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First published online August 31, 2004
Journal of Experimental Biology 207, 3477-3482 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01168

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The role of discontinuous gas exchange in insects: the chthonic hypothesis does not hold water

Allen G. Gibbs^1,* and Robert A. Johnson²

¹ Department of Ecology and Evolutionary Biology, 1041 E. Lowell Street, University of Arizona, Tucson, AZ 85721 USA
² School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, USA

^* Author for correspondence (e-mail: agibbs{at}arl.arizona.edu)

Accepted 29 June 2004

Insects breathe through valved openings (spiracles) in their cuticle. Many insects open and close their spiracles in a cyclic pattern (discontinuous gas-exchange cycles, or DGC). These cycles were observed over half a century ago, their hypothesized function being to minimize loss of water from the tracheal system. However, numerous recent studies have found that respiration accounts for a small fraction of total water loss, and that insects stop performing DGC at times when this pattern would be most useful. Thus, the importance of cyclic gas exchange for water conservation has been challenged. The leading alternative is the chthonic hypothesis, which proposes that DGC originated in insects from hypercapnic (high CO₂) environments (e.g. burrows) to aid in release of carbon dioxide. By keeping the spiracles closed, insects would concentrate CO₂ and increase the gradient for outward diffusion of CO₂. CO₂ would be released rapidly when the spiracles opened, and respiratory water loss would be reduced. The chthonic hypothesis therefore predicts that DGC minimizes the ratio of respiratory water loss to CO₂ release relative to other modes of gas exchange. We tested the chthonic hypothesis by simultaneously measuring water loss and CO₂ release in reproductive females (queens) of the seed-harvester ant Pogonomyrmex barbatus, a burrowing species from North American deserts. Queens used one of three patterns of gas exchange, discontinuous, cyclic and continuous. We resolved the problem of separating cuticular transpiration and respiratory water loss for individuals that used continuous gas exchange by developing a regression method that can be used across all patterns of gas exchange. The ratio of respiratory water loss to CO₂ release did not differ among ants using different patterns of gas exchange, in contrast to the expectation of the chthonic hypothesis. Metabolic rate, however, varied with gas-exchange pattern, and was lowest for individuals that used discontinuous gas exchange, intermediate for individuals using cyclic gas exchange, and highest for individuals using continuous gas exchange.

Key words: discontinuous gas-exchange cycle, queen mating stage, Pogonomyrmex barbatus, respiratory water loss, seed-harvester ant

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