Tolerance to cold and desiccation in overwintering terrestrial arthropods

Early studies on cold hardiness in insects and other terrestrial arthropods were concerned with freeze tolerance and supercooling, as well as the survival of overwintering stages at low temperatures. The role of glycerol and other low molecular weight cryoprotectant substances in freeze tolerant and freeze susceptible insects were intensively studied. The discovery of macromolecular antifreeze substances in a number of insects species opened for an increased understanding of the nature of cold hardiness. Thermal hysteresis proteins (THPs) secreted into the haemolymph depress the freezing point by several degrees C. Presently, workers in this field are studying the cellular mechanisms regulating the levels of THP in insects. Most freeze tolerant insects contain potent ice nucleating agents (INAs), that induce extracellular ice formation at relatively high subzero temperatures, thus preventing harmful effects of ice formation within cells. In contrast, some species tolerate freezing following a high degree of supercooling. The physiology of survival and mortality during chilling in both freeze susceptible and freeze tolerant species is poorly understood. More empirical data on prolonged exposures to cold, including seasonal variations, are needed. Insects in frozen soils are subjected to lower water vapour pressures that in an unfrozen environment. Under these conditions supercooled animals will easily loose water, but the accumulation of glycerol and similar substances reduce evaporation. It has been suggested that tolerance to low temperatures and resistance to desiccation are overlapping adaptations. Anaerobiosis during enclosure in ice is apparently another important aspect of overwintering strategies.