from the postlarval stages can be integrated in the systematic analysis.