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First published online February 1, 2008
Journal of Experimental Biology 211, 563-567 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.010041

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The hygric hypothesis does not hold water: abolition of discontinuous gas exchange cycles does not affect water loss in the ant Camponotus vicinus

John R. B. Lighton^1,2,* and Robbin J. Turner²

¹ Department of Biological Sciences, University of Nevada at Las Vegas, Las Vegas, NV 89154-4004, USA
² Sable Systems International, 6340 S. Sandhill Road, Suite 4, Las Vegas, NV 89120, USA

View larger version (15K): [in this window] [in a new window]   Fig. 1. The effect of gradual hypoxia on an alate Camponotus vicinus ant, mass 79.5 mg. Baselines for H2O and CO2 are equal to zero and are denoted by `b'. As hypoxia progresses, the interval between O phases (sharp upward spikes) increases until the discontinuous gas exchange cycle (DGC) ceases altogether below 8 kPa PO2. Rate of CO2 production (CO2) is the innermost of the two left-hand axis scales.

View larger version (4K): [in this window] [in a new window]   Fig. 2. The effect of hypoxia on the rate of CO2 production (CO2) in the F phase of the discontinuous gas exchange cycle (DGC) in Camponotus vicinus. As oxygen partial pressure (PO2) declines, spiracular area increases to compensate for the reduced trans-spiracular O2 concentration gradient, elevating CO2 output in the F phase as a byproduct. Consequently, the time taken to reach the hypercapnic threshold for the next O phase is increased, reducing DGC frequency (see Fig. 3). PO2 explains 91% of the variance in F phase CO2 (F1,9=90.1, P<10–5). Overall CO2 of the entire DGC remains constant across PO2s (see text). Error bars in this and subsequent graphs denote standard errors.

View larger version (5K): [in this window] [in a new window]   Fig. 3. The effect of hypoxia on the frequency of the discontinuous gas exchange cycle (DGC) in Camponotus vicinus. Hypoxia delays the initiation of the O phase (see Fig. 2), lowering DGC frequency. Partial pressure of O2 (PO2) explains 94% of the variance in DGC frequency (F1,9=150.4, P<10–6).

View larger version (5K): [in this window] [in a new window]   Fig. 4. The effect of hypoxia on water loss rate in the ant Camponotus vicinus. While the discontinuous gas exchange cycle (DGC) was expressed during the hypoxic ramp, overall water loss rate decreased significantly (see text); r2=0.673, F1,9=18.54, P=0.002. T = termination of the DGC at 8.44 kPa (Table 1). T–2s.d. = two standard deviations below the termination point. T–3s.d. = three standard deviations below the termination point. Hypoxia has no significant effect on water loss rate (WLR) over the indicated range, even when the DGC is not expressed (see text).