The preparation of insects for a cold, winter period has traditionally been the primary focus of research on the response of insects to low temperatures. As we have mentioned previously, this can be considered a long-term (weeks to months) programmed response to declining temperatures, prolonged periods of cold, or, less commonly, to changing photoperiod or dietary cues. Although this programmed response to cold is not always associated with diapause, the two 'programmes' are often intimately related, and this relationship (or the lack thereof) has been explored in detail by Denlinger (1991, see also Denlinger and Lee 1998; Denlinger 2002).
It is widely appreciated that the response of insects to cold is complex, and differs between dia-pausing and non-diapausing individuals (Lee and Denlinger 1985), individuals at different times of the year (van der Merwe et al. 1997), and in different ontogenetic stages (Vernon and Vannier 1996; McDonald et al. 2000; Klok and Chown 2001), and between populations in different years (Kukal and Duman 1989). Nonetheless, in their response to temperatures below the melting point of their body fluids, insects have regularly been classified either as freeze intolerant (freeze avoiding, freeze susceptible), or freezing tolerant (freeze tolerant) (Lee 1991; Somme 1999). The former group of species cannot survive the formation of ice within their bodies, and therefore have evolved a suite of measures to prevent ice formation. In contrast, freezing tolerant species can withstand ice formation, usually only in the extracellular fluids, and in turn have a suite of characteristics that enables them to survive such ice formation. Although variation about these 'strategies' has long been recognized, and has become a recurrent theme in recent reviews, this classification system is still widely adopted, though in a considerably modified form. Perhaps the most significant change to these categories has been the addition of cryoprotective dehydration as a third strategy by which insects can survive subzero temperatures (Holmstrup et al. 2002; Sinclair et al. 2003b). In this instance, the few species that adopt this strategy lose water to the surrounding environment, so resulting in an increase in the concentration of their body fluids and a decline in their melting point (to equilibration with the ambient temperature). In effect, they cannot freeze.
In this section, we examine the modified cold hardiness classification, as well as the characteristics of each of the major classes of cold hardiness. In doing so, we recognize that the field of insect cold hardiness has grown substantially since Asahina and Salt's work in the 1960s (Asahina 1969; Ring and Riegert 1991), and that even a review of the recent reviews amounts to a formidable task. Thus, we provide only a brief overview of the major characteristics of freeze intolerant and freezing tolerant species. Additional information can be found in several large reviews and books (Somme 1982, 1999; Zachariassen 1985; Cannon and Block 1988; Block 1990; Lee and Denlinger 1991; Storey and Storey 1996; Denlinger and Lee 1998; Lee and Costanzo 1998; Ramlov 2000; Duman 2001).
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