We examined and quantified the discontinuous ventilation cycle (DVC) characteristics of unfed nymphs and adults, as well as engorged nymphal and engorged diapausing and non-diapausing female adult life-stages, of the African tortoise tick Amblyomma marmoreum (Koch). All engorged stages ventilated continuously, with little evidence of active spiracular control. Unfed nymphs and adults ventilated discontinuously; at low activity and standard metabolic rate (SMR) levels, mean DVC duration was approximately 0.4 h in nymphs (mean mass 0.7 mg) and 2.8 h in female adults (mean mass 70 mg). SMR, measured as rate of CO2 production (V(dot)CO2; 0.064 mul mg-1 h-1 and 0.019 mul mg-1 h-1, respectively), was almost tenfold lower than that estimated for spiders of equivalent mass. In adults, the DVC was modulated to accommodate changing V(dot)CO2 chiefly by changes in DVC frequency. Modulation of other DVC characteristics was bimodal; at low V(dot)CO2 (below the ‘SMR threshold’), burst volumes were large and not correlated with V(dot)CO2, but the rate of CO2 emission during the burst was modulated by V(dot)CO2. Above the SMR threshold, burst volumes were small and tightly correlated with V(dot)CO2. No fluttering-spiracle phase could be detected, but CO2 bursts were triggered at low volumes above the SMR threshold, suggesting that hypoxia in addition to hypercapnia may initiate the termination of DVCs in the burst phase, rather than initiating the flutter phase as in insects. To explain this bimodal modulation of the DVC by V(dot)CO2 (and hence V(dot)O2) above and below the SMR threshold, we hypothesize that, below the SMR threshold, unfed ixodid ticks - with their very low SMR and large surface area/volume ratio - may obtain significant amounts of O2 by transcuticular or other putative non-spiracular avenues of O2 uptake (larval ticks obtain all their O2 in this way).

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