Shivering through a winter in the extreme latitudes, it's sometimes hard to believe that some creatures survive these hardships by allowing themselves to freeze. Yet many do, and the larva of the sub-Antarctic caterpillar Pringleophaga marioni probably suffers several freezing events over the course of its long lifetime. Wondering how insects can survive freezing over a small range of temperatures, yet perish when frozen to lower temperatures, Brent Sinclair and his colleagues from the University of Stellenbosch, South Africa, decided to monitor the caterpillar's metabolic rate as the insects cooled (p. 1287), monitoring the insect's metabolism in real time with open-flow respirometry.

The team dropped the environmental temperature well below 0°C, and tracked the insect's metabolism at various sub-zero temperatures. When the temperature fell to –0.6°C, Sinclair, Jaco Klok and Steven Chown noticed that the insect's metabolic rate suddenly fell. The team realised that this was the same temperature where the insects fall into a chill-coma, which they suspect may be caused by the failure of the vital Na+/K+ ATPase pump in neurones.

Cooling the insects further, the team discovered that the insect's body fluids froze at –4.6°C. Despite being frozen solid, the team detected metabolic activity from the insects. Even after the insects were frozen dead, and later thawed, the team were able to detect metabolic activity. Sinclair suspects that individual cells may still function even when the whole caterpillar is dead, suggesting that for these insects `mortality may be a result of a breakdown in processes at the organismal, rather than the cellular, level' he says.

Sinclair, B. J., Klok, C. J. and Chown, S. L.(
). Metabolism of the sub-Antarctic caterpillar Pringleophaga marioni during cooling, freezing and thawing.
J. Exp. Biol.