Respiratory dynamics of discontinuous gas exchange in the tracheal system of the desert locust,Schistocerca gregaria

Abstract

SUMMARYGas exchange dynamics in insects is of fundamental importance to understanding evolved variation in breathing patterns, such as discontinuous gas exchange cycles (DGCs). Most insects do not rely solely on diffusion for the exchange of respiratory gases but may also make use of respiratory movements (active ventilation) to supplement gas exchange at rest. However, their temporal dynamics have not been widely investigated. Here, intratracheal pressure, V CO2 and body movements of the desert locust Schistocerca gregaria were measured simultaneously during the DGC and revealed several important aspects of gas exchange dynamics. First, S. gregaria employs two different ventilatory strategies, one involving dorso-ventral contractions and the other longitudinal telescoping movements. Second, although a true spiracular closed (C)-phase of the DGC could be identified by means of subatmospheric intratracheal pressure recordings, some CO 2 continued to be released. Third, strong pumping actions do not necessarily lead to CO 2 release and could be used to ensure mixing of gases in the closed tracheal system, or enhance water vapour reabsorption into the haemolymph from fluid-filled tracheole tips by increasing the hydrostatic pressure or forcing fluid into the haemocoel. Finally, this work showed that the C-phase of the DGC can occur at any pressure. These results provide further insights into the mechanistic basis of insect gas exchange.

Oxygen diffusion limitation triggers ventilatory movements during spiracle closure when insects breathe discontinuously

“…The above results indicate that VMs are triggered when respiratory gas diffusion rate in the tracheal system during the interburst does not match tissue demands. This supports the hypothesis that VMs serve to mix tracheal gas contents during intermittent gas exchange with the environment (Groenewald et al, 2012).…”

“…Respiratory gas exchange during the O phase may or may not have to be supported by VMs (Chown et al, 2006). Interestingly, a recent study provided evidence for active VMs during the C phase of DGCs in the desert locust [Schistocerca gregaria (Forskål 1775)] (Groenewald et al, 2012). One of the suggested adaptive explanations for this behaviour was that VMs during the C phase in DGCs provide a convective transport mechanism that enhances oxygen transfer to the tissues.…”

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

“…2B). This suggests that small amounts of CO 2 may still diffuse through incompletely closed spiracles, a finding previously described in orthopterans (Groenewald et al, 2012;Hadley and Quinlan, 1993;Nespolo et al, 2007). Measuring subatmospheric intratracheal pressure in individuals that employ DGE (Groenewald et al, 2012), with a particular focus on the closed phase where limited CO 2 is still released, may offer further insight into gas exchange patterns seen in Hemideina.…”

Oxygen safety margins set thermal limits in an insect model system

“…Humidified air may have increased the delivery of oxygen to the tissues under hypoxia, alleviating the oxygen supply constraint. Trimix increases the diffusivity of oxygen in nitrogen and thus enhances the conductance of the tracheae -assuming that a significant part of gas exchange is diffusive (but see discussions in Wobschall and Hetz, 2004;Socha et al, 2010;Groenewald et al, 2012) -which thus allowed an increase in spiracle activity and aerobic scope.…”

“…Lowering the oxygen concentration, adding moisture to any gas mixture or changing the composition of the gas will affect one or several aspects of the physical chemistry including the heat capacity, viscosity, density or diffusivity of the gas (supplementary material Table S1), which may influence organismal heat tolerance. Assuming that a significant part of gas exchange is diffusive (but see discussions in Wobschall and Hetz, 2004;Socha et al, 2010;Groenewald et al, 2012), increases in viscosity or density of gas mixture are predicted to increase the ventilatory workload, whereas an increase in diffusivity (e.g. trimix gas composition, supplementary material Table S1) is likely to enhance the conductance of the tracheae.…”