Metabolic responses of Glossina pallidipes (Diptera: Glossinidae) puparia exposed to oxygen and temperature variation: Implications for population dynamics and subterranean life

Abstract

Understanding the factors affecting insect gas exchange in subterranean environments is critical to understanding energy budgets and predicting mortality under field conditions. Here, we examine the metabolic rate (MR) responses of tsetse puparia, which remain underground for ca. 1 month in this life-stage, to varying oxygen and temperature. First, the effects of temperature and oxygen on puparial MR were investigated by ramping temperature from 15 to 35°C under 10, 21 or 40% O2. Overall, temperature was the dominant effect on puparial MR although O2 had small but significant impacts. Second, critical O2 concentration (PCRIT) for MR of puparia was examined across a range of oxygen concentrations (0–40%). PCRIT was 6% O2 which is similar to PCRIT in other basal arthropods but relatively high for inactive or subterranean insects. Third, we asked if puparia exposed to anoxia might experience oxygen debt, potentially indicative of anaerobic metabolism or cellular repair. Metabolic responses to anoxia were limited or insignificant, but MR was marginally elevated (∼15%) in anoxia-exposed (4h) puparia by 12h post-anoxia. Finally, we examined the ability of puparia to withstand water submersion, thus simulating flooding conditions frequently experienced in tropical soil habitats. Puparia were unable to survive submersion for >24h suggesting limited flooding tolerance. These novel results suggest that soil conditions experienced by puparia should not be limiting for MR, except possibly under high temperature-low O2 conditions. Due to a large safety margin between PCRIT and soil oxygen levels and limited effects of oxygen on metabolism during temperature ramping experiments, we suggest that Glossina pallidipes puparia are not particularly susceptible to oxygen availability in their natural environment. However, soil flooding associated with tropical rainfall likely imposes strong selection on tsetse populations and may have had important effects for tsetse energy budgets and evolution.