...The stresses that imperil winter survival by insects are two: (1) lack of food, and (2) injuries due to low temperature and/or freezing. the seasonal lack of food is dealt with in several ways including the accumulation of large reserves of lipid and carbohydrates in the fat body prior to winter hibernation, entry into a state of reduced metabolic rate (quiesence), and/or entry into a state of arrested development (diapause). These are common responses to seasonally inhospitable environments. The specific proglems of injury due to low temperature and/or freezing are dealt with by a range of adaptations that promote cold hardiness, the expression of such adaptations being dinely tuned to the winter microhabitat experience of each indicidual species. For species that must endure prolonged exposures to temperatures below the freezing point of body fluids, two general stragegies of winter hardiness are recognized: freeze avoidance and freeze tolerance. Freeze-avoiding (also called freeze-susceptible) insects maintain a liquid state by employing adaptations that permit extensive undercooling of body fluids. Freeze-tolerant insect, by contrast, withstand the formation of ice in extracellular body fluids and employ adaptations that aggressively regulate ice formation while prevention nucleation within cells. The two strategies seem diametrically opposed yet they share common elements, such as the use of thermal hysteresis proteins and of polyhydric alcohol cryoprotectants, as will be discussed later. Freeze avoidance is widespread among arthropods (e.f., spiders, mites, ticks as well as the insects) and many other invertebrates whereas freeze tolerance has a more select distribution (primarily occurring among Coleoptera, Lepidoptera, Diptera and Hymenoptera) as well as a small group of intertidal marine invertebrates and a few species of terrestrially hibernating amphibians and reptiles. Freeze avoidance obviously suits the needs of species that are winter-active under the snow and is also the less stressful strategy in metabolic terms. Freeze-tolerant species, by contrast, must deal with the potential for physical injury by ice crystals, the osmotic stresses involved in the rapid redistribution of water and solutes across the cell membrane during freeze and thawing, and the long periods of ischemia caused by extracellular freezing. However, for freeze-avoiding animals the undercooled state is a metastable one and if ice forms below the freezing point of body fluids, due to either a decreas in temperature below the supercooling point or to inoculation from environmental ice propagating across the body surface, the subsequent freezing is lethal. Insect cold hardiness has been the subject of many excellent reviews over the years. The present review is not intended to be exhaustive but will highlight new advances in understinding the biochemistry of freeze avoidance versus freeze tolerance in insects. We sill begin with a brief overview of the adaptations involved as they are presently understood." Contains very informative sections on synthesis of low molecular weight cryoprotectants and metabolism and energetics at low temperature.
Biochemical adaptations for winter survival in insects