Cold only paralyzes some insects –

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Just a few short weeks ago, on a warm afternoon, all sorts of insects were gathered along my windowsills looking for warm places to overwinter. Some insects overwinter as eggs, often laid in protected areas or below the frost line to avoid the coldest temperatures. Some insects can survive the winter by supercooling their blood and freezing almost solid. Many insects can even survive sudden cold snaps: they stop moving, are paralyzed by the cold, but when warmed can often move again within minutes. Insects that live in the north have co-evolved with the seasons and can handle seasonal temperature change with relative ease.

Insects are cold-blooded animals, known as ectotherms — organisms that regulate their body temperature by exchanging heat with their surroundings. As such, they are vulnerable to changes in temperature. Different species of insects can tolerate different temperatures, but when it gets too cold insects will enter a state of paralysis called a chill-coma. They aren’t dead, they just can’t move.

When chilled, insects lose the ability to regulate water and salt balances in their muscles, and without the proper concentrations of water and salt, muscles can’t function so they go into a chill-coma. You can see this on a cold morning when insects wait motionless for the day to warm up enough to allow them to move.

A paper published last week in the journal Proceedings of the National Academy of Sciences (“Re-establishment of ion homeostasis during chill-coma recovery in the cricket Gryllus pennsylvanicus,” MacMillan et al.) described how field crickets recover from a chill-coma. In a nutshell, recovery involves re-establishing the correct balance of salts in muscles. Recovery can happen quite quickly; after warming, a cricket can regain movement in a matter of minutes. However, while a recovering cricket might be able to walk or a wasp fly, full recovery takes some time. This study found that the metabolism of crickets was boosted (sometimes doubled) for at least a couple hours after waking up from the chill-coma. In other words, the cricket is expending extra energy re-establishing the normal salt balance in its muscles; recovering from a chill-coma is metabolically costly.

Why is this sort of information of interest?

The time it takes to recover from a chill-coma is often used as a measure of the cold tolerance of an insect. Understanding the mechanisms that underlie chill-comas can help us predict the effects of climate change on local insect populations. For example, species adapted to northern conditions are often more cold-tolerant than southern species; the onset of chill-coma usually occurs at lower temperatures in northern species than in their southern counterparts. Warming temperatures might allow less-hardy species to survive and thrive and displace their cold-hardy northern neighbors.


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