Many insects exchange respiratory gases cyclically and discontinuously. A typical discontinuous gas exchange cycle (DGC) starts with a closed-spiracle (C) phase, during which little external gas exchange takes place, followed by a fluttering-spiracle (F) phase, which is usually dominated by diffusive oxygen uptake. The DGC is terminated by an open-spiracle (O) phase, during which accumulated CO2 escapes. This review critically examines the applicability of the DGC to insect gas exchange in general, discusses the primary mechanisms of gas exchange in the F and O phases, evaluates the widespread hypothesis that the DGC lowers respiratory water loss rates adaptively, and proposes new hypotheses concerning the evolutionary genesis of the DGC in insects and other tracheate arthropods.

Summary 1.The nutrient supply network model of the metabolic theory of ecology predicts that metabolic rate scales as mass 0·75 at all hierarchical levels.2. An alternative, cell size, model suggests that the scaling of metabolic rate is a by-product of the way in which body size changes, by cell size or number, or some combination thereof. It predicts a scaling exponent of mass 0·75 at the widest interspecific level, but values of mass 0·67 − 1·0 for lower taxonomic groups or within species. 3. Here these predictions are tested in insects using 391 species for the interspecific analysis, and the size-polymorphic workers of eight ant species at the intraspecific level. In the latter, the contribution of ommatidium size and number to variation in body length, which is closely related to eye size, is used to assess the relative contributions of changes in cell size and number to variation in body size. 4. Before controlling for phylogeny, metabolic rate scaled interspecifically as mass 0·82 . Following phylogenetic correction, metabolic rate scaled as mass 0·75 . 5. By contrast, the intraspecific scaling exponents varied from 0·67 to 1·0. Moreover, in the species where metabolic rate scaled as mass 1·0 , cell size did not contribute significantly to models of body size variation, only cell number was significant. Where the scaling exponent was < 1·0, cell size played an increasingly important role in accounting for size variation. 6. Data for one of the largest groups of organisms on earth are therefore inconsistent with the nutrient supply network model, but provide support for the cell size alternative.

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Many adult and diapausing pupal insects exchange respiratory gases discontinuously in a three-phase discontinuous gas exchange cycle (DGC). We summarize the known biophysical characteristics of the DGC and describe current research on the role of convection and diffusion in the DGC, emphasizing control of respiratory water loss. We summarize the main theories for the evolutionary genesis (or, alternatively, nonadaptive genesis) of the DGC: reduction in respiratory water loss (the hygric hypothesis), optimizing gas exchange in hypoxic and hypercapnic environments (the chthonic hypothesis), the hybrid of these two (the chthonic-hygric hypothesis), reducing the toxic properties of oxygen (the oxidative damage hypothesis), the outcome of interactions between O(2) and CO(2) control set points (the emergent property hypothesis), and protection against parasitic invaders (the strolling arthropods hypothesis). We describe specific techniques that are being employed to measure respiratory water loss in the presence or absence of the DGC in an attempt to test the hygric hypothesis, such as the hyperoxic switch and H(2)O/CO(2) regression, and summarize specific areas of the field that are likely to be profitable directions for future research.