Some insects can withstand extremely low temperatures, even below freezing point of water, to as low as -20°C.
Insects can be separated into three categories depending on their cold-hardiness:
In the case of insects that require high temperatures or occur in warm tropical habitats, the cause of death is not clear but may involve a critical upset of normal metabolism. Death may be due to the accumulation of toxic products ordinarily eliminated at normal temperatures or perhaps to the inability of insect at low temperatures to utilize certain of its food materials.
When tissues freeze, most insects die because of tissue dehydration or mechanical injury by ice crystals. If the ice crystals are confined to the extracellular fluids, many insects can survive the low temperatures. The growth of ice crystals results in a gradual withdrawal of water from cells, and prolonged freezing may cause cellular dehydration death. Generally, extracellular freezing precedes intracellular ice formation. Some insects can withstand ice crystal formation within their cells.
On the other hand, insects that are susceptible to freezing are killed rapidly by intracellular freezing. Shortly thereafter, the insects cells are covered with extracellular ice and the insects darken almost instantaneously. In quick freezing, rapid formation of numerous minute ice crystals occurs and is called "flashing".
Frozen insects may not die immediately after thawing (melting). They may live for brief period with their hearts beating. Apparently freezing the cells results in melanin production. Those that survive freezing often have difficulty in molting or completing their metamorphosis, which are the most common sublethal injuries from freezing.