Abstract
The adaptive significance of insect gas exchange patterns, in particular the importance of the discontinuous gas exchange cycle (DGC), has been extensively debated by insect physiologist for decades. The DGC is characterized by the release of bursts of CO sub(2) from the insect (open phase), followed by extended periods of spiracular closure (closed phase) and a period of time when spiracles open and close very rapidly (flutter phase). A variety of hypotheses have been set forth to explain the DGC, however, in this dissertation we focused on the water conservation and oxidative damage hypotheses to elucidate factors that might influence gas exchange patterns in insects. Using flow-through respirometry, we measured CO sub(2) release in a variety of insects (Rhodnius prolixus, Gromphadorhina portentosa and Aquarius remigis) after altering metabolic rates (using temperature or feeding) and ambient humidity. Since discriminating between the closed and flutter phase of the DGC is difficult, we developed a novel and objective method of determining periods of spiracular closure using hyperoxic conditions. We employed this method in our studies to discriminate between the different gas exchange patterns. When the metabolic rate of insects was altered with temperature we saw that at the lowest temperature (lowest metabolic rate) insects displayed the DGC. As temperature was slowly increased, insects transitioned from the DGC, to a cyclic and finally to a continuous gas exchange pattern. The duration of spiracular closure decrease with increased metabolic rate. These same results were observed when metabolic rates were altered with feeding. Ambient humidity did not have an effect on insect gas exchange pattern. If metabolic rates were not changed, insects showed the same gas exchange pattern during humid (>85% RH) and dry conditions. This was true for terrestrial and semi-aquatic insects. During humid conditions insects employed the DGC, cyclic and continuous gas exchange as metabolic rates were altered. We found that metabolic rate, and not ambient humidity, influences the gas exchange pattern of insects. Our results support the hypothesis that insects transition from one pattern to the next as a result of an interaction between oxygen demand and oxygen supply.