Differences in critical thermal maxima and mortality across life stages of the mealworm beetle Tenebrio molitor

Thermal 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 (CTmax), the temperature at which activity becomes uncontrolled. Thermolimit respirometry is a new technique that allows CTmax to be quantified in small animals, such as insects, as the point of spiracular failure by measuring CO2 release from the animal as temperature increases. Although prior studies have reported a characteristic pattern of CO2 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 CTmax and patterns of CO2 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 CO2 release in different life stages of an insect species during thermolimit respirometry. Our results show that CTmax 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 CTmax 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.