Woolly bear caterpillars are cold-blooded, so they can’t regulate their internal temperature. To make matters worse, water expands when it freezes. If a cell gets too cold, the water inside that cell will freeze and enlarge, rupturing the cell membrane. So, these clever caterpillars undergo an amazing biochemical process to avoid that nasty fate. First, they fill their cells with glycerol, an alcohol compound that lowers the freezing point. It’s a bit like salting the pavement to prevent them from freezing over. Then, they squeeze out some of the water from inside their cells, so it freezes in-between their cells rather than inside their cell membrane.
During the winter, the woolly bear caterpillar will, in fact, freeze over. But, thanks to their nifty antifreeze trick, their cells remain perfectly healthy. When the weather warms, the caterpillars safely thaw. They’re kind of like the Captain America of insects!
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