All climate change scenarios predict an increase in both global temperature means and the magnitude of seasonal and diel temperature variation. The nonlinear relationship between temperature and biological processes means that fluctuating temperatures lead to physiological, life history, and ecological consequences for ectothermic insects that diverge from those predicted from constant temperatures. Fluctuating temperatures that remain within permissive temperature ranges generally improve performance. By contrast, those which extend to stressful temperatures may have either positive impacts, allowing repair of damage accrued during exposure to thermal extremes, or negative impacts from cumulative damage during successive exposures. We discuss the mechanisms underlying these differing effects. Fluctuating temperatures could be used to enhance or weaken insects in applied rearing programs, and any prediction of insect performance in the field—including models of climate change or population performance—must account for the effect of fluctuating temperatures.

A common physiological response of organisms to environmental stresses is the increase in expression of heat shock proteins (Hsps). In insects, this process has been widely examined for heat stress, but the response to cold stress has been far less studied. In the present study, we focused on 11 Drosophila melanogaster Hsp genes during the stress exposure and recovery phases. The temporal gene expression of adults was analyzed during 9 h of cold stress at 0 °C and during 8 h of recovery at 25 °C. Increased expression of some, but not all, Hsp genes was elicited in response to cold stress. The transcriptional activity of Hsp genes was not modulated during the cold stress, and peaks of expression occurred during the recovery phase. On the basis of their response, we consider that Hsp60, Hsp67Ba and Hsc70‐1 are not cold‐inducible, whereas Hsp22, Hsp23, Hsp26, Hsp27, Hsp40, Hsp68, Hsp70Aa and Hsp83 are induced by cold. This study suggests the importance of the recovery phase for repairing chilling injuries, and highlights the need to further investigate the contributions of specific Hsp genes to thermal stress responses. Parallels are drawn between the stress response networks resulting from heat and cold stress.

Summary: 1. To cope with stressful environmental temperatures, organisms can enhance thermotolerance when exposed to sub‐lethal temperatures before thermal stress, a phenomenon referred to as thermal acclimation. Acclimation includes different forms (developmental, gradual or rapid) that vary in ecological importance depending on patterns of diurnal and seasonal thermal variation.2. Here, we complete a comprehensive assessment of how the different forms of acclimation based on simulated field temperatures affect cold tolerance in Drosophila melanogaster under different levels of cold stress (−4·5 °C/2 h and 0 °C/10 h).3. We predict that (i) combinations of acclimation treatments may be particularly beneficial and (ii) benefits of different acclimation types may differ for acute vs. chronic cold stress. We also investigate whether distinct forms of acclimation promote differential molecular responses to stress.4. Acclimation treatments had very large effects on cold tolerance and resulted in phenotypes ranging from sensitive to tolerant individuals within the specific cold stress applied (−4·5 °C/2 h and 0 °C/10 h). Acclimation also influenced expression of several genes (Hsp23, Hsp70, Hsp40, Hsp68, Starvin and Frost) during recovery from cold stress but effects depended on the nature of the acclimation treatment.5. Cumulative effects occurred between different forms of acclimation, and these as well as the different molecular responses point to different underlying mechanisms.6. These results highlight that combined acclimation treatments may strongly impact field stress resistance.

Among terrestrial organisms, arthropods are especially susceptible to dehydration, given their small body size and high surface area to volume ratio. This challenge is particularly acute for polar arthropods that face near-constant desiccating conditions, as water is frozen and thus unavailable for much of the year. The molecular mechanisms that govern extreme dehydration tolerance in insects remain largely undefined. In this study, we used RNA sequencing to quantify transcriptional mechanisms of extreme dehydration tolerance in the Antarctic midge,Belgica antarctica,the world’s southernmost insect and only insect endemic to Antarctica. Larvae ofB. antarcticaare remarkably tolerant of dehydration, surviving losses up to 70% of their body water. Gene expression changes in response to dehydration indicated up-regulation of cellular recycling pathways including the ubiquitin-mediated proteasome and autophagy, with concurrent down-regulation of genes involved in general metabolism and ATP production. Metabolomics results revealed shifts in metabolite pools that correlated closely with changes in gene expression, indicating that coordinated changes in gene expression and metabolism are a critical component of the dehydration response. Finally, using comparative genomics, we compared our gene expression results with a transcriptomic dataset for the Arctic collembolan,Megaphorura arctica.AlthoughB. antarcticaandM. arcticaare adapted to similar environments, our analysis indicated very little overlap in expression profiles between these two arthropods. Whereas several orthologous genes showed similar expression patterns, transcriptional changes were largely species specific, indicating these polar arthropods have developed distinct transcriptional mechanisms to cope with similar desiccating conditions.