Supercooling and winter survival in terrestrial arthropods

1. Supercooling as a means of winter survival is known from a large number of freezing-intolerant insects overwintering in the egg, larval, pupal and adult stages, and from mites, spiders, scorpions and centipedes among other arthropods. 2. Increased ability to supercool is an adaptation to the environment, and most pronounced in species from exposed habitats. 3. Differences in supercooling ability within species occur in populations from different geographical areas. 4. The ability to supercool is conveniently measured as the supercooling point (SCP), which is the temperature at which spontaneous freezing occurs during gradual cooling. 5. Since the chances of spontaneous freezing depend on both time and temperature, ice formation may occur at higher temperatures during prolonged exposures. 6. The SCP itself is influenced by several factors. (a). In feeding stages freezing may start in the gut, induced by nucleating agents in the food; in many species the gut is evacuated in preparation for the winter. (b). Contact with moisture may rapidly result in inoculated ice formation through the cuticle, as shown for a number of insects. (c). A decrease in water content may influence supercooling since concentrations of solutes are increased. (d). The lowering of SCP in a number of overwintering insects and cryptostigmatid mites is related to the accumulation of cryoprotective substances during the autumn and winter; glycerol is the most common one. but increased levels of sorbitol, mannitol, sugars and other low molecular substances may also be of importance. 7. In general it appears that the SCP is lowered more than the corresponding melting point when concentrations are increased. 8. The production of cryoprotective substances may be induced by low temperatures, and is enhanced in species overwintering in diapause. 9. Seasonal variations in the ability to supercool depend mainly on the concentration of cryoprotective substances in non-feeding stages, and on a combination of cryoprotectants and feeding status in feeding stages.