Insects generally maintain a low endotracheal partial pressure of oxygen. In the case of termites, we present evidence that this respiratory adaptation is also exaptive for the creation of a favorable microhabitat for oxygen-sensitive protist endosymbionts in the guts of termites. We present evidence that the oxygen flush accompanying the O phase of the discontinuous gas exchange cycle (DGC) is absent in termites; that termites actively "guard" internal hypoxia by reducing spiracular conductance in the presence of hyperoxia; and that this feature of insect gas exchange, proving as it does an exception to the rule of the insect DGC, suggests that the selective pressures responsible for the evolution of the DGC (and its subsequent loss in some taxa) are not as simple as the customary theories concerning reduction of respiratory water loss rates would suggest.