Doctoral Degrees (Conservation Ecology and Entomology)

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Browsing Doctoral Degrees (Conservation Ecology and Entomology) by browse.metadata.advisor "Chown, Steven L."

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    Discontinuous gas exchange in Orthoptera – mechanisms and hypotheses
    (Stellenbosch : Stellenbosch University, 2014-12) Groenewald, Berlize; Terblanche, J. S.; Chown, Steven L.; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Respiratory and ventilatory dynamics in insects are of fundamental importance to understanding evolved variation in gas exchange patterns, such as the discontinuous gas exchange cycle (DGC). However, the evolutionary origin and maintenance of the DGC in tracheate arthropods are poorly understood and highly controversial. Possible reasons for the occurrence of the DGC include water savings (hygric hypothesis) or the prevention of oxygen toxicity (oxidative damage hypothesis). Research presented in this dissertation aimed to examine the variation or modulation of the discontinuous gas exchange pattern, the environmental factors that potentially drive this relationship, and the competing hypotheses by: 1) examining the relative importance of convection vs diffusion during the DGC; 2) testing the oxidative damage and hygric hypotheses; 3) investigating ventilatory movements over the entire DGC; 4) examining intratracheal pressure patterns; and 5) investigating prioritisation of abiotic factors in the gas exchange control cascade. To accomplish these aims two model species of Orthoptera (Acrididae) which show discontinuous gas exchange cycles (DGCs) were chosen: the desert locust, Schistocerca gregaria, and a grasshopper commonly associated with wetlands, Paracinema tricolor. Firstly, I found that in S. gregaria, there was no clear intratracheal pressure pattern that accompanies the DGC. This shows that the gas exchange dynamics of Orthoptera differ substantially from those of Lepidoptera, in which a distinctive intratracheal pressure pattern accompanies the DGC, highlighting that the Lepidoptera cannot serve as a general model for all insects showing the DGC. Secondly, I found that in P. tricolor, a hierarchy of abiotic factors influence the DGC, rather than only a single hypothesis or factor explaining the occurrence of the DGC. For S. gregaria, gas exchange is most efficient at slightly hypoxic to normoxic conditions. At these conditions, diffusive gas exchange dominates, meaning that no body movements are necessary to aid gas exchange, making this mode of gas exchange less energetically costly than active convection. In conclusion, these results confirm that the maintenance of the DGC is affected by multiple abiotic factors. The findings of this dissertation have significant implications for understanding the mechanistic basis and energetic cost of gas exchange in insects.