Ice nucleation and freezing tolerance in New Zealand alpine and lowland weta, Hemideina spp. (Orthoptera; Stenopelmatidae)

The alpine tree weta Hemidiena maori Pictet et Saussure (Orthoptera: Stenopelmatidae) is a large, flightless insect found above the treeline on many of the mountain ranges of the South Island of New Zealand. The population found on the Rock and Pillar Range, Central Otago has been identified as freezing tolerant with a haemolymph ice nucleating agent. The ability of H. maori to survive freezing is compared to the lowland weta Hemideina thoracica Walker and H. crassidens Blanchard, both of which are able to survive the formation of some ice in their bodies. Mortality is associated with time spent frozen in H. thoracica, and it is hypothesized that this species is killed when a critical proportion of its body water is frozen. All five subalpine and alpine populations of H. maori surveyed were found to be freezing tolerant. Comparison of temperatures of first nucleation and mean supercooling point of haemolymph droplets suggest that haemolymph ice nucleating activity varies between populations of H. maori. Hemideina maori collected from the Mt Cook region appear to lack a haemolymph ice nucleator. This population is nevertheless freezing tolerant, suggesting that the haemolymph ice nucleating agent described in H. maori is not essential for freezing tolerance. Hemideina crassidens and H. ricta Hutton, both of which are found in lowland habitats, also had high mean supercooling point and temperatures of first nucleation of haemolymph droplets, suggesting that these species also have a haemolymph ice nucleator. Comparison of ice nucleation characteristics of haemolymph and faecal material (representing gut contents) suggests that gut nucleators in H. maori may be at least as efficient as the haemolymph nucleator. It is concluded that freezing tolerance is probably not an adaptation to the alpine environment. This highlights the need for inter- and intraspecific comparative studies if physiological data are to be used to draw evolutionary conclusions.