Physiological Entomology (2012) 37, 354–359 DOI: 10.1111/j.1365-3032.2012.00851.x Impact of different acclimation temperatures and duration on the chill coma temperature and oxygen consumption in the tenebrionid beetle Alphitobius diaperinus DAVID RENAULT 1 ,ALICEBIJOU 1 and F R ´ ED ´ ERIC HERVANT 2 1 Universit´ e de Rennes 1, Rennes Cedex, France and 2 Ecologie des Hydrosyst` emes Naturels et Anthropis´ es, Universit´ e de Lyon 1, Villeurbanne Cedex, France Abstract. When the ambient temperature is lowered to an insect’s lower thermal limit, the insect enters into chill coma. Chill coma temperature and chill coma recovery can vary within species as a result of thermal acclimation, although the physiological basis of the onset of chill coma remains poorly understood. The present study investigates how the temperature of acclimation (0, 5, 10, 15 and 20 ◦ C for 2 or 7 days) affects chill coma temperature and oxygen consumption in adult Alphitobius diaperinus Panzer (Coleoptera: Tenebrionidae). It is hypothesized that the threshold decline in metabolic rate corresponds to the entry into chill coma. Oxygen consumption (as a proxy of metabolism) is measured across the chill coma temperature threshold, and a strong decline in oxygen consumption is expected at entry into chill coma. The acclimation decreases the chill coma temperature significantly from 6.6 ± 1.1 ◦ C in control insects to 3.1 ± 0.7 ◦ C in those acclimated to 10 ◦ C. The change in metabolic rate (Q 10 ) after acclimation to temperatures ranging from 10 to 20 ◦ C is 3.7. Despite acclimation, the metabolic rate of A. diaperinus conforms to Arrhenius kinetics, suggesting that the response of this beetle does not show metabolic compensation. The data suggest the existence of a threshold decline in metabolic rate during cooling that coincides with the temperature at which an insect goes into chill coma. Key words. Critical thermal limit, insect, low temperature. Introduction The body temperature of insects generally tracks the temper- ature of their environment (Hochachka & Somero, 2002). As the ambient temperature falls, metabolic rate decreases, and insects become progressively slower to respond and move until they become unresponsive, entering a reversible state called chill coma (Vannier, 1994). When the temperature is low- ered to an insect’s lower thermal limit, the entry into chill coma is a four-step process that includes: the onset of chill coma (altered locomotory performance), the loss of sponta- neous movements (insects become inactive), the loss of motor coordination (insects are not able to stand on their legs) and no Correspondence: David Renault, Universit´ e de Rennes 1, UMR CNRS 6553 Ecobio, 263 Avenue du Gal Leclerc, CS 74205, 35042 Rennes Cedex, France. Tel.: +33 2 23 23 66 27; e-mail: david. renault@univ-rennes1.fr further limb movement (entry into chill coma) (Hazell & Bale, 2011). The loss of spontaneous movements (Klok & Chown, 2003) and the loss of movement coordination (Terblanche et al., 2005) are referred to as the critical thermal minimum (CT min ), whereas chill coma temperature is defined as the temperature at which muscle control is lost and all locomo- tory activities are interrupted (Hazell & Bale, 2011; MacMillan & Sinclair, 2011). Chill coma temperature varies both within species (resulting from acclimation and acclimatization) and between species (as a result of evolutionary adaptation) (Klok & Chown, 1997). It is important to understand the conditions for chill coma onset, which determine the thermal window in which active insects can feed, breed and avoid predators (Chown & Nicolson, 2004). Furthermore, chill coma recovery, which is the time required for recovery to normal activity, represents an estimate of the capacity to reset neurological and other physiological functions when the insects are returned to 354 © 2012 The Royal Entomological Society