A conceptual model of how chilling differentially affects chill-susceptible and chill tolerant insects.
(A) Chill susceptible insects maintain large [Na+] gradients, such that Na+ is a dominant extracellular osmolyte. At low temperatures, net Na+ leak across the gut epithelia causes a disruption of water balance, as extracellular water moves to the gut lumen. This disruption is partly driven by the effects of low temperature on the Malpighian tubules, which secrete a greater proportion of Na+ into the gut lumen relative to K+ in the cold (Fig. 4). Ultimately, the loss of hemolymph volume and impaired ability to clear K+ concentrate K+ in the extracellular space, causing cell depolarization and cell death (which likely causes further K+ leak into the extracellular space from compromised cells in a positive-feedback loop32). By contrast, chill tolerant insects (B) maintain lower Na+ gradients (making water balance less dependent on Na+) and defend the ratio of ions secreted by the Malpighian tubules in the cold, despite overall slowing of secretion rates at low temperatures. As a consequence of these and likely several other adaptations, the tolerant species maintain organismal water and ion balance at low temperatures (or even bolster existing gradients; Fig. 2) and recover from the same cold stress with little to no observable injury.