How plastic are upper thermal limits? A comparative study in tsetse (family: Glossinidae) and wider Diptera

Critical thermal maximum (CT_max) describes the upper thermal tolerance of an animal where biological functions start to fail. A period of acclimation can enhance CT_max through plasticity, potentially buffering animals from extreme temperatures caused by climate change. Basal and acclimated CT_max vary within and between species and may be explained by traits related to thermal physiology, such as body size and sex. Differences in CT_max have not been established among species of tsetse fly (Glossina spp.), vectors of animal and human African trypanosomiasis. Here, we investigated basal CT_max and its plasticity for five tsetse species following adult acclimation at constant 25 or 30 °C for five days. We then set our findings in context using a meta-analysis on 33 species of Diptera. We find that, of the five tsetse species considered, only Glossina palpalis gambiensis and Glossina brevipalpis exhibited plasticity of CT_max, with an increase of 0.12 °C and 0.10 °C per 1 °C acclimation respectively. Within some species, higher basal CT_max values were associated with larger body size and being female, while variation in plasticity (i.e., response to the acclimation temperature) could not be explained by sex or size. Our broader meta-analysis across Diptera revealed overall CT_max plasticity of 0.06 °C per 1 °C acclimation, versus a similar 0.05 °C mean increase in tsetse. In contrast, there was greater CT_max plasticity in males compared to females in Diptera. Our study highlights that CT_max and its plasticity varies even among closely related species. Broader patterns across groups are not always reflected at a finer resolution; we thus emphasise the need for detailed experimental studies across a wide range of insect species to capture their capacity to cope with rapidly warming temperatures.