The abundance and success of widely distributed species across variable environments make them suitable models for exploring which traits will be important for resilience to climate change. Using a widespread mosquito species, Culex tarsalis, we have investigated population-level variation in the critical thermal maximum (CT(max)) and the metabolic response to temperature (MR-T). Adult female C. tarsalis were sampled from three sites representing thermally distinct habitats in California, and flow-through respirometry was used to determine CT(max) and MR-T relationships. CT(max) differed significantly among the three populations and correlated positively with maximum temperatures at each site but not with mean temperatures. Culex tarsalis from our cool-temperature, high-altitude site had significantly higher metabolic rates at each test temperature compared with the two populations from warmer sites, consistent with previous examples of thermal compensation in ectothermic animals inhabiting cold climates. The MR-T slope was steepest in mosquitoes inhabiting the site with the lowest temperature variability, while shallower slopes were exhibited by mosquitoes from the two sites with higher thermal variability. Our results show the extent to which local populations may differentiate within their respective environments and suggest that plasticity in thermal tolerance traits may play a role in mediating resilience to climate change. Furthermore, our study highlights the importance of thermal variability and extremes rather than average temperatures for the evolution of thermal traits.

SUMMARYThermal limits to activity profoundly affect the abundance and distribution of ectothermic animals. Upper thermal limits to activity are typically reported as the critical thermal maximum (CT max ), the temperature at which activity becomes uncontrolled. Thermolimit respirometry is a new technique that allows CT max to be quantified in small animals, such as insects, as the point of spiracular failure by measuring CO 2 release from the animal as temperature increases. Although prior studies have reported a characteristic pattern of CO 2 release for insects during thermolimit respirometry trials, no studies have been carried out to determine the universality of this pattern across development, or at what point death occurs along this pattern. Here, we compared the CT max and patterns of CO 2 release among three life stages of a beetle species, Tenebrio molitor, and mapped heat death onto these patterns. Our study is the first to report distinct patterns of CO 2 release in different life stages of an insect species during thermolimit respirometry. Our results show that CT max was significantly higher in adult beetles than in either larvae or pupae (P<0.001) and, similarly, death occurred at higher temperatures in adults than in larvae and pupae. We also found that death during heating closely follows CT max in these animals, which confirms that measuring the loss of spiracular control with thermolimit respirometry successfully identifies the point of physiological limitation during heat stress. Supplementary material available online at

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Researchers utilizing thermolimit respirometry to study insect thermal tolerance have previously reported an unexplained surge of carbon dioxide release by insects following death at high temperatures. This phenomenon has been referred to as the “post‐mortal peak” (PMP). In some insects, the CO2 release rate during the PMP may be up to 50% higher than the maximal rate achieved in the live insect. We have observed the PMP in fruit flies, mosquitoes, crickets, cockroaches, and beetles. While it has been verified that the PMP occurs after death, the cause of death does play a role in the appearance of the PMP. We have observed the PMP only when an insect dies due to high temperature stress. Furthermore, the PMP does not occur in the absence of atmospheric oxygen, or if the dead insect is subjected to cyanide prior to the initiation of the peak. On the basis of those results, we hypothesize that the CO2 released in the PMP derives from the insect’s mitochondria. We are currently simultaneously measuring CO2 release and oxygen uptake during thermal ramping in order to clarify the source and function of the PMP. Understanding this event may provide insights into the physiological implications of heat stress in insects and the timeline of biochemical events before and after death at high temperatures. Grant Funding Source: Supported by The National Science Foundation