Modality in the supercooling points of cold tolerant but freezing intolerant terrestrial arthropods has proved a pragmatically reliable means of distinguishing between summer and winter cold hardiness in such species. This paper proposes an ecologically realistic method of modal analysis which may either be used in lieu of the traditional separation of supercooling points into "high" and "low" groups, or as a complementary assessment of the risk of freezing mortality. Instead of a posteriori determinations of modal break points, animal supercooling points are assigned a priori to one of four categories of cold hardiness: (1) summer cold-hardy; (2) semi-cold-hardy; (3) cold-hardy; and (4) winter cold-hardy. Each category is identified by the temperature range within which arthropods can be expected to freeze. The temperature ranges assigned to each category are based on a conservative, but realistic, assessment of the temperatures at which animals can be expected to freeze at a given point in the season. The approach has greater discriminatory power than traditional bimodal descriptors (i.e."summer" and "winter" cold-hardy), as well as allowing animal supercooling points to be related to the temperatures they actually experience in their habitats. Thus, for example, animals considered "summer" cold-hardy according to conventional analysis may actually be "semi-cold-hardy" with supercooling points well within the safety margin of minimum ambient temperatures.
Chill tolerance (time of survival at -5°C) increased in non-diapausing (reproducing) adults of Pyrrhocoris apterus after a gradual, 4-week-long decrease in ambient temperature from 25° to 0°C. The level of chill tolerance attained after cold-acclimation was considerably lower than that in similarly cold-acclimated diapausing adults. Some physiological changes accompanied the cold-acclimation, irrespective of developmental state (diapause vs. reproduction). They were: A decreased oxygen consumption, loss of body water, an increased haemolymph osmolality, an increased proportion of phosphatidylethanolamines vs. a decreased proportion of phosphatidylcholines in membrane phospholipids, and an increased proportion of linoleic vs. a decreased proportion of oleic acid in phosphatidylethanolamines. Such changes could contribute to the limited potential for cold-acclimation found in non-diapausing insects. Other physiological changes appeared to require the induction of diapause prior to cold-acclimation. They were: Down regulation of ice nucleators resulting in a lowering of the individual supercooling point, synthesis and accumulation of specific "winter" polyols, an increased proportion of palmitic acid in membrane phospholipids; and regulation of the concentrations of Na+ and K+ in the haemolymph. The potential contributions of these changes to the cold hardiness of P. apterus are discussed.
Cold tolerance of the eggs of the grasshopper, Chorthippus fallax (Zubovsky), was examined in the laboratory. Egg supercooling points varied from -6°C to -32.4°C and could be divided into two groups. The supercooling points of the higher SCP group ranged from -6°C to -14°C and those of lower SCP group from -21.8°C to -32.4°C. Although low temperature acclimation could slightly decrease the supercooling points of eggs, the effect was not significant for all embryonic developmental stages or acclimation periods. The supercooling capacity was obviously different between pre-diapause, diapause and post-diapause embryonic stages. The mean supercooling points of pre-diapause and diapause eggs were similar; -28.8 ± 1.6°C and -30.7 ± 1.0°C for non-acclimated eggs and -29.5 ± 1.3°C and -31.18 ± 0.8°C for acclimated eggs respectively. However, the mean supercooling points of post-diapause eggs were significantly higher; -12.9 ± 5.6°C for non-acclimated and -13.5 ± 4.5°C for acclimated eggs respectively. The survival rates of diapause eggs at > -25°C were not significantly different from that at 25°C, but survival rates at < -30°C decreased significantly. The lethal temperature (Ltemp50) for a 12 hrs exposure was -30.1°C, and the lethal time (Ltime50) at -25°C was 44 days. Since the SCPs of diapause eggs was similar to their Ltemp50, we may consider the supercooling capacity of such eggs is a good indicator of their cold hardiness and the species is a true freeze avoiding insect. Based on the analysis of local winter temperature data, pre-diapause and diapause, low SCP eggs can safely survive severe winters, but not the post-diapause, high SCP eggs. The importance of the overwintering strategy and the relationship between diapause and cold hardiness of this species is discussed.