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.