Winter is a time for rest; life enters a quiet state of dormancy as plants die back, animals take shelter or migrate, and the snow (finally, the snow!) muffles what little noise is left in the world.
But while life feels at a standstill, especially in the deepest throes of winter, brief moments of observation reveal that not everyone has settled down. You can still catch a flurry of activity at a well-stocked bird feeder, and the neighborhood deer still torment my dog as they saunter past our living room windows. And while you’ll have a more challenging time finding them, you might even spot some arthropods in the snow.
Insects and their arthropod relatives are not entirely relegated to underground dormancy during winter. You won’t be met with the familiar din of buzzing insects when you step outside, but taking a moment to scan the ground may reveal some surprising discoveries. A somewhat common sighting this time of year is the minuscule snow flea, a soil-dwelling insect relative that emerges en masse on warm days to form immense congregations on the ground, where their dark bluish bodies are easily spotted against a snowy backdrop. I like to call them “winter blues.”
Snow fleas are not actual fleas but a group of arthropods known as springtails. Once considered an order of insects, springtails have since had their insect status entirely revoked and were recategorized into their own class, Collembola. One of the major defining characteristics of springtails and their allies is their enclosed mouthparts, whereas insects have external mouthparts. The common name “snow flea” refers to their impressive jumping skills; springtails use a forked appendage called a furcula to hurl themselves into the air. The furcula is held tightly against the underside of the abdomen and, once released, snaps against the substrate and sends the little critter flying, sometimes at 161 thousandths of a second. But like their insect relatives, springtails are ectotherms, making them cold-blooded animals that cannot generate their own body heat. Ectothermic animals face obvious challenges in temperate climates like ours, where seasonality causes significant temperature fluctuations.
Temperatures in northern climates, as we well know, are subject to drop well below freezing, and in that case, many arthropods (including snow fleas) employ a strategy known as “freeze avoidance.” Essentially, the insect prevents ice formation in bodily fluids through a process known as “supercooling.”
For ice formation to occur, ice crystals need a nucleation site. Ice crystals won’t form if they don’t have something to “grab” onto. Water and other liquids can be cooled far beyond their freezing point without a nucleation source; water, for example, can reach -42°C (-43.6°F) and remain liquid if no nucleation source is introduced. This process is adequately named “supercooling;” arthropods use it to depress the temperature at which their body fluids will freeze.
As the weather turns colder, arthropods begin cold-hardening, the process by which they prepare for colder temperatures. In freeze-avoidant insects, this process involves removing all potential ice nucleation sources from their body, including food particles and bacteria. Some stop eating, while others force themselves to molt, shedding the entire lining of their midgut in the process.
Some insects completely alter their metabolism by using cryoprotectants to supercool their body fluids. “Cryoprotectant” is a catch-all term referring to the various substances produced by ectothermic animals to protect their tissues from freezing. The most common cryoprotectant insects produce is glycerol, which renders their body fluids too viscous to form ice crystals at freezing temperatures.
Finally, one of the most fascinating freeze-avoidant strategies is the production of antifreeze proteins (AFPs – or, if you want to impress your friends, thermal hysteresis factors (THFs)). All freeze-avoidant arthropods produce antifreeze proteins. They are a family of proteins that prevent freezing by binding to the surface of ice crystals and preventing them from growing. The production of antifreeze proteins is triggered by day length; as daylight decreases and insects experience a shorter photoperiod, the production of antifreeze proteins ramps up and peaks during the darkest months of winter.
As the days lengthen and we experience short bursts of warm, sunny weather, be on the lookout for these winter-hardy arthropods and more, as others will start emerging from their slumber before we know it.