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

Adrian L V Davis,Steven L Chown

doi:10.1242/jeb.00603

Abstract

Respiratory 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 mass(0.721), but did not differ from that expected from geometric considerations alone. By contrast, respiratory water loss scaled as mass(0.531), 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.

Highlights

The significance of the contribution of respiratory transpiration to total water loss in insects is controversial
Because respiratory transpiration is mostly restricted to a short O-period, it has long seemed obvious that discontinuous gas exchange has evolved to reduce water loss
The alternatives to these null expectations are straightforward, in the case of Discontinuous gas exchange cycles (DGCs) period characteristics there are two, non-exclusive alternative hypotheses. These are that covariation between C- and/or Fperiod duration and water loss rate is negative (Lighton, 1990; Davis et al, 1999), and/or that covariation between O-period duration and water loss rate is positive (Lighton, 1990; Duncan et al, 2002a). We examine these three major hypotheses in an analysis of water loss in five species of Scarabaeus dung beetles that exhibit discontinuous gas exchange (Davis et al, 1999)

Summary

Introduction

The significance of the contribution of respiratory transpiration to total water loss in insects is controversial. Several studies have reported considerable variation in the characteristics (especially the duration) of the C-, F- and O-periods among species from different habitats, suggesting that modulating the characteristics of the periods is a significant way in which respiratory water loss might be altered (Lighton, 1988a, 1990, 1991; Lighton et al, 1993; Davis et al, 1999; Bosch et al, 2000; Duncan and Dickman, 2001; Duncan et al, 2002a).

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