Water loss and metabolic water in starving Argas reflexus nymphs (Acari: Argasidae)

Basic components of the water balance were determined in A. reflexus unfed second instar nymphs kept at 30 degrees C and different relative humidities in darkness. At 2.5% and 32.5% RH, A. reflexus survived for 170 +/- 60 and 246 +/- 71 days, respectively. At 56%, 75.5% and 96.5% RH, the experiment was terminated after 168 days without any mortality. The initial water content of hydrated A. reflexus kept at 85% RH was 70.3 +/- 1.8% while the water content was close to 50% in ticks dehydrated at less than or equal to 32.5% RH at less than or equal to 14 days before each tick's death, respectively. Ticks dehydrated at 56% RH for 164 days regained water by active water vapour uptake at 96.5% RH, but no net water gain was observed in ticks kept at constant RHs less than or equal to 75.5%. A, reflexus showed extraordinarily low water loss rates. The whole body permeability (cuticular permeability plus respiratory water loss) was calculated as 78.1 +/- 22.9 ng cm(-2) cuticle h(-1) hPa(-1) saturation deficit (corresponding to 58.6 +/- 17.2 ng cm(-2) h(-1) Torr(-1)), the lowest value recorded for any arthropod species so far. Dry mass loss in fasting ticks was generally below 0.005% of tick dry mass per h, confirming a low level of metabolism, a typical attribute of resting ticks, also for A. reflexus. In spite of that low metabolism, A. reflexus partly compensated net water loss to ambient air by concurrent metabolic water production. Single specimens exhibited almost equilibrium water contents close to 70% even after 168 days at 30 degrees C and at a constant 56% or 75.5% RH. Unexpectedly, however, dry mass loss per unit time was significantly higher at 32.5% and especially at 2.5% RH than at RHs greater than or equal to 56% suggesting that A. reflexus increased its oxidative metabolism under dehydrating conditions and thus delayed lethal dehydration. Interestingly, the dry mass of dead ticks was almost identical irrespective of whether the ticks had previously been kept at 2.5% and 32.5% RH, i.e. irrespective of their different survival periods. This raises the question whether the ticks had died by dehydration or by exhaustion of energy reserves. (C) 1998 Elsevier Science Ltd. All rights reserved.