Frozen flies? Mosquito popsicles? It really happens

I guess I’ll have to buy flies now. I have a student who wants to do research on pesticide resistance. All the pests are frozen this time of year, so we’ll have to raise our own Musca domestica. They’re the common housefly we spend our summers trying to get rid of and now I’m raising them! We have to do strange things in science sometimes.

You may be asking yourself, “Where did all the Musca go?” Or not.

Well, most of the flies are dead. Then there are some kinds that are still alive, but they are not hibernating, mind you. They are frozen. It is surprising to learn that many insects can withstand freezing. Some fruit fly species, the praying mantis, all native bee larvae, and most gall flies are examples. But how can they do this?

A good average estimate of the amount of water in living things is 70 percent, and water is an interesting molecule. I have discussed before the structure of the water molecule, how it packs close together as it gets cold, and then expands slightly when it actually forms an ice crystal.

The expansion of the water molecule when frozen is what damages live tissue. As living tissue cools, the cytoplasm of the cells gets thicker and denser as the molecules move together. Then as the tissue freezes, the ice crystals expand. Since the cell is 70 percent water, this causes the cell to enlarge. This process breaks the cell membrane and the cell is destroyed.

Insects survive freezing with two strategies. First, many insects have relatively large body cavities. Their limbs are very thin, hold little of their body mass, and therefore hold little water.  As temperatures drop in the fall, the insect moves water out of its cells and into the body cavity.  There the water can freeze without damaging cell membranes.

Secondly, as cold weather approaches, many insects increase their stores of sugar by eating a sugar-rich diet. In the case of the Culex mosquito, an important disease vector, that means drinking more nectar from other sources of plant juices. Insects also produce several kinds of organic alcohols as they metabolize the sugars. Both sugar and alcohol act as kinds of antifreeze to stop any water left in the cell from forming expanded ice crystals.

Of course, removing the water from the cells also helps concentrate the antifreeze compounds.  The contents of the cell slowly turn into a kind of slush instead of ice. As the animals thaw, the process is reversed. Water floods back into the cell, diluting the antifreeze compounds, and allows the insects to return to activity once temperatures are warm enough to permit movement.

Have you ever seen a mosquito bouncing around the inside of your windshield on a warm, sunny, January day? You can really impress your family and friends if you squint at it real hard and then declare, “Hmmm. That’s a female Culex mosquito.” If they press the issue, you can launch into an explanation about pointed abdomens and white bands on the hind tarsi. But the truth is, at least in the Rocky Mountain West, only the female Culex mosquitoes overwinter as adults. The one you’re seeing just crawled up from under your dashboard where she had hidden herself for the winter. Then she thawed out and got quite confused by the greenhouse-effect of your car’s window.

Most insects don’t overwinter as adults but their eggs may be subject to freezing. Insect eggs are high in fat content and low on water and therefore no ice crystals are formed. The embryo within is metabolically inactive until the temperature warms to an appropriate degree.

But now you know why I have to buy houseflies for our experiments this winter. It’s kind of a bother, but one advantage of experimenting with houseflies is that you can just flush the failed experiments and nobody cares.

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Gary McCallister is professor of biology at Mesa State College.


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