Photo of the Week – February 8, 2018

Some aquatic insects can survive being encased in ice – water boatmen, for example, or dragonfly larvae.  But what happens if they are frozen near the surface of a pond and the ice around them melts (or sublimates), leaving them exposed to the air when they thaw out?  This is what I was wondering last weekend, as I poked around the icy wetland at our family prairie.

A frozen dragonfly larva in the wetland of our family prairie.

As I wandered around our wetland, I found several dragonfly larvae and a couple other aquatic insects frozen at or near the surface of the ice.  I’m still trying to puzzle out how they got there.  My best guess is that they must have been swimming near the surface as the water around them neared its freezing point.  Maybe they got cold enough they couldn’t swim back down before the water around them froze?  Regardless, there they were, right at the surface.  In some cases, they were partially exposed to the air as the ice was melting and/or sublimating from around them.

Another dragonfly larva – this one was upside down when it froze.

Dragonfly larvae breathe through gills, which I assume means they can’t survive for long out of water.  They can apparently survive being frozen, at least for a while, but I assume they only survive if they thaw out underwater where they can breathe.  If they thaw out on top of a frozen pond, that seems like a really bad outcome…  If so, the larvae I was seeing were either already dead or doomed to be so.

Gusts of wind were blowing snow across this partially exposed dragonfly larva on the surface of the ice.

I’m still not sure why the larvae would have been swimming near the water’s surface as it froze, or if that’s what actually happened.  It’s not an isolated incident – I find insects near the frozen surface of wetlands and ponds pretty frequently.  Anyone have a great explanation for what’s happening?

This entry was posted in Prairie Insects, Prairie Photography and tagged , , , by Chris Helzer. Bookmark the permalink.

About Chris Helzer

Chris Helzer is the Director of Science for The Nature Conservancy in Nebraska. His main role is to evaluate and capture lessons from the Conservancy’s land management and restoration work and then share those lessons with other landowners – both private and public. In addition, Chris works to raise awareness about the importance of prairies and their conservation through his writing, photography, and presentations to various groups. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press. He lives in Aurora, Nebraska with his wife Kim and their children.

19 thoughts on “Photo of the Week – February 8, 2018

  1. P.S. My smart wife pointed out that it’s possible the larvae were dead and floated to the surface. I asked why they would float when they were dead if they didn’t float when they were alive, and she reminded me that gases from decomposer organisms often make carcasses float even if they sink initially. It’s a pretty good theory…

  2. This is fabulous, Chris. Your final photo, snow gusting across an ice-entombed larva, is especially spectacular. Thank you for provoking so much thought and reflection, both biologically and artistically! You do much in connecting our prairies to ourselves.

  3. One thought did come to me when I read this sentence of yours: “Maybe they got cold enough they couldn’t swim back down before the water around them froze?”

    When we had our recent experience with ice and frigid temperatures here on the Gulf coast, there were many hundreds of turtles that were cold-stunned. Below 50F/10C, turtles become hypothermic and unable to swim, so they float up to the surface, where they’re subject to boat strikes, predation, or washing onto the shore. If temperatures drop rapidly, turtles that have come into the shallows of the bays often can’t make it to the deeper waters of the Gulf in time to avoid problems.

    There’s a good article about the phenomenon here.

  4. Maybe the thermal mass from the earth keeps the deeper water warmer in comparison to the surface and when the larvae swim to the surface they freeze??? thought provoking…..
    Keep us thinking

  5. We love your work and wander about our maritime Canada winter landscape asking ourselves the same sorts of questions…but coming from a line of scientists, and both being curious as chimps, we speculated on if one could collect a few, thaw them out, and see what happens. The photos are amazing…the last particularly brought up images in my mind of the Franklin expedition and I felt great empathy for this tiny (voracious predator) and its challenging life.

  6. I’m thinking along the same lines as marianwhit (in terms of collecting and thawing). However, the ones that are exposed, the tissue looks to be very pale, which would suggest death. I can’t add anything to your question about what an insect larva would be doing swimming so close to the surface that it would be frozen. Wondered if there possibly was some action of freezing that would push enclosed items higher in the water table? I can’t think of any except for water/ice loss due to evaporation. I’m from the humid East, so not familiar with the extremes in your study area. I have observed chorus frogs frozen briefly in ice, which thawed out just fine. Also a colleague has studied freezing in turtle nests, but those weren’t encased in ice, just frozen (not sure how solid, but they did a test with turtle hatchlings in a freezer later) and they did thaw out successfully.

  7. As a teenager I spent a couple summers on a lake in lower Michigan. One of the highlights was watching dragonfly larvae hatch. Our local kind would climb up onto a dock or plant growing up out of the water, then begin their exquisite dance of cracking their “shell” and pulling in themselves out of it, bit by bit. Feet, then body, wings unfolding and drying, body telescoping out to its full lengths. Amazing.Then, finally, taking flight and going on into it’s next, radically different, phase of life.
    How and when does that itch to climb out of the water hit them? I’m wondering if your frozen larvae had that irresistible urge to come to the surface and climb out to transform at a bad moment in time and froze in the process? I’ve read that sometimes their wing-drying came be interrupted by rainstorms to a bad end. Sudden freezes could have the same effect. Though I like the idea that maybe they could thaw out and finish the process.
    Thanks for your photos and columns. They take me “home” and out of the city.

  8. There is higher oxygen concentration at the surface, but it’s also colder than further down and they may have gone up for air and gotten too chilled to go back down. Though cold water holds more oxygen than warm and I wouldn’t really expect them to have needed to go up in the first place…

  9. The water freezes from the top down, thus it is likely that the insect swam to the surface, probably being slow because of the cold temperature, was frozen in place and the water froze underneath. Honey bees on cold winter days with snow on the ground may leave the hive, hit the cold outside and drop into the snow near the hive. If one recovers them quickly enough they seem to revive. I have returned some to the hive, but do not know if their long term survival was impacted.

  10. Pingback: Thawing Frozen Bugs; The Grand Experiment | The Prairie Ecologist


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