BRADLEY, T.J.; Univ. of California, Irvine: Discontinuous Gas Exchange in Insects Serves to Keep Oxygen Out

The respiratory system of insects is composed of a branching system of cuticle-lined tracheae. At the external openings of the tracheal system, spiracular valves control the degree of gas exchange between the tracheal lumina and the external atmosphere. Many insects exhibit prolonged periods of discontinuous gas exchange (DGC). These are characterized by a period of full spiracular closure (the closed phase), followed by a period in which the spiracles open briefly and erratically (the flutter phase), followed by a period when the spiracles are fully open (the open phase). This respiratory pattern has been observed in diverse groups of insects and even some other tracheate arthropods. Although the pattern has been intensely studied, the evolutionary explanation for the occurrence of DGC remains controversial. The most widely accepted explanation is that DGC reduces the loss of water from the respiratory system. It has also been proposed that it facilitates carbon dioxide excretion. I propose that the closed phase of the DGC serves to lower the partial pressure of oxygen at the level of the cells. The open phase is initiated by the accumulation of carbon dioxide in the insect. The insects� respiratory system is so efficient at delivering oxygen that toxic levels are provided to the cells during the open phase. The ensuing closed phase corrects this problem. The flutter phase serves to maintain the proper oxygen concentration at the level of the cells. The respiratory pattern of insects is driven therefore by two opposing control systems. One monitors carbon dioxide and serves to open the spiracles when carbon dioxide levels exceed a specific threshold. The second monitors oxygen and closes the spiracles to maintain a low level of oxygen appropriate for the protection of the cells from oxidative damage.