Insects breathe discontinuously to avoid oxygen toxicity

Abstract

The respiratory organs of terrestrial insects consist of tracheal tubes with external spiracular valves that control gas exchange. Despite their relatively high metabolic rate, many insects have highly discontinuous patterns of gas exchange, including long periods when the spiracles are fully closed. Two explanations have previously been put forward to explain this behaviour: first, that this pattern serves to reduce respiratory water loss¹, and second, that the pattern may have initially evolved in underground insects as a way of dealing with hypoxic or hypercapnic conditions². Here we propose a third possible explanation based on the idea that oxygen is necessary for oxidative metabolism but also acts as a toxic chemical that can cause oxidative damage of tissues even at relatively low concentrations. At physiologically normal partial pressures of CO₂, the rate of CO₂ diffusion out of the insect respiratory system is slower than the rate of O₂ entry; this leads to a build-up of intratracheal CO₂. The spiracles must therefore be opened at intervals to rid the insect of accumulated CO₂, a process that exposes the tissues to dangerously high levels of O₂. We suggest that the cyclical pattern of open and closed spiracles observed in resting insects is a necessary consequence of the need to rid the respiratory system of accumulated CO₂, followed by the need to reduce oxygen toxicity.