Cold-hardy insects fall into two groups. Freeze-avoiding species use antifreeze compounds to supercool their body fluids and die if ice forms. Freeze-tolerant species do the opposite: they allow ice to nucleate in the extracellular spaces under control, while keeping the interior of their cells liquid.
The mountain stone weta belongs to the second group. Under rock slabs on exposed tors it experiences repeated hard frosts, and it freezes and thaws through the winter, having been observed to survive being frozen for extended periods. Ice-nucleating agents start freezing at a relatively high subzero temperature, which keeps ice crystals small and outside the cells, and cryoprotectant solutes limit the osmotic damage as water leaves the cells into the growing ice.
The limits are not fully mapped. Reported survival covers repeated freeze-thaw cycles over a winter, but the lowest survivable temperature and the number of cycles the tissues can take before damage accumulates depend on conditions, and field measurements are sparse for an insect that spends winter under rock.