Gas exchange patterns and water loss rates in the Table Mountain cockroach,Aptera fusca(Blattodea: Blaberidae)

Abstract

SUMMARYThe 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 CO 2 and H 2 O release (V CO2 and V H2O , 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 V CO2 , V H2O and V H2O per unit V CO2 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. V CO2 , V H2O , RWL and cuticular water loss). However, we found for A. fusca that V H2O 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. Supplementary material available online at

The effect of discontinuous gas exchange on respiratory water loss in grasshoppers (Orthoptera: Acrididae) varies across an aridity gradient

“…These values are in agreement with previously reported values for Melanoplus sanguinipes (15%; Rourke, 2000) and for Romalea guttata and Taeniopoda eques (<10%; Quinlan and Hadley, 1993). Significantly lower proportions of RWL in T. pulchripennis may support a role for modified DGC characteristics in reducing water loss in this xeric-adapted species (Tables 1, 2) (see Groenewald et al, 2013, and references therein). Moreover, calculation of RWL based on water vapour emission during the burst phase only (see Materials and methods) underestimated RWL for the Ocneropsis sp.…”

“…Alternatively, the elimination rate of CO 2 accumulated in the haemolymph during the interburst may depend on carbonic anhydrase activity during the burst. Therefore, variation in carbonic anhydrase activity could be responsible for interspecific variation in burst duration if the rate of bicarbonate conversion back to CO 2 is a limiting step in CO 2 Groenewald et al, 2013). Oxygen consumption rates were calculated for the three study grasshopper species by assuming a respiratory exchange ratio of 0.84 (Lighton, 2008).…”

“…Employing DGC is also associated with increased desiccation and starvation resistance (Schimpf et al, 2012). Lower rates of RWL are associated with DGC compared with continuous exchange in lepidopteran larvae (Williams et al, 2010), but findings from similar studies are contradictory to predictions of the hygric hypothesis in ants (Gibbs and Johnson, 2004;Lighton et al, 2004;Lighton and Turner, 2008) and cockroaches (Groenewald et al, 2013). Moreover, earlier studies provided the first experimental challenge to the hygric hypothesis, showing that DGC is abolished in dehydrated insects when body water conservation is most essential Quinlan and Hadley, 1993).…”

Water loss in tree weta (Hemideina): adaptation to the montane environment and a test of the melanisation–desiccation resistance hypothesis

“…While respiratory water loss as a function of metabolic rate can vary widely within and among insect species (Groenewald et al, 2013), water loss can be reduced through changes in respiratory patterns (Chown, 2002). Continuous gas exchange (CGE), where spiracles do not close in a coordinated manner, is the most prevalent pattern (Marais et al, 2005).…”

Metabolism and water loss rate of the haematophagous insect,Rhodnius prolixus: effect of starvation and temperature

“…*Calculated using Meeh's formula (S=k×W 0.667 ), where k=8.1 (species-specific constant) (Wigglesworth, 1945) and W is mass (in g). Groenewald et al, 2013).…”