The respiratory basis of locomotion in Drosophila

The respiratory system of insects has evolved to satisfy the oxygen supply during rest and energetically demanding processes such as locomotion. Flapping flight in particular is considered a key trait in insect evolution and requires an increase in metabolic activity of 10-15-fold the resting metabolism. Two major trade-offs are associated with the extensive development of the tracheal system and the function of spiracles in insects: the risk of desiccation because body water may leave the tracheal system when spiracles open for gas exchange and the risk of toxic tracheal oxygen levels at low metabolic activity. In resting animals there is an ongoing debate on the function and evolution of spiracle opening behavior, focusing mainly on discontinuous gas exchange patterns. During locomotion, large insects typically satisfy the increased respiratory requirements by various forms of ventilation, whereas in small insects such as Drosophila diffusive processes are thought to be sufficient. Recent data, however, have shown that during flight even small insects employ ventilatory mechanisms, potentially helping to balance respiratory currents inside the tracheal system. This review broadly summarizes our current knowledge on breathing strategies and spiracle function in the genus Drosophila, highlighting the gas exchange strategies in resting, running and flying animals.