Discontinuous gas exchange in Orthoptera – mechanisms and hypotheses

[摘要] ENGLISH ABSTRACT: Respiratory and ventilatory dynamics in insects are of fundamental importance to understandingevolved 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 poorlyunderstood and highly controversial. Possible reasons for the occurrence of the DGC include watersavings (hygric hypothesis) or the prevention of oxygen toxicity (oxidative damage hypothesis).Research presented in this dissertation aimed to examine the variation or modulation of thediscontinuous gas exchange pattern, the environmental factors that potentially drive thisrelationship, and the competing hypotheses by: 1) examining the relative importance of convectionvs diffusion during the DGC; 2) testing the oxidative damage and hygric hypotheses; 3)investigating ventilatory movements over the entire DGC; 4) examining intratracheal pressurepatterns; 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 discontinuousgas exchange cycles (DGCs) were chosen: the desert locust, Schistocerca gregaria, and agrasshopper 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 showsthat the gas exchange dynamics of Orthoptera differ substantially from those of Lepidoptera, inwhich a distinctive intratracheal pressure pattern accompanies the DGC, highlighting that theLepidoptera cannot serve as a general model for all insects showing the DGC. Secondly, I foundthat in P. tricolor, a hierarchy of abiotic factors influence the DGC, rather than only a singlehypothesis or factor explaining the occurrence of the DGC. For S. gregaria, gas exchange is mostefficient at slightly hypoxic to normoxic conditions. At these conditions, diffusive gas exchangedominates, meaning that no body movements are necessary to aid gas exchange, making this modeof gas exchange less energetically costly than active convection. In conclusion, these resultsconfirm that the maintenance of the DGC is affected by multiple abiotic factors. The findings of thisdissertation have significant implications for understanding the mechanistic basis and energetic costof gas exchange in insects.

[发布机构] Stellenbosch University