Browsing by Author "Groenewald, Berlize"

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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.
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    Gas exchange patterns and water loss rates in the Table Mountain cockroach, Aptera fusca (Blattodea: Blaberidae)
    (Company of Biologists, 2013) Groenewald, Berlize; Bazelet, Corinna S.; Potter, C. Paige; Terblanche, John S.
    The importance of metabolic rate and/or spiracle modulation for saving respiratory water is contentious. One major explanation for gas exchange pattern variation in terrestrial insects is to effect a respiratory water loss (RWL) saving. To test this, we measured the rates of CO2 and H2O release (Embedded Image and Embedded Image, respectively) in a previously unstudied, mesic cockroach, Aptera fusca, and compared gas exchange and water loss parameters among the major gas exchange patterns (continuous, cyclic, discontinuous gas exchange) at a range of temperatures. Mean Embedded Image, Embedded Image and Embedded Image per unit Embedded Image did not differ among the gas exchange patterns at all temperatures (P>0.09). There was no significant association between temperature and gas exchange pattern type (P=0.63). Percentage of RWL (relative to total water loss) was typically low (9.79±1.84%) and did not differ significantly among gas exchange patterns at 15°C (P=0.26). The method of estimation had a large impact on the percentage of RWL, and of the three techniques investigated (traditional, regression and hyperoxic switch), the traditional method generally performed best. In many respects, A. fusca has typical gas exchange for what might be expected from other insects studied to date (e.g. Embedded Image, Embedded Image, RWL and cuticular water loss). However, we found for A. fusca that Embedded Image expressed as a function of metabolic rate was significantly higher than the expected consensus relationship for insects, suggesting it is under considerable pressure to save water. Despite this, we found no consistent evidence supporting the conclusion that transitions in pattern type yield reductions in RWL in this mesic cockroach.
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    A hierarchy of factors influence discontinuous gas exchange in the grasshopper Paracinema tricolor (Orthoptera: Acrididae)
    (The Company of Biologists, 2014-07) Groenewald, Berlize; Chown, Steven L.; Terblanche, John S.
    The evolutionary origin and maintenance of discontinuous gas exchange (DGE) in tracheate arthropods are poorly understood and highly controversial. We investigated prioritization of abiotic factors in the gas exchange control cascade by examining oxygen, water and haemolymph pH regulation in the grasshopper Paracinema tricolor. Using a full-factorial design, grasshoppers were acclimated to hypoxic or hyperoxic (5% O2, 40% O2) gas conditions, or dehydrated or hydrated, whereafter their CO2 release was measured under a range of O2 and relative humidity (RH) conditions (5%, 21%, 40% O2 and 5%, 60%, 90% RH). DGE was significantly less common in grasshoppers acclimated to dehydrating conditions compared with the other acclimations (hypoxia, 98%; hyperoxia, 100%; hydrated, 100%; dehydrated, 67%). Acclimation to dehydrating conditions resulted in a significant decrease in haemolymph pH from 7.0±0.3 to 6.6±0.1 (mean ± s.d., P=0.018) and also significantly increased the open (O)-phase duration under 5% O2 treatment conditions (5% O2, 44.1±29.3 min; 40% O2, 15.8±8.0 min; 5% RH, 17.8±1.3 min; 60% RH, 24.0±9.7 min; 90% RH, 20.6±8.9 min). The observed acidosis could potentially explain the extension of the O-phase under low RH conditions, when it would perhaps seem more useful to reduce the O-phase to lower respiratory water loss. The results confirm that DGE occurrence and modulation are affected by multiple abiotic factors. A hierarchical framework for abiotic factors influencing DGE is proposed in which the following stressors are prioritized in decreasing order of importance: oxygen supply, CO2 excretion and pH modulation, oxidative damage protection and water savings.
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    High metabolic and water-loss rates in caterpillar aggregations : evidence against the resource-conservation hypothesis
    (Company of Biologists, 2013) Schoombie, Ruben E.; Boardman, Leigh; Groenewald, Berlize; Glazier, Douglas S.; Van Daalen, Corne E.; Clusella-Trullas, Susana; Terblanche, John S.
    Several hypotheses have been proposed for explaining animal aggregation, including energy or water conservation. However, these physiological hypotheses have not been well investigated. Here, we report the effects of aggregation on metabolic (Embedded Image) and evaporative water-loss rates (Embedded Image) of the gregarious caterpillar Eutricha capensis, by comparing individuals and groups of individuals (N=10–100). Contrary to findings from previous physiological studies, we did not find an advantage to aggregation: unexpectedly, Embedded Image and Embedded Image did not decrease with increasing group size. Embedded Image and Embedded Image generally remained constant or increased in larger groups relative to individuals. The amount of water lost per unit of CO2 exchanged (Embedded Image: Embedded Image ratio) showed a marked increase in grouped caterpillars, particularly in larger groups. Other benefits of aggregation (e.g. reduced predation or increased growth rates) likely outweigh these potential costs, because individuals of E. capensis aggregate voluntarily despite no obvious energetic or hygric advantage, and other potentially confounding group effects (e.g. increased thermoregulatory advantage or whole-animal activity) are inconsequential. The results of this study provide an important exception to physiological studies reporting enhanced energy or water conservation in animal groups.