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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

^* Author for correspondence (e-mail: john{at}johnlighton.org)

Accepted 3 December 2007

The discontinuous gas exchange cycle (DGC) of insects and other tracheate arthropods temporally decouples oxygen uptake and carbon dioxide emission and generates powerful concentration gradients for both gas species between the outside world and the tracheal system. Although the DGC is considered an adaptation to reduce respiratory water loss (RWL) – the `hygric hypothesis' – it is absent from many taxa, including xeric ones. The `chthonic hypothesis' states that the DGC originated as an adaptation to gas exchange in hypoxic and hypercapnic, i.e. underground, environments. If that is the case then the DGC is not the ancestral condition, and its expression is not necessarily a requirement for reducing RWL. Here we report a study of water loss rate in the ant Camponotus vicinus, measured while its DGC was slowly eliminated by gradual hypoxia (hypoxic ramp de-DGCing). Metabolic rate remained constant. The DGC ceased at a mean P_O2 of 8.4 kPa. RWL in the absence of DGCs was not affected until P_O2 declined below 3.9 kPa. Below that value, non-DGC spiracular regulation failed, accompanied by a large increase in RWL. Thus, the spiracular control strategy of the DGC is not required for low RWL, even in animals that normally express the DGC.

Key words: discontinuous gas exchange, DGC, insect respiration, respiratory water loss