The physiology of cold hardiness in terrestrial arthropods

Insects and other terrestrial arthropods are widely distributed in temperate and polar regions and overwinter in a variety of habitats. Some species are exposed to very low ambient temperatures, while others are protected by plant litter and snow. As may be expected from the enormous diversity of terrestrial arthropods, many different overwintering strategies have evolved. Time is an important factor. Temperate and polar species are able to survive extended periods at freezing temperatures, while summer adapted species and tropical species may be killed by short periods even above the freezing point. Some insects survive extracellular ice formation, while most species, as well as all spiders, mites and springtails are freeze intolerant and depend on supercooling to survive. Both the degree of freeze tolerance and supercooling increase by the accumulation of low molecular weight cryoprotectant substances, e.g. glycerol. Thermal hysteresis proteins (antifreeze proteins) stabilise the supercooled state of insects and may prevent the inoculation of ice from outside through the cuticle. Recently, the amino acid sequences of these proteins have been revealed. Due to potent ice nucleating agents in the haemolymph most Freeze tolerant insects freeze at relatively high temperatures, thus preventing harmful effects of intracellular freezing. Doe to the low water vapour pressure in frozen environments, supercooled terrestrial arthropods are at a risk of desiccation. Glycerol and other low molecular weight substances may protect against dehydration as well as against cold. In the arctic springtail Onychiurus arcticus, freezing is avoided due to dehydration in equilibrium with the ambient freezing temperature. Tn some frozen habitats terrestrial arthropods are enclosed by ice and survive an oxygen deficiency by anaerobic metabolism. Suggestions for further research include investigating the nature of freeze tolerance, the physiology of prolonged exposures to cold, and the relation between desiccation, anaerobiosis and cold hardiness.