Spiracle structure in ticks (Ixodida: Anactinotrichida: Arachnida): resume, taxonomic and functional significance

Spiracle and tracheal structure in the extant Ixodida is revised and shown to comprise 29 distinct component characters, some of which are common to all Anactinotrichida, while others are unique to the Ixodida or one of its six component clades of ((Argasidae Nuttalliellidae) (Prostriata Metastriata)). Structural variation both between and within families is based upon combinations of minor differences in the component characters, only one of which, spiracular position, proved to be incongruent within the most parsimonious cluster- and tree-analysis solutions. Tracheal airflow in ticks is mediated via passive diffusion gradients. In the argasid spiracle, both aeropyles and ostium are functional, although the latter is only opened briefly during infrequent periods of activity. The ixodid ostium is sealed and all gas exchange takes place via an enlarged sieveplate which reduces transpiration via small aeropyles, an underlying dense array of pedicels and possibly hygroscopic sub-atrial glands. Changes in spiracular morphology from a more ‘ancestral’ argasid type to a more ‘derived’ ixodid type are correlated with changes in tick behaviour, particularly with increased activity associated with the change from nidicoly to host-seeking.