Discontinuous gas exchange and the significance of respiratory water loss in scarabaeine beetles

Abstract

SUMMARYRespiratory water loss in insects is a controversial topic. Whilst earlier studies considered respiratory transpiration a significant component of overall water loss, to the extent that it was thought to be responsible not only for the evolution of discontinuous gas exchange cycles (DGCs) but also for variation in DGC patterns, later work repeatedly questioned its importance. In particular, investigations of the proportional contribution of respiratory transpiration to total water loss in species showing DGCs suggested that respiratory transpiration was unlikely to be important in these species. In turn, these studies have been criticized on analytical grounds. In this study we investigated variation in cuticular and respiratory water loss rates in five Scarabaeus dung beetle species, all of which show discontinuous gas exchange cycles, to ascertain the significance of respiratory water loss using modern analytical techniques. In particular, we determined whether there is variation in water loss rates amongst these beetles, whether both respiratory and cuticular water loss rates contribute significantly to variation in the former, and whether metabolic rate variation and variation in the duration of the DGC periods contribute significantly to variation in respiratory water loss rate. Total water loss rate varied such that species from arid areas had the lowest rates of water loss, and both cuticular and spiracular transpiration contributed significantly to variation in overall water loss rate. Moreover, variation in metabolic rate and in the duration of the DGC periods contributed significantly to variation in respiratory water loss rate. By contrast, examination of proportional water loss revealed little other than that it varies between 6.5% and 21%, depending on the species and the temperature at which it was examined. Cuticular water loss scaled as mass0.721, but did not differ from that expected from geometric considerations alone. By contrast, respiratory water loss scaled as mass0.531, suggesting that gas exchange takes place by diffusion and convection. Our results provide direct evidence that respiratory water loss forms a significant component of water balance, and that changes in both metabolic rate and DGC characteristics contribute to modulation of respiratory water loss.

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

“…A meta-analysis of DGC characteristics, habitat aridity and temperature provided support for the hygric hypothesis (White et al, 2007). Comparative studies also showed that xeric species modulate DGC characteristics to restrict RWL (Duncan and Byrne, 2000;Chown and Davis, 2003). To the best of our knowledge, work by Chown and Davis (2003) provides the only comparative study examining RWL associated with DGC, demonstrating that xeric species have reduced RWL rates correlated with DGC characteristics, e.g.…”

“…Comparative studies also showed that xeric species modulate DGC characteristics to restrict RWL (Duncan and Byrne, 2000;Chown and Davis, 2003). To the best of our knowledge, work by Chown and Davis (2003) provides the only comparative study examining RWL associated with DGC, demonstrating that xeric species have reduced RWL rates correlated with DGC characteristics, e.g. longer interburst phase, shorter burst phase and/or shorter cycle length.…”

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

“…Thus, more comparative studies focusing on the importance of RWL (e.g. Chown and Davis, 2003) should be encouraged in order to appreciate the real importance and processes of selection to reduce the spiracular component of WLR in insects, especially in small ectotherms, such as insects, whose metabolic rate and WLR are more susceptible to increasing temperature and declining rainfall, as predicted in many regions because of global warming (Chown, 2011;Chown et al, 2011).…”

“…The contribution of respiratory water loss (RWL) to dehydration has been analysed mostly in insects showing discontinuous gas exchange (DGE) (e.g. Chown and Davis, 2003;Lighton, 1992;Quinlan and Lighton, 1999). There are two RESEARCH ARTICLE 1 Departamento de Biodiversidad y Biología Experimental, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina.…”

“…The contribution of respiratory water loss (RWL) to dehydration has been analysed mostly in insects showing discontinuous gas exchange (DGE) (e.g. Chown and Davis, 2003;Lighton, 1992;Quinlan and Lighton, 1999 techniques that enable CWL and RWL to be distinguished in insects with continuous gas exchange: the regression method (Gibbs and Johnson, 2004) and the hyperoxic switch method . Using these techniques, it was observed that spiracular control under continuous gas exchange can modulate RWL as effectively as DGE (Schilman et al, 2005;Gray and Chown, 2008).…”