(This Buffalo News column was first published on February 24, 1992.)
When the temperature drops, you and I rely on mechanical heating systems to keep us warm, praying all the while that those systems will not malfunction and leave us at nature's mercy. We all know of homes where, in the absence of the owners, the heating system has broken down, pipes have burst, and extraordinary damage has been done. Thus we also know that, should we lose heat when the temperature is well below freezing, we must stoke up the fireplace and prepare to drain our water pipes.
Animals are lucky. They don't have to worry about such complications. All they have to do is survive that cold first hand.
Marianne Moore and Richard Lee have written an interesting article in American Entomologist about how land and water insects respond to winter cold. Here are some of their observations.
Of course, a few insects simply avoid cold temperatures. Like some of us, monarch butterflies fly up to 1000 miles south. Other insects migrate downward. Some burrow into leaf litter or deeper into the earth below. Several water insects dig into pond sediments. Ground and ladybird beetles join large groups in rotting logs in order to share body warmth.
There is an interesting trade-off in burrowing into the ground. The deeper the insect digs, the warmer it finds its surroundings. But the insect doesn't have a calendar handy and needs the signal of spring's warming temperatures. It can't dig too deep for then it wouldn't awaken in time to utilize its full season in the sun. For this reason most burrowing insects are found in the top six inches of soil.
It may seem odd that snow serves as an insulator. A thick blanket of snow can raise the temperature at ground level dozens of degrees over that of the air above, but of course only up to the 32° temperature of snow itself. Thus a winter like this one with little snow cover is tougher on insects than one with deep snow.
Ice too provides insulation. The temperature of water below the ice of a stream, pond, or lake is above freezing and increases with depth. Some aquatic insects like mayfly nymphs take advantage of this and retreat from shallow to deeper water as winter progresses.
But the biology of a few insects provides its own response to cold. Goldenrod gall worms withstand temperatures of –60°, in the high arctic some insects spend the winter on exposed rocks, and the larvae of midges can survive encased in ice. The midges may find this necessary because, after prolonged cold, some shallow ponds freeze to the bottom.
For those of us who can never stay warm enough in winter, those facts are impressive. Even more impressive is how they are accomplished.
The secret of such insects' ability to withstand sub-zero temperatures is in their body's ability to "cold harden," a complex physiological and biochemical process through which their body synthesizes glycerol. In simpler terms they convert up to a quarter of their body fluids into antifreeze, providing them exactly the same cold defense that we give our car radiator.
But insects still cannot survive if their body fluids freeze. (This finding should send a message to those who believe what they read in science fiction about maintaining human life in frozen form.) And even those midges die in surface ice. Recall that ice expands as it freezes and hard ice crushes them. It is only in softer ice, called anchor ice, that they can sleep until spring melt revives them.
Interestingly, food in insects' stomachs can freeze and injure them too. It doesn't contain glycerol.
This winter with its warm and cold spells alternating is a difficult one for insects because it is forcing them to use more energy as their metabolism increases and decreases.
Let's hope it's killing off those mosquitoes now hiding in the leaf litter.-- Gerry Rising
Marianne V. Moore and Richard E. Lee, Jr., "Surviving the Big Chill: Overwintering Strategies of Aquatic and Terrestrial Insects," American Entomologist 37, 2 (Summer, 1991): 111-118.