Notes from Underground: Towards Ultimate Hypotheses of Cyclic, Discontinuous Gas-Exchange in Tracheate Arthropods

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Abstract
SYNOPSIS. The Discontinuous Gas-exchange Cycle or DGC is generally thought to have evolved primarily as a means of reducing respiratory water loss rates in tracheate arthropods. However, several lines of evidence suggest that this supposition is oversimplified. I suggest that the DGC originated as an adaptation to the hypoxic and hypercapnic environments characteristic of underground burrows, rather than primarily as an adaptation to reduce respiratory water loss rates. This suggestion is based on a consideration of trans-spiracular oxygen and carbon dioxide partial pressure gradients in such environments, and the concomitant importance of decoupling oxygen and carbon dioxide exchange. The occurrence and/ or absence of the DGC in sundry arthropod taxa is discussed, and diverse phylogenetic and other arguments are advanced for the inferred distribution thereof.
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