Discontinuous gas-exchange cycle characteristics are differentially affected by hydration state and energy metabolism in gregarious and solitarious desert locusts

Abstract

The abolishment of discontinuous gas exchange cycles (DGC) in severely dehydrated insects casts doubt on the generality of the hygric hypothesis, which posits that DGC evolved as a water conservation mechanism. We followed DGC characteristics in the two density-dependent phases of the desert locust, Schistocerca gregaria, throughout exposure to an experimental treatment of combined dehydration and starvation stress, and subsequent rehydration. We hypothesized that, under stressful conditions, the more stress-resistant gregarious locusts would maintain DGC longer than solitarious locusts. However, we found no phase-specific variations in body water content, water loss rates (total and respiratory), and timing of stress-induced DGC abolishment. Likewise, locusts of both phases reemployed DGC after having ingested comparable volumes of water when rehydrated. Despite comparable water management performances, the effect of exposure to stressful experimental conditions on DGC characteristics varied significantly between gregarious and solitarious locusts. Interburst duration, affected by the ability to buffer CO2, was significantly reduced in dehydrated solitarious, compared to gregarious, locusts. Moreover, despite similar rehydration levels, only gregarious locusts recovered their initial CO2 accumulation capacity, indicating that cycle characteristics are affected by factors other than haemolymph volume. Haemolymph protein measurements and calculated respiratory quotients suggest that catabolism of haemolymph proteins may contribute to a reduced haemolymph buffering capacity, and thus a compromised ability for CO2 accumulation, in solitarious locusts. Nevertheless, DGC was lost at similar hydration states in the two phases, suggesting that DGC is terminated as a result of inadequate oxygen supply to the tissues.